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PURPOSE: Electronic portal imaging devices (EPIDs) have been widely utilized for patient-specific quality assurance (PSQA) and their use for transit dosimetry applications is emerging. Yet there are no specific guidelines on the potential uses, limitations, and correct utilization of EPIDs for these purposes. The American Association of Physicists in Medicine (AAPM) Task Group 307 (TG-307) provides a comprehensive review of the physics, modeling, algorithms and clinical experience with EPID-based pre-treatment and transit dosimetry techniques. This review also includes the limitations and challenges in the clinical implementation of EPIDs, including recommendations for commissioning, calibration and validation, routine QA, tolerance levels for gamma analysis and risk-based analysis. METHODS: Characteristics of the currently available EPID systems and EPID-based PSQA techniques are reviewed. The details of the physics, modeling, and algorithms for both pre-treatment and transit dosimetry methods are discussed, including clinical experience with different EPID dosimetry systems. Commissioning, calibration, and validation, tolerance levels and recommended tests, are reviewed, and analyzed. Risk-based analysis for EPID dosimetry is also addressed. RESULTS: Clinical experience, commissioning methods and tolerances for EPID-based PSQA system are described for pre-treatment and transit dosimetry applications. The sensitivity, specificity, and clinical results for EPID dosimetry techniques are presented as well as examples of patient-related and machine-related error detection by these dosimetry solutions. Limitations and challenges in clinical implementation of EPIDs for dosimetric purposes are discussed and acceptance and rejection criteria are outlined. Potential causes of and evaluations of pre-treatment and transit dosimetry failures are discussed. Guidelines and recommendations developed in this report are based on the extensive published data on EPID QA along with the clinical experience of the TG-307 members. CONCLUSION: TG-307 focused on the commercially available EPID-based dosimetric tools and provides guidance for medical physicists in the clinical implementation of EPID-based patient-specific pre-treatment and transit dosimetry QA solutions including intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) treatments.
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PURPOSE: At the Netherlands Cancer Institute--Antoni van Leeuwenhoek Hospital in vivo dosimetry using an electronic portal imaging device (EPID) has been implemented for almost all high-energy photon treatments of cancer with curative intent. Lung cancer treatments were initially excluded, because the original back-projection dose-reconstruction algorithm uses water-based scatter-correction kernels and therefore does not account for tissue inhomogeneities accurately. The aim of this study was to test a new method, in aqua vivo EPID dosimetry, for fast dose verification of lung cancer irradiations during actual patient treatment. METHODS: The key feature of our method is the dose reconstruction in the patient from EPID images, obtained during the actual treatment, whereby the images have been converted to a situation as if the patient consisted entirely of water; hence, the method is termed in aqua vivo. This is done by multiplying the measured in vivo EPID image with the ratio of two digitally reconstructed transmission images for the unit-density and inhomogeneous tissue situation. For dose verification, a comparison is made with the calculated dose distribution with the inhomogeneity correction switched off. IMRT treatment verification is performed for each beam in 2D using a 2D γ evaluation, while for the verification of volumetric-modulated arc therapy (VMAT) treatments in 3D a 3D γ evaluation is applied using the same parameters (3%, 3 mm). The method was tested using two inhomogeneous phantoms simulating a tumor in lung and measuring its sensitivity for patient positioning errors. Subsequently five IMRT and five VMAT clinical lung cancer treatments were investigated, using both the conventional back-projection algorithm and the in aqua vivo method. The verification results of the in aqua vivo method were statistically analyzed for 751 lung cancer patients treated with IMRT and 50 lung cancer patients treated with VMAT. RESULTS: The improvements by applying the in aqua vivo approach are considerable. The percentage of γ values ≤1 increased on average from 66.2% to 93.1% and from 43.6% to 97.5% for the IMRT and VMAT cases, respectively. The corresponding mean γ value decreased from 0.99 to 0.43 for the IMRT cases and from 1.71 to 0.40 for the VMAT cases, which is similar to the accepted clinical values for the verification of IMRT treatments of prostate, rectum, and head-and-neck cancers. The deviation between the reconstructed and planned dose at the isocenter diminished on average from 5.3% to 0.5% for the VMAT patients and was almost the same, within 1%, for the IMRT cases. The in aqua vivo verification results for IMRT and VMAT treatments of a large group of patients had a mean γ of approximately 0.5, a percentage of γ values ≤1 larger than 89%, and a difference of the isocenter dose value less than 1%. CONCLUSIONS: With the in aqua vivo approach for the verification of lung cancer treatments (IMRT and VMAT), we can achieve results with the same accuracy as obtained during in vivo EPID dosimetry of sites without large inhomogeneities.
