Catching errors with in vivo EPID dosimetry.

The potential for detrimental incidents and the ever increasing complexity of patient treatments emphasize the need for accurate dosimetric verification in radiotherapy. For this reason, all curative treatments are verified, either pretreatment or in vivo, by electronic portal imaging device (EPID) dosimetry in the Radiation Oncology Department of The Netherlands Cancer Institute-Antoni van Leeuwenhoek hospital, Amsterdam, The Netherlands. Since the clinical introduction of the method in January 2005 until August 2009, treatment plans of 4337 patients have been verified. Among these plans, 17 serious errors were detected that led to intervention. Due to their origin, nine of these errors would not have been detected with pretreatment verification. The method is illustrated in detail by the case of a plan transfer error detected in a 5 x 5 Gy intensity-modulated radiotherapy (IMRT) rectum treatment. The EPID reconstructed dose at the isocenter was 6.3% below the planned value. Investigation of the plan transfer chain revealed that due to a network transfer error, the plan was corrupted. 3D analysis of the acquired EPID data revealed serious underdosage of the planning target volume: On average 11.6%, locally up to 20%. This report shows the importance of in vivo (EPID) dosimetry for all treatment plans as well as the ability of the method to assess the dosimetric impact of deviations found.

Prediction of DVH parameter changes due to setup errors for breast cancer treatment based on 2D portal dosimetry.

Electronic portal imaging devices (EPIDs) are increasingly used for portal dosimetry applications. In our department, EPIDs are clinically used for two-dimensional (2D) transit dosimetry. Predicted and measured portal dose images are compared to detect dose delivery errors caused for instance by setup errors or organ motion. The aim of this work is to develop a model to predict dose-volume histogram (DVH) changes due to setup errors during breast cancer treatment using 2D transit dosimetry. First, correlations between DVH parameter changes and 2D gamma parameters are investigated for different simulated setup errors, which are described by a binomial logistic regression model. The model calculates the probability that a DVH parameter changes more than a specific tolerance level and uses several gamma evaluation parameters for the planning target volume (PTV) projection in the EPID plane as input. Second, the predictive model is applied to clinically measured portal images. Predicted DVH parameter changes are compared to calculated DVH parameter changes using the measured setup error resulting from a dosimetric registration procedure. Statistical accuracy is investigated by using receiver operating characteristic (ROC) curves and values for the area under the curve (AUC), sensitivity, specificity, positive and negative predictive values. Changes in the mean PTV dose larger than 5%, and changes in V90 and V95 larger than 10% are accurately predicted based on a set of 2D gamma parameters. Most pronounced changes in the three DVH parameters are found for setup errors in the lateral-medial direction. AUC, sensitivity, specificity, and negative predictive values were between 85% and 100% while the positive predictive values were lower but still higher than 54%. Clinical predictive value is decreased due to the occurrence of patient rotations or breast deformations during treatment, but the overall reliability of the predictive model remains high. Based on our predictive model, 2D transit dosimetry measurements can now directly be translated in clinically more relevant DVH parameter changes for the PTV during conventional breast treatment. In this way, the possibility to design decision protocols based on extracted DVH changes is created instead of undertaking elaborate actions such as repeated treatment planning or 3D dose reconstruction for a large group of patients.

Error detection during VMAT delivery using EPID-based 3D transit dosimetry.

PURPOSE: To investigate the effectiveness of an EPID-based 3D transit dosimetry system in detecting deliberately introduced errors during VMAT delivery. METHODS: An Alderson phantom was irradiated using four VMAT treatment plans (one prostate, two head-and-neck and one lung case) in which delivery, thickness and setup errors were introduced. EPID measurements were performed to reconstruct 3D dose distributions of "error" plans, which were compared with "no-error" plans using the mean gamma (γmean), near-maximum gamma (γ1%) and the difference in isocenter dose (ΔDisoc) as metrics. RESULTS: Out of a total of 42 serious errors, the number of errors detected was 33 (79%), and 27 out of 30 (90%) if setup errors are not included. The system was able to pick up errors of 5 mm movement of a leaf bank, a wrong collimator rotation angle and a wrong photon beam energy. A change in phantom thickness of 1 cm was detected for all cases, while only for the head-and-neck plans a 2 cm horizontal and vertical shift of the phantom were alerted. A single leaf error of 5 mm could be detected for the lung plan only. CONCLUSION: Although performed for a limited number of cases and error types, this study shows that EPID-based 3D transit dosimetry is able to detect a number of serious errors in dose delivery, leaf bank position and patient thickness during VMAT delivery. Errors in patient setup and single leaf position can only be detected in specific cases.

