Transit and non-transit 3D EPID dosimetry versus detector arrays for patient specific QA.

PURPOSE: Despite their availability and simplicity of use, Electronic Portal Imaging Devices (EPIDs) have not yet replaced detector arrays for patient specific QA in 3D. The purpose of this study is to perform a large scale dosimetric evaluation of transit and non-transit EPID dosimetry against absolute dose measurements in 3D. METHODS: After evaluating basic dosimetric characteristics of the EPID and two detector arrays (Octavius 1500 and Octavius 1000SRS ), 3D dose distributions for 68 VMAT arcs, and 10 IMRT plans were reconstructed within the same phantom geometry using transit EPID dosimetry, non-transit EPID dosimetry, and the Octavius 4D system. The reconstructed 3D dose distributions were directly compared by γ-analysis (2L2 = 2% local/2 mm and 3G2 = 3% global/2 mm, 50% isodose) and by the percentage difference in median dose to the high dose volume (%∆HDVD 50 ). RESULTS: Regarding dose rate dependency, dose linearity, and field size dependence, the agreement between EPID dosimetry and the two detector arrays was found to be within 1.0%. In the 2L2 γ-comparison with Octavius 4D dose distributions, the average γ-pass rate value was 92.2 ± 5.2%(1SD) and 94.1 ± 4.3%(1SD) for transit and non-transit EPID dosimetry, respectively. 3G2 γ-pass rate values were higher than 95% in 150/156 cases. %∆HDVD 50 values were within 2% in 134/156 cases and within 3% in 155/156 cases. With regard to the clinical classification of alerts, 97.5% of the treatments were equally classified by EPID dosimetry and Octavius 4D. CONCLUSION: Transit and non-transit EPID dosimetry are equivalent in dosimetric terms to conventional detector arrays for patient specific QA. Non-transit 3D EPID dosimetry can be readily used for pre-treatment patient specific QA of IMRT and VMAT, eliminating the need of phantom positioning.

Accuracy requirements and uncertainties in radiotherapy: a report of the International Atomic Energy Agency.

BACKGROUND: Radiotherapy technology continues to advance and the expectation of improved outcomes requires greater accuracy in various radiotherapy steps. Different factors affect the overall accuracy of dose delivery. Institutional comprehensive quality assurance (QA) programs should ensure that uncertainties are maintained at acceptable levels. The International Atomic Energy Agency has recently developed a report summarizing the accuracy achievable and the suggested action levels, for each step in the radiotherapy process. Overview of the report: The report seeks to promote awareness and encourage quantification of uncertainties in order to promote safer and more effective patient treatments. The radiotherapy process and the radiobiological and clinical frameworks that define the need for accuracy are depicted. Factors that influence uncertainty are described for a range of techniques, technologies and systems. Methodologies for determining and combining uncertainties are presented, and strategies for reducing uncertainties through QA programs are suggested. The role of quality audits in providing international benchmarking of achievable accuracy and realistic action levels is also discussed. RECOMMENDATIONS: The report concludes with nine general recommendations: (1) Radiotherapy should be applied as accurately as reasonably achievable, technical and biological factors being taken into account. (2) For consistency in prescribing, reporting and recording, recommendations of the International Commission on Radiation Units and Measurements should be implemented. (3) Each institution should determine uncertainties for their treatment procedures. Sample data are tabulated for typical clinical scenarios with estimates of the levels of accuracy that are practically achievable and suggested action levels. (4) Independent dosimetry audits should be performed regularly. (5) Comprehensive quality assurance programs should be in place. (6) Professional staff should be appropriately educated and adequate staffing levels should be maintained. (7) For reporting purposes, uncertainties should be presented. (8) Manufacturers should provide training on all equipment. (9) Research should aid in improving the accuracy of radiotherapy. Some example research projects are suggested.

Portal dosimetry in wedged beams.