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Algoritmos , Neoplasias Pulmonares/radioterapia , Radiometria/instrumentação , Radiometria/métodos , Planejamento da Radioterapia Assistida por Computador/métodos , Radioterapia Assistida por Computador/métodos , Ecrans Intensificadores para Raios X , Humanos , Dosagem RadioterapêuticaRESUMO
PURPOSE: Electronic portal imaging devices (EPIDs) are increasingly used for IMRT dose verification, both pretreatment and in vivo. In this study, an earlier developed backprojection model has been modified to avoid the need for patient-specific transmission measurements and, consequently, leads to a faster procedure. METHODS: Currently, the transmission, an essential ingredient of the backprojection model, is estimated from the ratio of EPID measurements with and without a phantom/patient in the beam. Thus, an additional irradiation to obtain "open images" under the same conditions as the actual phantom/patient irradiation is required. However, by calculating the transmission of the phantom/patient in the direction of the beam instead of using open images, this extra measurement can be avoided. This was achieved by using a model that includes the effect of beam hardening and off-axis dependence of the EPID response on photon beam spectral changes. The parameters in the model were empirically obtained by performing EPID measurements using polystyrene slab phantoms of different thickness in 6, 10, and 18 MV photon beams. A theoretical analysis to verify the sensitivity of the model with patient thickness changes was performed. The new model was finally applied for the analysis of EPID dose verification measurements of step-and-shoot IMRT treatments of head and neck, lung, breast, cervix, prostate, and rectum patients. All measurements were carried out using Elekta SL20i linear accelerators equipped with a hydrogenated amorphous silicon EPID, and the IMRT plans were made using PINNACLE software (Philips Medical Systems). RESULTS: The results showed generally good agreement with the dose determined using the old model applying the measured transmission. The average differences between EPID-based in vivo dose at the isocenter determined using either the new model for transmission and its measured value were 2.6 +/- 3.1%, 0.2 +/- 3.1%, and 2.2 +/- 3.9% for 47 patients treated with 6, 10, and 18 MV IMRT beams, respectively. For the same group of patients, the differences in mean gamma analysis (3% maximum dose, 3 mm) were 0.16 +/- 0.26%, 0.21 +/- 0.24%, and 0.02 +/- 0.12%, respectively. For a subgroup of 11 patients, pretreatment verification was also performed, showing similar dose differences at the isocenter: -1.9 +/- 0.9%, -1.4 +/- 1.2%, and -0.4 +/- 2.4%, with somewhat lower mean gamma difference values: 0.01 +/- 0.09%, 0.01 +/- 0.07%, and -0.09 +/- 0.10%, respectively. Clinical implementation of the new model would save 450 h/yr spent in measurement of open images. CONCLUSIONS: It can be concluded that calculating instead of measuring the transmission leads to differences in the isocenter dose generally smaller than 2% (2.6% for 6 MV photon beams for in vivo dose) and yielded only slightly higher gamma-evaluation parameter values in planes through the isocenter. Hence, the new model is suitable for clinical implementation and measurement of open images can be omitted.
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Equipamentos e Provisões Elétricas , Radiometria/instrumentação , Radioterapia Assistida por Computador/instrumentação , Radioterapia de Intensidade Modulada/métodos , Calibragem , Humanos , Processamento de Imagem Assistida por Computador , Masculino , Neoplasias/radioterapia , Fótons/uso terapêuticoRESUMO
Treatment plans are usually designed, optimized, and evaluated based on the total 3D dose distribution, motivating a total 3D dose verification. The purpose of this study was to develop a 2D transmission-dosimetry method using an electronic portal imaging device (EPID) into a simple 3D method that provides 3D dose information. In the new method, the dose is reconstructed within the patient volume in multiple planes parallel to the EPID for each gantry angle. By summing the 3D dose grids of all beams, the 3D dose distribution for the total treatment fraction is obtained. The algorithm uses patient contours from the planning CT scan but does not include tissue inhomogeneity corrections. The 3D EPID dosimetry method was tested for IMRT fractions of a prostate, a rectum, and a head-and-neck cancer patient. Planned and in vivo-measured dose distributions were within 2% at the dose prescription point. Within the 50% isodose surface of the prescribed dose, at least 97% of points were in agreement, evaluated with a 3D gamma method with criteria of 3% of the prescribed dose and 0.3 cm. Full 3D dose reconstruction on a 0.1 x 0.1 x 0.1 cm3 grid and 3D gamma evaluation took less than 15 min for one fraction on a standard PC. The method allows in vivo determination of 3D dose-volume parameters that are common in clinical practice. The authors conclude that their EPID dosimetry method is an accurate and fast tool for in vivo dose verification of IMRT plans in 3D. Their approach is independent of the treatment planning system and provides a practical safety net for radiotherapy.
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Algoritmos , Radiometria/métodos , Radioterapia de Intensidade Modulada , Humanos , Masculino , Modelos Teóricos , Neoplasias Orofaríngeas/radioterapia , Imagens de Fantasmas , Neoplasias da Próstata/radioterapia , Dosagem Radioterapêutica , Planejamento da Radioterapia Assistida por Computador , Neoplasias Retais/radioterapia , Tomografia Computadorizada por Raios XRESUMO
As radiotherapy becomes more complicated, dose and geometry verification become more necessary. The aim of this study was to use back-projected EPID-based 3D in vivo dosimetry and cone-beam CT (CBCT) to obtain a complete account of the entire treatment for a select patient group. Nine hypo-fractionated rectum IMRT patient plans were investigated. The absolute dose was reconstructed at multiple planes using patient contours and EPID images acquired for all fields during treatment. The meso-rectal fat (m-R) was re-delineated on daily CBCT scans, acquired prior to each fraction. The total accumulated dose was determined by mapping the m-R surface of each fraction to the planned m-R surface. Average planned and measured isocentre dose ratios were 0.98 (+/-0.01SD). 3D gamma analysis (3% maximum dose and 3mm) revealed mean gamma, gamma(mean)=0.35 (+/-0.03 SD), maximum 1% of gamma points, gamma(max1%)=1.02 (+/-0.14SD) and the percentage of points with gamma < or = 1, P(gamma < or = 1)=99% (range [96%, 100%]), averaged over all patients. CBCT m-R volumes varied by up to 20% of planned volumes, but remained in the high dose region. Over-dosage of up to 4.5% in one fraction was measured in the presence of gas pockets. By combining EPID dosimetry with CBCT geometry information, the total dose can be verified in 3D in vivo and compared with the planned dose distribution. This method can provide a safety net for advanced treatments involving dose escalation, as well as a full account of the delivered dose to specific volumes, allowing adaptation of the treatment from the original plan if necessary.