Characterization of the a-Si EPID in the unity MR-linac for dosimetric applications.

Electronic portal imaging devices (EPIDs) are frequently used in external beam radiation therapy for dose verification purposes. The aim of this study was to investigate the dose-response characteristics of the EPID in the Unity MR-linac (Elekta AB, Stockholm, Sweden) relevant for dosimetric applications under clinical conditions. EPID images and ionization chamber (IC) measurements were used to study the effects of the magnetic field, the scatter generated in the MR housing reaching the EPID, and inhomogeneous attenuation from the MR housing. Dose linearity and dose rate dependencies were also determined. The magnetic field strength at EPID level did not exceed 10 mT, and dose linearity and dose rate dependencies proved to be comparable to that on a conventional linac. Profiles of fields, delivered with and without the magnetic field, were indistinguishable. The EPID center had an offset of 5.6 cm in the longitudinal direction, compared to the beam central axis, meaning that large fields in this direction will partially fall outside the detector area and not be suitable for verification. Beam attenuation by the MRI scanner and the table is gantry angle dependent, presenting a minimum attenuation of 67% relative to the 90° measurement. Repeatability, observed over two months, was within 0.5% (1 SD). In order to use the EPID for dosimetric applications in the MR-linac, challenges related to the EPID position, scatter from the MR housing, and the inhomogeneous, gantry angle-dependent attenuation of the beam will need to be solved.

Three-dimensional heart dose reconstruction to estimate normal tissue complication probability after breast irradiation using portal dosimetry.

Irradiation of the heart is one of the major concerns during radiotherapy of breast cancer. Three-dimensional (3D) treatment planning would therefore be useful but cannot always be performed for left-sided breast treatments, because CT data may not be available. However, even if 3D dose calculations are available and an estimate of the normal tissue damage can be made, uncertainties in patient positioning may significantly influence the heart dose during treatment. Therefore, 3D reconstruction of the actual heart dose during breast cancer treatment using electronic imaging portal device (EPID) dosimetry has been investigated. A previously described method to reconstruct the dose in the patient from treatment portal images at the radiological midsurface was used in combination with a simple geometrical model of the irradiated heart volume to enable calculation of dose-volume histograms (DVHs), to independently verify this aspect of the treatment without using 3D data from a planning CT scan. To investigate the accuracy of our method, the DVHs obtained with full 3D treatment planning system (TPS) calculations and those obtained after resampling the TPS dose in the radiological midsurface were compared for fifteen breast cancer patients for whom CT data were available. In addition, EPID dosimetry as well as 3D dose calculations using our TPS, film dosimetry, and ionization chamber measurements were performed in an anthropomorphic phantom. It was found that the dose reconstructed using EPID dosimetry and the dose calculated with the TPS agreed within 1.5% in the lung/heart region. The dose-volume histograms obtained with EPID dosimetry were used to estimate the normal tissue complication probability (NTCP) for late excess cardiac mortality. Although the accuracy of these NTCP calculations might be limited due to the uncertainty in the NTCP model, in combination with our portal dosimetry approach it allows incorporation of the actual heart dose. For the anthropomorphic phantom, and for fifteen patients for whom CT data were available to test our method, the average difference between the NTCP values obtained with our method and those resulting from the dose distributions calculated with the TPS was 0.1% +/- 0.3% (1 SD). Most NTCP values were 1%-2% lower than those obtained using the method described by Hurkmans et al. [Radiother. Oncol. 62, 163-171 (2002)], using the maximum heart distance determined from a simulator image as a single pre-treatment parameter. A similar difference between the two methods was found for twelve patients using in vivo EPID dosimetry; the average NTCP value obtained with EPID dosimetry was 0.9%, whereas an average NTCP value of 2.2% was derived using the method of Hurkmans et al. The results obtained in this study show that EPID dosimetry is well suited for in vivo verification of the heart dose during breast cancer treatment, and can be used to estimate the NTCP for late excess cardiac mortality. To the best of our knowledge, this is the first study using portal dosimetry to calculate a DVH and NTCP of an organ at risk.