Portal dosimetry using electronic portal imaging devices (EPIDs) is often applied to verify high-energy photon beam treatments. Due to the change in photon energy spectrum, the resulting dose values are, however, not very accurate in the case of wedged beams if the pixel-to-dose conversion for the situation without wedge is used. A possible solution would be to consider a wedged beam as another photon beam quality requiring separate beam modeling of the dose calculation algorithm. The aim of this study was to investigate a more practical solution: to make aSi EPID-based dosimetry models also applicable for wedged beams without an extra commissioning effort of the parameters of the model. For this purpose two energy-dependent wedge multiplication factors have been introduced to be applied for portal images taken with and without a patient/phantom in the beam. These wedge multiplication factors were derived from EPID and ionization chamber measurements at the EPID level for wedged and nonwedged beams, both with and without a polystyrene slab phantom in the beam. This method was verified for an EPID dosimetry model used for wedged beams at three photon beam energies (6, 10, and 18 MV) by comparing dose values reconstructed in a phantom with data provided by a treatment planning system (TPS), as a function of field size, depth, and off-axis distance. Generally good agreement, within 2%, was observed for depths between dose maximum and 15 cm. Applying the new model to EPID dose measurements performed during ten breast cancer patient treatments with wedged 6 MV photon beams showed that the average isocenter underdosage of 5.3% was reduced to 0.4%. Gamma-evaluation (global 3%/3 mm) of these in vivo data showed an increase in percentage of points with γ ≤ 1 from 60.2% to 87.4%, while γmean reduced from 1.01 to 0.55. It can be concluded that, for wedged beams, the multiplication of EPID pixel values with an energy-dependent correction factor provides good agreement between dose values determined by an EPID and a TPS, indicating the usefulness of such a practical solution.

AAPM Task Group Report 307: Use of EPIDs for Patient-Specific IMRT and VMAT QA.

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.

In aqua vivo EPID dosimetry.

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.

Extending in aqua portal dosimetry with dose inhomogeneity conversion maps for accurate patient dose reconstruction in external beam radiotherapy.

Background and purpose: In aqua dosimetry with electronic portal imaging devices (EPIDs) allows for dosimetric treatment verification in external beam radiotherapy by comparing EPID-reconstructed dose distributions (EPID_IA) with dose distributions calculated with the treatment planning system in water-equivalent geometries. The main drawback of the method is the inability to estimate the dose delivered to the patient. In this study, an extension to the method is presented to allow for patient dose reconstruction in the presence of inhomogeneities. Materials and methods: EPID_IA dose distributions were converted into patient dose distributions (EPID_IA_MC) by applying a 3D dose inhomogeneity conversion, defined as the ratio between patient and water-filled patient dose distributions computed using Monte Carlo calculations. EPID_IA_MC was evaluated against dose distributions calculated with a collapsed cone convolution superposition (CCCS) algorithm and with a GPU-based Monte Carlo dose calculation platform (GPUMCD) using non-transit EPID measurements of 25 plans. In vivo EPID measurements of 20 plans were also analyzed. Results: In the evaluation of EPID_IA_MC, the average γ-mean values (2% local/2mm, 50% isodose volume) were 0.70 ± 0.14 (1SD) and 0.66 ± 0.10 (1SD) against CCCS and GPUMCD, respectively. Percentage differences in median dose to the planning target volume were within 3.9% and 2.7%, respectively. The number of in vivo dosimetric alerts with EPID_IA_MC was comparable to EPID_IA. Conclusions: EPID_IA_MC accommodates accurate patient dose reconstruction for treatment disease sites with significant tissue inhomogeneities within a simple EPID-based direct dose back-projection algorithm, and helps to improve the clinical interpretation of both pre-treatment and in vivo dosimetry results.

Reduction of systematic dosimetric uncertainties in volumetric modulated arc therapy triggered by patient-specific quality assurance.