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Tomografia Computadorizada de Feixe Cônico , Fracionamento da Dose de Radiação , Imageamento Tridimensional , Planejamento da Radioterapia Assistida por Computador , Radioterapia de Intensidade Modulada , Humanos , Radiometria , Dosagem Radioterapêutica , Neoplasias Retais/radioterapiaRESUMO
PURPOSE: To investigate the feasibility of replacing pretreatment verification with in vivo electronic portal imaging device (EPID) dosimetry for prostate intensity-modulated radiotherapy (IMRT). METHODS AND MATERIALS: Dose distributions were reconstructed from EPID images, inside a phantom (pretreatment) or the patient (five fractions in vivo) for 75 IMRT prostate plans. Planned and EPID dose values were compared at the isocenter and in two dimensions using the gamma index (3%/3 mm). The number of measured in vivo fractions required to achieve similar levels of agreement with the plan as pretreatment verification was determined. The time required to perform both methods was compared. RESULTS: Planned and EPID isocenter dose values agreed, on average, within +/-1% (1 SD) of the total plan for both pretreatment and in vivo verification. For two-dimensional field-by-field verification, an alert was raised for 10 pretreatment checks with clear but clinically irrelevant discrepancies. Multiple in vivo fractions were combined by assessing gamma images consisting of median, minimum and low (intermediate) pixel values of one to five fractions. The "low" gamma values of three fractions rendered similar results as pretreatment verification. Additional time for verification was approximately 2.5 h per plan for pretreatment verification, and 15 min +/- 10 min/fraction using in vivo dosimetry. CONCLUSIONS: In vivo EPID dosimetry is a viable alternative to pretreatment verification for prostate IMRT. For our patients, combining information from three fractions in vivo is the best way to distinguish systematic errors from non-clinically relevant discrepancies, save hours of quality assurance time per patient plan, and enable verification of the actual patient treatment.
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Algoritmos , Neoplasias da Próstata/radioterapia , Planejamento da Radioterapia Assistida por Computador/métodos , Radioterapia de Intensidade Modulada/métodos , Protocolos Clínicos , Análise Custo-Benefício , Estudos de Viabilidade , Humanos , Masculino , Imagens de Fantasmas , Dosagem Radioterapêutica , Planejamento da Radioterapia Assistida por Computador/instrumentação , Radioterapia de Intensidade Modulada/instrumentação , Fatores de TempoRESUMO
PURPOSE: To predict the three-dimensional dose distribution of our total body irradiation technique, using a commercial treatment planning system (TPS). In vivo dosimetry, using metal oxide field effect transistors (MOSFETs) and thermoluminescence detectors (TLDs), was used to verify the calculated dose distributions. METHODS AND MATERIALS: A total body computed tomography scan was performed and loaded into our TPS, and a three-dimensional-dose distribution was generated. In vivo dosimetry was performed at five locations on the patient. Entrance and exit dose values were converted to midline doses using conversion factors, previously determined with phantom measurements. The TPS-predicted dose values were compared with the MOSFET and TLD in vivo dose values. RESULTS: The MOSFET and TLD dose values agreed within 3.0% and the MOSFET and TPS data within 0.5%. The convolution algorithm of the TPS, which is routinely applied in the clinic, overestimated the dose in the lung region. Using a superposition algorithm reduced the calculated lung dose by approximately 3%. The dose inhomogeneity, as predicted by the TPS, can be reduced using a simple intensity-modulated radiotherapy technique. CONCLUSIONS: The use of a TPS to calculate the dose distributions in individual patients during total body irradiation is strongly recommended. Using a TPS gives good insight of the over- and underdosage in a patient and the influence of patient positioning on dose homogeneity. MOSFETs are suitable for in vivo dosimetry purposes during total body irradiation, when using appropriate conversion factors. The MOSFET, TLD, and TPS results agreed within acceptable margins.