A global calibration model for a-Si EPIDs used for transit dosimetry.

Electronic portal imaging devices (EPIDs) are not only applied for patient setup verification and detection of organ motion but are also increasingly used for dosimetric verification. The aim of our work is to obtain accurate dose distributions from a commercially available amorphous silicon (a-Si) EPID for transit dosimetry applications. For that purpose, a global calibration model was developed, which includes a correction procedure for ghosting effects, field size dependence and energy dependence of the a-Si EPID response. In addition, the long-term stability and additional buildup material for this type of EPID were determined. Differences in EPID response due to photon energy spectrum changes have been measured for different absorber thicknesses and field sizes, yielding off-axis spectrum correction factors based on transmission measurements. Dose measurements performed with an ionization chamber in a water tank were used as reference data, and the accuracy of the dosimetric calibration model was determined for a large range of treatment conditions. Gamma values using 3% as dose-difference criterion and 3 mm as distance-to-agreement criterion were used for evaluation. The field size dependence of the response could be corrected by a single kernel, fulfilling the gamma evaluation criteria in case of virtual wedges and intensity modulated radiation therapy fields. Differences in energy spectrum response amounted up to 30%-40%, but could be reduced to less than 3% using our correction model. For different treatment fields and (in)homogeneous phantoms, transit dose distributions satisfied in almost all situations the gamma criteria. We have shown that a-Si EPIDs can be accurately calibrated for transit dosimetry purposes.

Intensity-modulated vs. conformal radiotherapy of parotid gland tumors: potential impact on hearing loss.

In 3-dimensional (3D) conformal radiotherapy of parotid gland tumors, little effort is made to avoid the auditory system or the oral cavity. Damage may occur when the ear is located inside the treatment field. The purpose of this study was to design and evaluate an intensity-modulation radiotherapy (IMRT) class solution, and to compare this technique to a 3D conformal approach with respect to hearing loss. Twenty patients with parotid gland cancer were retrospectively planned with 2 different techniques using the original planning target volume (PTV). First, a conventional technique using a wedged beam pair was applied, yielding a dose distribution conformal to the shape of the PTV. Next, an IMRT technique using a fluence map optimization with predefined constraints was designed. A dose of 66 Gy in the PTV was given at the International Commission on Radiation Units and Measures (ICRU) dose prescription point. Dose-volume histograms of the PTV and organs at risk (OARs), such as auditory system, oral cavity, and spinal cord, were compared. The dose in the OARs was lower in the IMRT plans. The mean volume of the middle ear receiving a dose higher than 50 Gy decreased from 66.5% to 33.4%. The mean dose in the oral cavity decreased from 19.4 Gy to 16.6 Gy. The auditory system can be spared if the distance between the inner ear and the PTV is 0.6 cm or larger, and if the overlap between the middle ear and the PTV is smaller than 10%. The maximum dose in the spinal cord was below 40 Gy in all treatment plans. The mean volume of the PTV receiving less than 95% of the prescribed dose increased in the IMRT plan slightly from 3.3% to 4.3 % (p = 0.01). The mean volume receiving more than 107% increased from 0.9% to 2.5% (p = 0.02). It can be concluded that the auditory system, as well as the oral cavity, can be spared with IMRT, but at the cost of a slightly larger dose inhomogeneity in the PTV. The IMRT technique can therefore, in most cases, be recommended as the treatment of choice for the irradiation of parotid tumors.