BACKGROUND AND PURPOSE: Dosimetric patient-Specific Quality Assurance (PSQA) data contain in addition to cases with alerts, many cases without alerts. The aim of this study was to present a procedure to investigate long-term trend analysis of the complete set of PSQA data for the presence of site-specific deviations to reduce underlying systematic dose uncertainties. MATERIALS AND METHODS: The procedure started by analysing a large set of prostate Volumetric Modulated Arc Therapy (VMAT) PSQA data obtained by comparing 3D electronic portal image device (EPID)_based in vivo dosimetry measurements with dose values predicted by the Treatment Planning System (TPS). If systematic deviations were present, several actions were required. These included confirmation of these deviations with an independent dose verification system for which a 2D detector array in a phantom was used, and analysing calculated with measured PSQA data, or delivery machine characteristics. Further analysis revealed that the under-dosage correlated with plan complexity and coincided with changes in clinically applied planning techniques. RESULTS: Prostate VMAT PSQA data showed an under-dosage gradual increasing to about 2% in 3 years, which was confirmed by the measurements with the 2D detector array in a phantom. The implementation of new beam fits in the TPS led to a reduction of the observed deviations. CONCLUSION: Long-term analysis of site-specific PSQA data is a useful method to monitor incremental changes in a radiotherapy department due to various changes in the treatment planning and delivery of prostate VMAT, and may lead to a reduction of systematic dose uncertainties in complex treatments.

Simplifying EPID dosimetry for IMRT treatment verification.

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.

Portal dosimetry of small unflattened beams.

We developed and validated a dedicated small field back-projection portal dosimetry model for pretreatment andin vivoverification of stereotactic plans entailing small unflattened photon beams. For this purpose an aSi-EPID was commissioned as a small field dosimeter. Small field output factors for 6 MV FFF beams were measured using the PTW microDiamond detector and the Agility 160-leaf MLC from Elekta. The back-projection algorithm developed in our department was modified to better model the small field physics. The feasibility of small field portal dosimetry was validated via absolute point dose differences w.r.t. small static beams, and 5 hypofractionated stereotactic VMAT clinical plans measured with the OCTAVIUS 1000 SRS array dosimeter and computed with the treatment planning system Pinnacle v16.2. Dose reconstructions using the currently clinically applied back-projection model were also computed for comparison. We found that the latter yields underdosage of about -8% for square beams with cross section near 10 mm x 10 mm and about -6% for VMAT treatments with PTV volumes smaller than about 2cm3. With the methods described in this work such errors can be reduced to less than the ±3.0% recommendations for clinical use. Our results indicate that aSi-EPIDs can be used as accurate small field radiation dosimeters, offering advantages over point dose detectors, the correct positioning and orientation of which is challenging for routine clinical QA.

Automatic dosimetric verification of online adapted plans on the Unity MR-Linac using 3D EPID dosimetry.

BACKGROUND AND PURPOSE: The Unity MR-Linac is equipped with an EPID, the images from which contain information about the dose delivered to the patient. The purpose of this study was to introduce a framework for the automatic dosimetric verification of online adapted plans using 3D EPID dosimetry and to present the obtained dosimetric results. MATERIALS AND METHODS: The framework was active during the delivery of 1207 online adapted plans corresponding to 127 clinical IMRT treatments (74 prostate, 19 rectum, 19 liver and 15 lymph node oligometastases). EPID reconstructed dose distributions in the patient geometry were calculated automatically and then compared to the dose distributions calculated online by the treatment planning system (TPS). The comparison was performed by γ-analysis (3% global/2mm/10% threshold) and by the difference in median dose to the high-dose volume (ΔHDVD50). 85% for γ-pass rate and 5% for ΔHDVD50 were used as tolerance limit values. RESULTS: 93% of the online plans were verified automatically by the framework. Missing EPID data was the reason for automation failure. 91% of the verified plans were within tolerance. CONCLUSION: Automatic dosimetric verification of online adapted plans on the Unity MR-Linac is feasible using in vivo 3D EPID dosimetry. Almost all online adapted plans were approved automatically by the framework. This newly developed framework is a major step forward towards the clinical implementation of a permanent safety net for the entire online adaptive workflow.

The effect of the choice of patient model on the performance of in vivo 3D EPID dosimetry to detect variations in patient position and anatomy.