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Imageamento Tridimensional/métodos , Planejamento da Radioterapia Assistida por Computador/métodos , Dosimetria Termoluminescente/instrumentação , Tomografia Computadorizada por Raios X/métodos , Irradiação Corporal Total/métodos , Algoritmos , Humanos , Óxidos , Imagens de Fantasmas , Radiometria/instrumentação , Dosagem Radioterapêutica , Dosimetria Termoluminescente/métodos , Transistores EletrônicosRESUMO
BACKGROUND AND PURPOSE: To analyse the results of routine EPID measurements for individualised patient dosimetry. MATERIALS AND METHODS: Calibrated camera-based EPIDs were used to measure the central field dose, which was compared with a dose prediction at the EPID level. For transit dosimetry, dose data were calculated using patient transmission and scatter, and compared with measured values. Furthermore, measured transit dose data were back-projected to an in vivo dose value at 5 cm depth in water (D(5)) and directly compared with D(5) from the treatment planning system. Dose differences per treatment session were calculated by weighting dose values with the number of monitor units per beam. Reported errors were categorised and analysed for approximately 37,500 images from 2511 patients during a period of 24 months. RESULTS: Pre-treatment measurements showed a mean dose difference per treatment session of 0.0+/-1.7% (1 SD). Transfer errors were detected and corrected prior to the first treatment session. An accelerator output variation of about 4% was found between two weekly QC measurements. Patient dosimetry showed mean transit and D(5) dose differences of -0.7+/-5.2% (1 SD) and -0.3+/-5.6% (1 SD) per treatment session, respectively. Dose differences could be related to set-up errors, organ motion, erroneous density corrections and changes in patient anatomy. CONCLUSIONS: EPIDs can be used routinely to accurately verify treatment parameter transfer and machine output. By applying transit and in vivo dosimetry, more insight can be obtained with respect to the different error sources influencing dose delivery to a patient.
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Planejamento da Radioterapia Assistida por Computador , Radioterapia/instrumentação , Neoplasias da Mama/radioterapia , Neoplasias de Cabeça e Pescoço/radioterapia , Humanos , Neoplasias Pulmonares/radioterapia , Neoplasias/radioterapia , Aceleradores de Partículas , Neoplasias Pélvicas/radioterapia , Dosagem Radioterapêutica , Tecnologia RadiológicaRESUMO
The gamma-evaluation method is a tool by which dose distributions can be compared in a quantitative manner combining dose-difference and distance-to-agreement criteria. Since its introduction, the gamma evaluation has been used in many studies and is on the verge of becoming the preferred dose distribution comparison method, particularly for intensity-modulated radiation therapy (IMRT) verification. One major disadvantage, however, is its long computation time, which especially applies to the comparison of three-dimensional (3D) dose distributions. We present a fast algorithm for a full 3D gamma evaluation at high resolution. Both the reference and evaluated dose distributions are first resampled on the same grid. For each point of the reference dose distribution, the algorithm searches for the best point of agreement according to the gamma method in the evaluated dose distribution, which can be done at a subvoxel resolution. Speed, computer memory efficiency, and high spatial resolution are achieved by searching around each reference point with increasing distance in a sphere, which has a radius of a chosen maximum search distance and is interpolated "on-the-fly" at a chosen sample step size. The smaller the sample step size and the larger the differences between the dose distributions, the longer the gamma evaluation takes. With decreasing sample step size, statistical measures of the 3D gamma distribution converge. Two clinical examples were investigated using 3% of the prescribed dose as dose-difference and 0.3 cm as distance-to-agreement criteria. For 0.2 cm grid spacing, the change in gamma indices was negligible below a sample step size of 0.02 cm. Comparing the full 3D gamma evaluation and slice-by-slice 2D gamma evaluations ("2.5D") for these clinical examples, the gamma indices improved by searching in full 3D space, with the average gamma index decreasing by at least 8%.
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Algoritmos , Raios gama/uso terapêutico , Neoplasias Pulmonares/radioterapia , Planejamento da Radioterapia Assistida por Computador/métodos , Simulação por Computador , Humanos , Neoplasias Pulmonares/diagnóstico por imagem , Dosagem Radioterapêutica , Tomografia Computadorizada por Raios XRESUMO
Treatment verification is a prerequisite for the verification of complex treatments, checking both the treatment planning process and the actual beam delivery. Pretreatment verification can detect errors introduced by the treatment planning system (TPS) or differences between planned and delivered dose distributions. In a previous paper we described the reconstruction of three-dimensional (3-D) dose distributions in homogeneous phantoms using an in-house developed model based on the beams delivered by the linear accelerator measured with an amorphous silicon electronic portal imaging device (EPID), and a dose calculation engine using the Monte Carlo code XVMC. The aim of the present study is to extend the method to situations in which tissue inhomogeneities are present and to make a comparison with the dose distributions calculated by the TPS. Dose distributions in inhomogeneous phantoms, calculated using the fast-Fourier transform convolution (FFTC) and multigrid superposition (MGS) algorithms present in the TPS, were verified using the EPID-based dose reconstruction method and compared to film and ionization chamber measurements. Differences between dose distributions were evaluated using the gamma-evaluation method (3%/3 mm) and expressed as a mean gamma and the percentage of points with gamma> 1 (P(gamma>1)). For rectangular inhomogeneous phantoms containing a low-density region, the differences between film and reconstructed dose distributions were smaller than 3%. In low-density regions there was an overestimation of the planned dose using the FFTC and MGS algorithms of the TPS up to 20% and 8%, respectively, for a 10 MV photon beam and a 3 x 3 cm2 field. For lower energies and larger fields (6 MV, 5 x 5 cm2), these differences reduced to 6% and 3%, respectively. Dose reconstruction performed in an anthropomorphic thoracic phantom for a 3-D conformal and an IMRT plan, showed good agreement between film data and reconstructed dose values (P(gamma>1) <6%). The algorithms of the TPS underestimated the dose in the low-dose regions outside the treatment field, due to an implementation error of the jaws and multileaf collimator of the linac in the TPS. The FFTC algorithm of the TPS showed differences up to 6% or 6 mm at the interface between lung and breast. Two intensity-modulated radiation therapy head and neck plans, reconstructed in a commercial phantom having a bone-equivalent insert and an air cavity, showed good agreement between film measurement, reconstructed and planned dose distributions using the FFTC and MGS algorithm, except in the bone-equivalent regions where both TPS algorithms underestimated the dose with 4%. Absolute dose verification was performed at the isocenter where both planned and reconstructed dose were within 2% of the measured dose. Reproducibility for the EPID measurements was assessed and found to be of negligible influence on the reconstructed dose distribution. Our 3-D dose verification approach is based on the actual dose measured with an EPID in combination with a Monte Carlo dose engine, and therefore independent of a TPS. Because dose values are reconstructed in 3-D, isodose surfaces and dose-volume histograms can be used to detect dose differences in target volume and normal tissues. Using our method, the combined planning and treatment delivery process is verified, offering an easy to use tool for the verification of complex treatments.