A back-projection algorithm in the presence of an extra attenuating medium: towards EPID dosimetry for the MR-Linac.

In external beam radiotherapy, electronic portal imaging devices (EPIDs) are frequently used for pre-treatment and for in vivo dose verification. Currently, various MR-guided radiotherapy systems are being developed and clinically implemented. Independent dosimetric verification is highly desirable. For this purpose we adapted our EPID-based dose verification system for use with the MR-Linac combination developed by Elekta in cooperation with UMC Utrecht and Philips. In this study we extended our back-projection method to cope with the presence of an extra attenuating medium between the patient and the EPID. Experiments were performed at a conventional linac, using an aluminum mock-up of the MRI scanner housing between the phantom and the EPID. For a 10 cm square field, the attenuation by the mock-up was 72%, while 16% of the remaining EPID signal resulted from scattered radiation. 58 IMRT fields were delivered to a 20 cm slab phantom with and without the mock-up. EPID reconstructed dose distributions were compared to planned dose distributions using the [Formula: see text]-evaluation method (global, 3%, 3 mm). In our adapted back-projection algorithm the averaged [Formula: see text] was [Formula: see text], while in the conventional it was [Formula: see text]. Dose profiles of several square fields reconstructed with our adapted algorithm showed excellent agreement when compared to TPS.

Comparison of ghosting effects for three commercial a-Si EPIDs.

Many studies have reported dosimetric characteristics of amorphous silicon electronic portal imaging devices (EPIDs). Some studies ascribed a non-linear signal to gain ghosting and image lag. Other reports, however, state the effect is negligible. This study compares the signal-to-monitor unit (MU) ratio for three different brands of EPID systems. The signal was measured for a wide range of monitor units (5-1000), dose-rates, and beam energies. All EPIDs exhibited a relative under-response for beams of few MUs; giving 4 to 10% lower signal-to-MU ratios relative to that of 1000 MUs. This under-response is consistent with ghosting effects due to charge trapping.

Clinical experience with EPID dosimetry for prostate IMRT pre-treatment dose verification.

The aim of this study was to demonstrate how dosimetry with an amorphous silicon electronic portal imaging device (a-Si EPID) replaced film and ionization chamber measurements for routine pre-treatment dosimetry in our clinic. Furthermore, we described how EPID dosimetry was used to solve a clinical problem. IMRT prostate plans were delivered to a homogeneous slab phantom. EPID transit images were acquired for each segment. A previously developed in-house back-projection algorithm was used to reconstruct the dose distribution in the phantom mid-plane (intersecting the isocenter). Segment dose images were summed to obtain an EPID mid-plane dose image for each field. Fields were compared using profiles and in two dimensions with the y evaluation (criteria: 3%/3 mm). To quantify results, the average gamma (gamma avg), maximum gamma (gamma max), and the percentage of points with gamma < 1(P gamma < 1) were calculated within the 20% isodose line of each field. For 10 patient plans, all fields were measured with EPID and film at gantry set to 0 degrees. The film was located in the phantom coronal mid-plane (10 cm depth), and compared with the back-projected EPID mid-plane absolute dose. EPID and film measurements agreed well for all 50 fields, with (gamma avg) =0.16, (gamma max)=1.00, and (P gamma < 1)= 100%. Based on these results, film measurements were discontinued for verification of prostate IMRT plans. For 20 patient plans, the dose distribution was re-calculated with the phantom CT scan and delivered to the phantom with the original gantry angles. The planned isocenter dose (plan(iso)) was verified with the EPID (EPID(iso)) and an ionization chamber (IC(iso)). The average ratio, (EPID(iso)/IC(iso)), was 1.00 (0.01 SD). Both measurements were systematically lower than planned, with (EPID(iso)/plan(iso)) and (IC(iso)/plan(iso))=0.99 (0.01 SD). EPID mid-plane dose images for each field were also compared with the corresponding plane derived from the three dimensional (3D) dose grid calculated with the phantom CT scan. Comparisons of 100 fields yielded (gamma avg)=0.39, gamma max=2.52, and (P gamma < 1)=98.7%. Seven plans revealed under-dosage in individual fields ranging from 5% to 16%, occurring at small regions of overlapping segments or along the junction of abutting segments (tongue-and-groove side). Test fields were designed to simulate errors and gave similar results. The agreement was improved after adjusting an incorrectly set tongue-and-groove width parameter in the treatment planning system (TPS), reducing (gamma max) from 2.19 to 0.80 for the test field. Mid-plane dose distributions determined with the EPID were consistent with film measurements in a slab phantom for all IMRT fields. Isocenter doses of the total plan measured with an EPID and an ionization chamber also agreed. The EPID can therefore replace these dosimetry devices for field-by-field and isocenter IMRT pre-treatment verification. Systematic errors were detected using EPID dosimetry, resulting in the adjustment of a TPS parameter and alteration of two clinical patient plans. One set of EPID measurements (i.e., one open and transit image acquired for each segment of the plan) is sufficient to check each IMRT plan field-by-field and at the isocenter, making it a useful, efficient, and accurate dosimetric tool.