PURPOSE: In vivo EPID dosimetry is meant to trigger on relevant differences between delivered and planned dose distributions and should therefore be sensitive to changes in patient position and patient anatomy. Three-dimensional (3D) EPID back-projection algorithms can use either the planning computed tomography (CT) or the daily patient anatomy as patient model for dose reconstruction. The purpose of this study is to quantify the effect of the choice of patient model on the performance of in vivo 3D EPID dosimetry to detect patient-related variations. METHODS: Variations in patient position and patient anatomy were simulated by transforming the reference planning CT images (pCT) into synthetic daily CT images (dCT) representing a variation of a given magnitude in patient position or in patient anatomy. For each variation, synthetic in vivo EPID data were also generated to simulate the reconstruction of in vivo EPID dose distributions. Both the planning CT images and the synthetic daily CT images could be used as patient model in the reconstructions yielding e D pCT and e D dCT EPID reconstructed dose distributions respectively. The accuracy of e D pCT and e D dCT reconstructions was evaluated against absolute dose measurements made in different phantom setups, and against dose distributions calculated by the treatment planning system (TPS). The comparison was performed by γ-analysis (3% local dose/2 mm). The difference in sensitivity between e D pCT and e D dCT reconstructions to detect variations in patient position and in patient anatomy was investigated using receiver operating characteristic analysis and the number of triggered alerts for 100 volumetric modulated arc therapy plans and 12 variations. RESULTS: e D dCT showed good agreement with both absolute point dose measurements (<0.5%) and TPS data (γ-mean = 0.52 ± 0.11). The agreement degraded with e D pCT , with the magnitude of the deviation varying with each specific case. e D dCT readily detected combined 3 mm translation setup errors in all directions (AUC = 1.0) and combined 3° rotation setup errors around all axes (AUC = 0.86) whereas e D pCT showed good detectability only for 12 mm translations (AUC = 0.85) and 9° rotations (AUC = 0.80). Conversely, e D pCT manifested a higher sensitivity to patient anatomical changes resulting in AUC values of 0.92/0.95 for a 6 mm patient contour expansion/contraction compared to 0.70/0.64 with e D dCT . Using |ΔPTVD50 | > 3% as clinical tolerance level, the percentage of alerts for 6 mm changes in patient contour were 85%/27% with e D pCT / e D dCT . CONCLUSIONS: With planning CT images as patient model, EPID dose reconstructions underestimate the dosimetric effects caused by errors in patient positioning and overestimate the dosimetric effects caused by changes in patient anatomy. The use of the daily patient position and anatomy as patient model for in vivo 3D EPID transit dosimetry improves the ability of the system to detect uncorrected errors in patient position and it reduces the likelihood of false positives due to patient anatomical changes.

In vivo dosimetry in external beam photon radiotherapy: Requirements and future directions for research, development, and clinical practice.

External beam radiotherapy with photon beams is a highly accurate treatment modality, but requires extensive quality assurance programs to confirm that radiation therapy will be or was administered appropriately. In vivo dosimetry (IVD) is an essential element of modern radiation therapy because it provides the ability to catch treatment delivery errors, assist in treatment adaptation, and record the actual dose delivered to the patient. However, for various reasons, its clinical implementation has been slow and limited. The purpose of this report is to stimulate the wider use of IVD for external beam radiotherapy, and in particular of systems using electronic portal imaging devices (EPIDs). After documenting the current IVD methods, this report provides detailed software, hardware and system requirements for in vivo EPID dosimetry systems in order to help in bridging the current vendor-user gap. The report also outlines directions for further development and research. In vivo EPID dosimetry vendors, in collaboration with users across multiple institutions, are requested to improve the understanding and reduce the uncertainties of the system and to help in the determination of optimal action limits for error detection. Finally, the report recommends that automation of all aspects of IVD is needed to help facilitate clinical adoption, including automation of image acquisition, analysis, result interpretation, and reporting/documentation. With the guidance of this report, it is hoped that widespread clinical use of IVD will be significantly accelerated.

A simple backprojection algorithm for 3D in vivo EPID dosimetry of IMRT treatments.

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.

Site-specific alert criteria to detect patient-related errors with 3D EPID transit dosimetry.