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Dosagem Radioterapêutica , Planejamento da Radioterapia Assistida por Computador , Algoritmos , Humanos , Método de Monte Carlo , Radiometria , Radioterapia de Intensidade Modulada , Reprodutibilidade dos TestesRESUMO
PURPOSE: To quantify the effect of set-up errors and respiratory motion on dose distributions for non-small cell lung cancer (NSCLC) treatment. METHODS AND MATERIALS: Irradiations of 5 NSCLC patients were planned with 3 techniques, two (conformal radiation therapy (CRT) and intensity modulated radiation therapy (IMRT1)) with a homogeneous dose in the planning target volume (PTV) and a third (IMRT2) with dose heterogeneity. Set-up errors were simulated for gross target volume (GTV) and organs at risk (OARs). For the GTV, the respiration was also simulated with a periodical motion around a varying average. Two configurations were studied for the breathing motion, to describe the situations of free-breathing (FB) and respiration-correlated (RC) CT scans, each with 2 amplitudes (5 and 10 mm), thus resulting in 4 scenarios (FB_5, FB_10, RC_5 and RC_10). Five thousand treatment courses were simulated, producing probability distributions for the dosimetric parameters. RESULTS: For CRT and IMRT1, RC_5, RC_10 and FB_5 were associated with a small degradation of the GTV coverage. IMRT2 with FB_10 showed the largest deterioration of the GTV dosimetric indices, reaching 7% for Dmin at the 95% probability level. Removing the systematic error due to the periodic breathing motion was advantageous for a 10 mm respiration amplitude. The estimated probability of radiation pneumonitis and acute complication for the esophagus showed limited sensitivity to geometrical uncertainties. Dmax in the spinal cord and the parameters predicting the risk of late esophageal toxicity were associated to a probability up to 50% of violating the dose tolerances. CONCLUSIONS: Simulating the effect of geometrical uncertainties on the individual patient plan should become part of the standard pre-treatment verification procedure.
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Carcinoma Pulmonar de Células não Pequenas/radioterapia , Neoplasias Pulmonares/radioterapia , Movimento , Radioterapia Conformacional , Respiração , Esôfago/efeitos da radiação , Humanos , Pulmão/efeitos da radiação , Dose Máxima Tolerável , Probabilidade , Lesões por Radiação/etiologia , Pneumonite por Radiação/etiologia , Dosagem Radioterapêutica , Radioterapia de Intensidade Modulada , Medula Espinal/efeitos da radiação , IncertezaRESUMO
BACKGROUND AND PURPOSE: Localisation images normally acquired to verify patient positioning also contain information about the patient's internal anatomy. The aim of this study was to investigate the anatomical changes observed in localisation images and examples of dosimetric consequences. PATIENTS AND METHODS: Localisation images were obtained weekly prior to radiotherapy with an electronic portal imaging device (EPID). A series of 'difference images' was created by subtracting the first localisation image from that of subsequent fractions. Images from 81 lung, 40 head and neck and 34 prostate cancer patients were classified according to the changes observed. Changes were considered relevant if the average pixel value over an area of at least 1cm(2) differed by more than 5%, to allow for variations in linac output and EPID signal. Two patients were selected to illustrate the dosimetric effects of relevant changes. Their plans were re-calculated with repeat CT scans acquired after 4 weeks of treatment and compared with the difference images of the corresponding days. RESULTS: Progressive changes were detected for 57% of lung and 37% of head and neck cancer patients studied. Random changes were observed in 37% of lung, 28% of head and neck and 82% of prostate cancer patients. For a lung case, an increase of 10.0% in EPID dose due to tumour shrinkage corresponded to an increase of 9.8% in mean lung dose. Gas pockets in the rectum region of the prostate case increased the EPID dose by 6.3%, and resulted in a decrease of the minimum dose to the planning target volume of 26.4%. CONCLUSIONS: Difference images are an efficient means of qualitatively detecting anatomical changes for various treatment sites in clinical practice. They can be used to identify changes for a particular patient, to indicate if the dose delivered to the patient would differ from planning and to detect if there is a need for re-planning.