Experimental verification of a portal dose prediction model.

Electronic portal imaging devices (EPIDs) can be used to measure a two-dimensional (2D) dose distribution behind a patient, thus allowing dosimetric treatment verification. For this purpose we experimentally assessed the accuracy of a 2D portal dose prediction model based on pencil beam scatter kernels. A straightforward derivation of these pencil beam scatter kernels for portal dose prediction models is presented based on phantom measurements. The model is able to predict the 2D portal dose image (PDI) behind a patient, based on a PDI without the patient in the beam in combination with the radiological thickness of the patient, which requires in addition a PDI with the patient in the beam. To assess the accuracy of portal dose and radiological thickness values obtained with our model, various types of homogeneous as well as inhomogeneous phantoms were irradiated with a 6 MV photon beam. With our model we are able to predict a PDI with an accuracy better than 2% (mean difference) if the radiological thickness of the object in the beam is symmetrically situated around the isocenter. For other situations deviations up to 3% are observed for a homogeneous phantom with a radiological thickness of 17 cm and a 9 cm shift of the midplane-to-detector distance. The model can extract the radiological thickness within 7 mm (maximum difference) of the actual radiological thickness if the object is symmetrically distributed around the isocenter plane. This difference in radiological thickness is related to a primary portal dose difference of 3%. It can be concluded that our model can be used as an easy and accurate tool for the 2D verification of patient treatments by comparing predicted and measured PDIs. The model is also able to extract the primary portal dose with a high accuracy, which can be used as the input for a 3D dose reconstruction method based on back-projection.

Quality assurance of EORTC trial 22922/10925 investigating the role of internal mammary--medial supraclavicular irradiation in stage I-III breast cancer: the individual case review.

To assess consistency among participants in an European Organisation for Research and Treatment of Cancer (EORTC) phase III trial randomising between irradiation and no irradiation of the internal mammary and medial supraclavicular (IM-MS) lymph nodes, all participating institutes were invited to send data from 3 patients in each arm as soon as they started accrual. The evaluation focused on eligibility, compliance with the radiotherapy guidelines, treatment techniques and dose prescription to the IM-MS region. Nineteen radiotherapy departments provided a total of 111 cases, all being eligible. Minor discrepancies were found in the surgery and pathology data in almost half the patients. Major radiotherapy protocol deviations were very limited: 2 cases of unwarranted irradiation of the supraclavicular region and a significant dose deviation to the internal mammary region in 5 patients. The most frequently observed minor protocol deviation was the absence of delineation of the target volumes in 80% of the patients. By detecting systematic protocol deviations in an early phase of the trial, recommendations made to all the participating institutes should improve the interinstitutional consistency and promote a high-quality treatment.

In vivo dosimetry during external photon beam radiotherapy.