PURPOSE: To assess the sensitivity of various EPID dosimetry alert indicators to patient-related variations and to determine alert threshold values that ensure excellent error detectability. METHODS: Our virtual dose reconstruction method uses in air EPID measurements to calculate virtual 3D dose distributions within a CT data set. Patient errors are introduced by transforming the plan-CT into an error-CT data set. Virtual patient dose distributions reconstructed using the plan-CT and the error-CT data set are compared to the planned dose distributions by γ(3%/3 mm) and DVH analysis using seven indicators: ΔDISOC , γ-mean, near γ-max, γ-pass rate, ΔPTVD 2 , ΔPTVD 50, and ΔPTVD 98 . Translation and rotation patient setup errors and uniform contour changes are studied for 104 VMAT plans of 4 treatment sites. Lung expansions and contractions to simulate changes in lung density are considered for 26 IMRT lung plans. A ROC curve is generated for each combination of error and indicator. For each ROC curve, the AUC value and the optimal alert threshold value of the indicator are determined. RESULTS: AUC values for γ-indicators and ΔPTVD 2 are consistently higher than for ΔDISOC and ΔPTVD 98 . For VMAT plans, error detectability to patient position shifts is worse for pelvic treatments and best for head-and-neck and brain plans. Excellent detectability is observed for 5 mm translations in head-and-neck plans (AUC = 0.94) and for 4° rotations in brain plans (AUC = 0.89). All sites but prostate show good-to-excellent detectability (AUC > 0.8) for 10 mm translations and 8° rotations and excellent detectability (AUC > 0.9) for ±6 mm patient contour changes. For head-and-neck, excellent detectability is obtained with γ-mean and γ-pass rate threshold values of around 0.63 and 83%, respectively. For brain and rectum, these threshold values are 0.53 and 90%, respectively. In IMRT lung plans, expansions of 3 mm and contractions of 6 mm are detected (AUC > 0.8). CONCLUSIONS: By combining virtual dose reconstructions with synthetic patient data, we developed a framework to assess the sensitivity of our 3D EPID transit dosimetry method to patient-related variations. The detectability of each introduced error is specific to the treatment site and indicator used. Optimal alert criteria can be determined to ensure excellent detectability for each combination of error type and indicator. The alert threshold values and the magnitude of the error that can be detected are site-specific. In situations where the minimum error that can be detected is larger than the clinically desirable action level, EPID transit dosimetry must be used in combination with IGRT procedures to ensure correct patient positioning and early detection of anatomy variations.

A literature review of electronic portal imaging for radiotherapy dosimetry.

Electronic portal imaging devices (EPIDs) have been the preferred tools for verification of patient positioning for radiotherapy in recent decades. Since EPID images contain dose information, many groups have investigated their use for radiotherapy dose measurement. With the introduction of the amorphous-silicon EPIDs, the interest in EPID dosimetry has been accelerated because of the favourable characteristics such as fast image acquisition, high resolution, digital format, and potential for in vivo measurements and 3D dose verification. As a result, the number of publications dealing with EPID dosimetry has increased considerably over the past approximately 15 years. The purpose of this paper was to review the information provided in these publications. Information available in the literature included dosimetric characteristics and calibration procedures of various types of EPIDs, strategies to use EPIDs for dose verification, clinical approaches to EPID dosimetry, ranging from point dose to full 3D dose distribution verification, and current clinical experience. Quality control of a linear accelerator, pre-treatment dose verification and in vivo dosimetry using EPIDs are now routinely used in a growing number of clinics. The use of EPIDs for dosimetry purposes has matured and is now a reliable and accurate dose verification method that can be used in a large number of situations. Methods to integrate 3D in vivo dosimetry and image-guided radiotherapy (IGRT) procedures, such as the use of kV or MV cone-beam CT, are under development. It has been shown that EPID dosimetry can play an integral role in the total chain of verification procedures that are implemented in a radiotherapy department. It provides a safety net for simple to advanced treatments, as well as a full account of the dose delivered. Despite these favourable characteristics and the vast range of publications on the subject, there is still a lack of commercially available solutions for EPID dosimetry. As strategies evolve and commercial products become available, EPID dosimetry has the potential to become an accurate and efficient means of large-scale patient-specific IMRT dose verification for any radiotherapy department.

The next step in patient-specific QA: 3D dose verification of conformal and intensity-modulated RT based on EPID dosimetry and Monte Carlo dose calculations.