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Diagnóstico por Imagem , Neoplasias de Cabeça e Pescoço/radioterapia , Neoplasias Pulmonares/radioterapia , Neoplasias da Próstata/radioterapia , Planejamento da Radioterapia Assistida por Computador , Feminino , Cabeça/anatomia & histologia , Neoplasias de Cabeça e Pescoço/diagnóstico , Humanos , Pulmão/anatomia & histologia , Neoplasias Pulmonares/diagnóstico , Masculino , Pescoço/anatomia & histologia , Próstata/anatomia & histologia , Neoplasias da Próstata/diagnóstico , Dosagem RadioterapêuticaRESUMO
BACKGROUND AND PURPOSE: To determine the factors converting the reading of a MOSFET detector placed on the patient's skin without additional build-up to the dose at the depth of dose maximum (D(max)) and investigate their feasibility for in vivo dose measurements in electron beams. MATERIALS AND METHODS: Factors were determined to relate the reading of a MOSFET detector to D(max) for 4 - 15 MeV electron beams in reference conditions. The influence of variation in field size, SSD, angle and field shape on the MOSFET reading, obtained without additional build-up, was evaluated using 4, 8 and 15 MeV beams and compared to ionisation chamber data at the depth of dose maximum (z(max)). Patient entrance in vivo measurements included 40 patients, mostly treated for breast tumours. The MOSFET reading, converted to D(max), was compared to the dose prescribed at this depth. RESULTS: The factors to convert MOSFET reading to D(max) vary between 1.33 and 1.20 for the 4 and 15 MeV beams, respectively. The SSD correction factor is approximately 8% for a change in SSD from 95 to 100 cm, and 2% for each 5-cm increment above 100 cm SSD. A correction for fields having sides smaller than 6 cm and for irregular field shape is also recommended. For fields up to 20 x 20 cm(2) and for oblique incidence up to 45 degrees, a correction is not necessary. Patient measurements demonstrated deviations from the prescribed dose with a mean difference of -0.7% and a standard deviation of 2.9%. CONCLUSION: Performing dose measurements with MOSFET detectors placed on the patient's skin without additional build-up is a well suited technique for routine dose verification in electron beams, when applying the appropriate conversion and correction factors.
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Neoplasias da Mama/radioterapia , Elétrons/uso terapêutico , Garantia da Qualidade dos Cuidados de Saúde , Radiometria/instrumentação , Planejamento da Radioterapia Assistida por Computador/instrumentação , Pele/efeitos da radiação , Calibragem , Estudos de Viabilidade , Humanos , Radiometria/normas , Dosagem Radioterapêutica , Semicondutores , Sensibilidade e Especificidade , Transistores EletrônicosRESUMO
BACKGROUND AND PURPOSE: To design a clinically reliable and efficient step-and-shoot IMRT delivery technique for the treatment of breast cancer using direct aperture optimization (DAO). Using DAO, segments are created and optimized within the same optimization process. PATIENTS AND METHODS: The DAO technique implemented in the Pinnacle treatment planning system, which is called direct machine parameter optimization (DMPO), was used to generate IMRT plans for twelve breast cancer patients. The prescribed dose was 50 Gy. Two DMPO plans were generated. The first approach uses DMPO only; the second technique combines DMPO with two predefined segments (DMPO(segm)), having shapes identical to the conventional tangential fields. The weight of these predefined segments is optimized simultaneously with DMPO. The DMPO plans were compared with normal two-step (TS) IMRT, creating segments after optimizing the intensity. RESULTS: Dose homogeneity within the target volume was 4.8+/-0.6, 4.3+/-0.5 and 3.8+/-0.5 Gy for the TS, DMPO and DMPO(segm) plans, respectively. Comparing the IMRT plans with an idealized dose distribution obtained using only beamlet optimization, the degradation of the dose distribution was less for the DMPO plans compared with the two-step IMRT approach. Furthermore, this degradation was similar for all patients, while for the two-step IMRT approach it was patient specific. CONCLUSIONS: An efficient step-and-shoot IMRT solution was developed for the treatment of breast cancer using DMPO combined with two predefined segments.
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Neoplasias da Mama/radioterapia , Radioterapia de Intensidade Modulada/métodos , Algoritmos , Humanos , Dosagem Radioterapêutica , Radioterapia de Intensidade Modulada/instrumentaçãoRESUMO
The use of electronic portal imaging devices (EPIDs) is a promising method for the dosimetric verification of external beam, megavoltage radiation therapy-both pretreatment and in vivo. In this study, a previously developed EPID back-projection algorithm was modified for IMRT techniques and applied to an amorphous silicon EPID. By using this back-projection algorithm, two-dimensional dose distributions inside a phantom or patient are reconstructed from portal images. The model requires the primary dose component at the position of the EPID. A parametrized description of the lateral scatter within the imager was obtained from measurements with an ionization chamber in a miniphantom. In addition to point dose measurements on the central axis of square fields of different size, we also used dose profiles of those fields as reference input data for our model. This yielded a better description of the lateral scatter within the EPID, which resulted in a higher accuracy in the back-projected, two-dimensional dose distributions. The accuracy of our approach was tested for pretreatment verification of a five-field IMRT plan for the treatment of prostate cancer. Each field had between six and eight segments and was evaluated by comparing the back-projected, two-dimensional EPID dose distribution with a film measurement inside a homogeneous slab phantom. For this purpose, the y-evaluation method was used with a dose-difference criterion of 2% of dose maximum and a distance-to-agreement criterion of 2 mm. Excellent agreement was found between EPID and film measurements for each field, both in the central part of the beam and in the penumbra and low-dose regions. It can be concluded that our modified algorithm is able to accurately predict the dose in the midplane of a homogeneous slab phantom. For pretreatment IMRT plan verification, EPID dosimetry is a reliable and potentially fast tool to check the absolute dose in two dimensions inside a phantom for individual IMRT fields. Film measurements inside a phantom can therefore be replaced by EPID measurements.
Assuntos
Intensificação de Imagem Radiográfica/métodos , Interpretação de Imagem Radiográfica Assistida por Computador/métodos , Radiometria/métodos , Dosagem Radioterapêutica , Algoritmos , Desenho de Equipamento , Dosimetria Fotográfica/métodos , Humanos , Aceleradores de Partículas , Imagens de Fantasmas , Interpretação de Imagem Radiográfica Assistida por Computador/instrumentação , Reprodutibilidade dos Testes , Espalhamento de Radiação , Sensibilidade e EspecificidadeRESUMO
The verification of intensity-modulated radiation therapy (IMRT) is necessary for adequate quality control of the treatment. Pretreatment verification may trace the possible differences between the planned dose and the actual dose delivered to the patient. To estimate the impact of differences between planned and delivered photon beams, a three-dimensional (3-D) dose verification method has been developed that reconstructs the dose inside a phantom. The pretreatment procedure is based on portal dose images measured with an electronic portal imaging device (EPID) of the separate beams, without the phantom in the beam and a 3-D dose calculation engine based on the Monte Carlo calculation. Measured gray scale portal images are converted into portal dose images. From these images the lateral scattered dose in the EPID is subtracted and the image is converted into energy fluence. Subsequently, a phase-space distribution is sampled from the energy fluence and a 3-D dose calculation in a phantom is started based on a Monte Carlo dose engine. The reconstruction model is compared to film and ionization chamber measurements for various field sizes. The reconstruction algorithm is also tested for an IMRT plan using 10 MV photons delivered to a phantom and measured using films at several depths in the phantom. Depth dose curves for both 6 and 10 MV photons are reconstructed with a maximum error generally smaller than 1% at depths larger than the buildup region, and smaller than 2% for the off-axis profiles, excluding the penumbra region. The absolute dose values are reconstructed to within 1.5% for square field sizes ranging from 5 to 20 cm width. For the IMRT plan, the dose was reconstructed and compared to the dose distribution with film using the gamma evaluation, with a 3% and 3 mm criterion. 99% of the pixels inside the irradiated field had a gamma value smaller than one. The absolute dose at the isocenter agreed to within 1% with the dose measured with an ionization chamber. It can be concluded that our new dose reconstruction algorithm is able to reconstruct the 3-D dose distribution in phantoms with a high accuracy. This result is obtained by combining portal dose images measured prior to treatment with an accurate dose calculation engine.
Assuntos
Processamento de Imagem Assistida por Computador , Imageamento Tridimensional/métodos , Radioterapia de Intensidade Modulada/métodos , Algoritmos , Dosimetria Fotográfica , Humanos , Modelos Estatísticos , Método de Monte Carlo , Aceleradores de Partículas , Imagens de Fantasmas , Radiometria , Dosagem Radioterapêutica , Radioterapia Assistida por ComputadorRESUMO
PURPOSE: To quantify the dose escalation achievable in the treatment of non-small-cell lung cancer (NSCLC) by allowing dose heterogeneity in the target volume or using intensity-modulated radiotherapy (IMRT), or both. METHODS AND MATERIALS: Computed tomography data and contours of 10 NSCLC patients with limited movements of the tumor and representing a broad spectrum of clinical cases were selected for this study. Four irradiation techniques were compared: two conformal (CRT) and two IMRT techniques, either prescribing a homogeneous dose in the planning target volume (PTV) (CRT(hom) and IMRT(hom)) or allowing dose heterogeneity (CRT(inhom) and IMRT(inhom)). The dose heterogeneity was allowed only toward high doses, i.e., the minimum dose in the target for CRT(inhom) and IMRT(inhom) could not be lower than for the corresponding homogeneous plan. The dose in the PTV was escalated (fraction size of 2.25 Gy) until either an organ at risk reached the maximum allowed dose or the mean PTV dose reached a maximum level set at 101.25 Gy. RESULTS: When small and convex tumors were irradiated, CRT(hom) could achieve the maximum dose of 101.25 Gy, whereas for bigger and/or concave PTVs the dose level achievable with CRT(hom) was significantly lower, in 1 case even below 60 Gy. The CRT(inhom) allowed on average a 6% dose escalation with respect to CRT(hom). The IMRT(hom) achieved in all except 1 case a mean PTV dose of at least 75 Gy. The gain in mean PTV dose of IMRT(hom) with respect to CRT(hom) ranged from 7.7 to 14.8 Gy and the IMRT(hom) plans were always more conformal than the corresponding CRT(hom) plans. The IMRT(inhom) provided an additional advantage over IMRT(hom) of at least 5 Gy. For all CRT plans the achievable dose was determined by the lung dose threshold, whereas for more than half of the IMRT plans the esophagus was the dose-limiting organ. The IMRT plans were deliverable with 10-12 segments per beam and did not produce an increase of lung volume irradiated at low doses (<20 Gy). CONCLUSIONS: The dose in NSCLC treatments can be escalated by loosening the constraints on maximum dose in the target volume or using IMRT, or both. For large and concave tumors, an average dose escalation of 6% and 17% was possible when dose heterogeneity and IMRT were applied alone. When they were combined, the average dose increase was as high as 35%. Intensity-modulated RT delivered in a static mode can produce homogeneous dose distributions in the target and does not lead to an increase of lung volume receiving (very) low doses, even down to 5 Gy.