BACKGROUND AND PURPOSE: A method was evaluated to reconstruct the 3D dose distribution in patients using their planning CT-scan in combination with a Monte Carlo calculation, and the energy fluence of the actual treatment beams measured pre-treatment with an EPID without the patient or a phantom in the beam. MATERIALS AND METHODS: Nine plans of lung cancer patients treated with a 3D conformal technique, calculated using a simple convolution algorithm (CA), as well as five IMRT treatments of head-and-neck cancer patients, calculated with a more advanced superposition algorithm (SA), were verified. Differences between planned and reconstructed dose distributions were quantified in terms of DVH parameters. RESULTS: For the lung cancer group, differences between the reconstructed mean PTV dose and the values calculated with the TPS were 5.0+/-4.2% (1SD) and -1.4+/-1.5% for the CA and SA algorithm, respectively. No large differences in the lung and spinal cord DVH parameters were found. For the IMRT treatments, the average dose differences in the PTV were generally below 3%. The reconstructed mean parotid gland dose was 3.2+/-1.2% lower, while the maximum spinal cord dose was on average 3.1+/-1.9% higher. CONCLUSIONS: EPID dosimetry combined with 3D dose reconstruction is a useful procedure for patient-specific QA of complex treatments. DVH parameters can be used to interpret the dose distribution delivered to the patient in the same way as during standard treatment plan evaluation.

3D in vivo dose verification of entire hypo-fractionated IMRT treatments using an EPID and cone-beam CT.

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.

Transition from a simple to a more advanced dose calculation algorithm for radiotherapy of non-small cell lung cancer (NSCLC): implications for clinical implementation in an individualized dose-escalation protocol.

BACKGROUND AND PURPOSE: To investigate the clinical consequences of the transition from a simple convolution algorithm (CA) to a more advanced superposition dose calculation algorithm (SA) in an individualized isotoxic dose-escalation protocol for NSCLC patients. MATERIAL AND METHODS: First, treatment plans designed according to ICRU50-criteria using the CA were recalculated using the SA, for 16 patients. Next, two additional plans were designed for each patient using only the SA: one with 95%-isodose coverage (ICRU50-criteria), the other allowing PTV coverage with 90%-isodose at the lung side. PTV dose was escalated to a maximum dose of 79.2Gy or lower when limited by either a mean lung dose (MLD) of 19Gy or a maximum spinal cord dose of 54Gy. Equivalent uniform doses (EUD) in the PTV were compared. RESULTS: Recalculation of the CA plans using the SA, showed PTV underdosage in the CA plans: the median PTV EUD was 61.3Gy (range 44.9-80.4Gy) and 55.5Gy (43.9-76.8Gy), for CA and SA, respectively (p<0.001). Redesigning plans using the SA resulted in an almost identical PTV EUD of 55.1Gy (43.7-79.2Gy). For the subgroup (N=9) with MLD as dose-limiting factor a gain in PTV EUD of 2.7+/-1.8Gy (p=0.008) was achieved using the 90%-isodose coverage plan. CONCLUSIONS: Plans calculated using the CA caused large PTV underdosage. Plans designed using the SA often lead to lower maximum achievable tumour doses due to higher MLD values. Allowing somewhat relaxed PTV coverage criteria increased the PTV dose again for MLD restricted cases. Consequently, in clinics where isotoxic individual dose-escalation is applied, implementation of an SA should be accompanied by accepting limited PTV underdosage in patients with MLD as the dose-limiting factor.

State of the art of in vivo dosimetry.

The complexity of modern radiotherapy requires a comprehensive quality assurance programme, including in vivo dosimetry. In this paper, the use of the detector systems most often used for in vivo dosimetry [diodes, thermoluminescence detectors, metal oxide field effect transistors and electronic portal imaging devices (EPIDs)] will be summarised. Although point detectors are useful for the verification of conventional 3-D conformal radiotherapy, the use of 2-D detector systems, such as EPIDs, is required for the verification of more complicated techniques, including intensity-modulated radiotherapy.

Replacing pretreatment verification with in vivo EPID dosimetry for prostate IMRT.

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.