Assuntos
Carcinoma Pulmonar de Células não Pequenas/radioterapia , Neoplasias Pulmonares/radioterapia , Carcinoma Pulmonar de Células não Pequenas/patologia , Esôfago/efeitos da radiação , Coração/efeitos da radiação , Humanos , Pulmão/efeitos da radiação , Neoplasias Pulmonares/patologia , Dose Máxima Tolerável , Método de Monte Carlo , Dosagem Radioterapêutica , Planejamento da Radioterapia Assistida por Computador , Medula Espinal/efeitos da radiaçãoRESUMO
BACKGROUND AND PURPOSE: IMRT necessitates extension of existing inter-centre quality assurance programs due to its increased complexity. We assessed the feasibility of an inter-centre verification method for different IMRT techniques. MATERIALS AND METHODS: Eight European radiotherapy institutions of the QUASIMODO network, have designed an IMRT plan for a horseshoe-shaped PTV surrounding a cylindrical OAR in a simplified pelvic phantom. All centres applied common plan objectives but used their own equipment for planning and delivery. They verified the delivery of this plan according to a common protocol with radiographic film and ionisation chamber measurements. The irradiated films, the results of the ionisation chamber measurements and the computed dose distributions were sent to one analysis centre that compared the measured and computed dose distributions with the gamma method and composite dose-area histograms. RESULTS: 4% (relative to the prescribed dose) and 3mm (distance-to-agreement) were decided feasible gamma criteria. The composite dose-area histograms showed a maximum local deviation of 3.5% in the mean dose of the PTV and 5% in the OAR. Systematic differences could be identified, and in some cases explained. CONCLUSIONS: This multi-centre dosimetric verification study demonstrated both the feasibility of a multi-centre quality assurance network to evaluate any IMRT planning and delivery system combination, as well as the validity of the methodology involved.
Assuntos
Redes Comunitárias , Garantia da Qualidade dos Cuidados de Saúde/organização & administração , Radioterapia/normas , Fracionamento da Dose de Radiação , Humanos , Modelos Teóricos , Neoplasias/radioterapia , Radiometria , Reprodutibilidade dos Testes , Interface Usuário-ComputadorRESUMO
BACKGROUND AND PURPOSE: To determine the effect of organ motion and set-up uncertainties on IMRT dose distributions for prostate. METHODS: For five patients, IMRT techniques were designed to irradiate the CTV (prostate plus seminal vesicles). Technique I delivered 78 Gy to PTV1 (CTV+10 mm margin). Technique II delivered 68 Gy to PTV1, and a 10 Gy boost to PTV2 (CTV+an anisotropic margin of 0 to 5 mm). Technique III delivered 68 Gy to PTV1 and simultaneously 78 Gy to PTV2. Uncertainties were simulated using population statistics of organ motion and set-up accuracy. The average TCP (TCPpop) of the CTV and average NTCP (NTCPpop) of the rectal wall were calculated. RESULTS: The planning TCP was a good predictor for TCPpop for Techniques I and II. Technique III was sensitive for geometrical uncertainties, reducing TCPpop by 0.8 to 2.4% compared to planning. NTCPpop was reduced for Technique III by a factor 2.6 compared to Technique I. For all plans, the planning NTCP was strongly correlated with NTCPpop. CONCLUSIONS: Dose distributions created with Techniques I and II are insensitive for geometrical uncertainties, while Technique III resulted in a reduction of TCPpop. This reduction can be compensated by a small dose escalation, while still resulting in an NTCPpop of the rectal wall that is lower or comparable to Technique I.
Assuntos
Neoplasias da Próstata/radioterapia , Planejamento da Radioterapia Assistida por Computador , Radioterapia de Intensidade Modulada , Humanos , Masculino , Movimento , Próstata/efeitos da radiação , Dosagem Radioterapêutica , Reto/efeitos da radiação , Glândulas Seminais/efeitos da radiaçãoRESUMO
BACKGROUND AND PURPOSE: The purpose of this work was a comparison of realistic IMRT plans based on the same CT-image data set and a common predefined set of dose objectives for the planning target volume and the organs at risk. This work was part of the larger European QUASIMODO IMRT verification project. MATERIALS AND METHODS: Eleven IMRT plans were produced by nine different European groups, each applying a representative set of clinically used IMRT treatment planning systems. The plans produced were to be deliverable in a clinically acceptable treatment time with the local technical equipment. All plans were characterized using a set of different quality measures such as dose-volume histograms, number of monitor units and treatment time. RESULTS: Only one plan was able to fulfil all dose objectives strictly; six plans failed some of the objectives but were still considered to be clinically acceptable; four plans were not able to reach the objectives. Additional quality scores such as the number of monitor units and treatment time showed large variations, which mainly depend on the delivery technique. CONCLUSION: The presented planning study showed that with nearly all presently available IMRT planning and delivery systems comparable dose distributions could be achieved if the planning goals are clearly defined in advance.