In this critical review of the current practice of patient dose verification, we first demonstrate that a high accuracy (about 1-2%, 1 SD) can be obtained. Accurate in vivo dosimetry is possible if diodes and thermoluminescence dosimeters (TLDs), the main detector types in use for in vivo dosimetry, are carefully calibrated and the factors influencing their sensitivity are taken into account. Various methods and philosophies for applying patient dose verification are then evaluated: the measurement of each field for each fraction of each patient, a limited number of checks for all patients, or measurements of specific patient groups, for example, during total body irradiation (TBI) or conformal radiotherapy. The experience of a number of centers is then presented, providing information on the various types of errors detected by in vivo dosimetry, including their frequency and magnitude. From the results of recent studies it can be concluded that in centers having modern equipment with verification systems as well as comprehensive quality assurance (QA) programs, a systematic error larger than 5% in dose delivery is still present for 0.5-1% of the patient treatments. In other studies, a frequency of 3-10% of errors was observed for specific patient groups or when no verification system was present at the accelerator. These results were balanced against the additional manpower and other resources required for such a QA program. It could be concluded that patient dose verification should be an essential part of a QA program in a radiotherapy department, and plays a complementary role to treatment-sheet double checking. As the radiotherapy community makes the transition from the conventional two-dimensional (2D) to three-dimensional (3D) conformal and intensity modulated dose delivery, it is recommended that new treatment techniques be checked systematically for a few patients, and to perform in vivo dosimetry a few times for each patient for situations where errors in dose delivery should be minimized.

Accuracy and precision of absorbed dose measurements for neutron therapy.

The occurrence of normal tissue complications and probability of tumor control are steep functions of absorbed dose. Consequently the delivery of the dose to the patient should be performed with a precision better than +/- 2% and an overall uncertainty less than +/- 5%. The sequence of dosimetry procedures to deliver the absorbed dose to the patient is analyzed with emphasis on the physical parameters involved in neutron dosimetry; the results of neutron dosimetry intercomparisons are summarized. The protocols for neutron dosimetry formulated by European and American physicists differ in a number of aspects, including the choice of the phantom material. For the treatment of a specific lesion, e.g., a tumor of the floor of the mouth, different treatment plannings have been suggested. Regarding the determination of total absorbed dose at a reference point in a phantom, the required overall uncertainty can be achieved for neutron energies up to 20 MeV. Because of differences in size, shape and composition between the phantom and the patient, somewhat larger uncertainties are to be anticipated for the actual treatment. Further experimental and theoretical studies are needed to obtain more reliable values for kerma in different elements and neutron sensitivity of the photon dosimeters for neutron energies in excess of 20 MeV.

The Amsterdam fast neutron therapy project: a final report.

In the period from February 1975 through September 1981 a total of 435 patients received radiotherapy with the 14 MeV d + T neutron generator, hospital based in the Netherlands Cancer Institute (the Antoni van Leeuwenhoek Hospital). Preliminary data on clinical results were published during the past few years. In this paper a final report is given of the program. The results can be summarized as follows: The neutron generator fulfilled the criteria for clinical use, that is it was reliable and had the required minimal output of 10(12) neutrons s-1. However, the dose distribution was more comparable with a 250 kV X-ray machine than with a modern accelerator. A number of physical parameters of importance for clinical neutron dosimetry have been determined for our therapy unit. These data, as well as the results of dosimetry intercomparisons in which our institute participated, contributed in the drafting of a European protocol for clinical neutron dosimetry. Pilot studies were carried out on different tumor sites, including head and neck, brain, pelvis, soft tissue and pulmonary metastases. In many patients local tumor control was seen, however, often concomitant with severe complications, especially in deep seated tumors. Randomized clinical trials were carried out for head and neck tumors (in collaboration with some other European centers) and for inoperable bladder and rectal tumors. No significant difference was observed in local tumor control or late morbidity between photon and neutron irradiation for the head and neck tumors. Also the results for pelvic tumors failed to demonstrate an advantage for neutron therapy. In this study two neutron arms were used with different dose schedules. As could be expected a higher local control rate was noticed for the higher neutron dose group, but concomitant with a higher complication rate. From our experience we have to conclude that treatment with our fast neutron treatment facility did not result in a benefit over photon irradiation. It seemed that the differential effect between tumor and normal tissues is smaller with fast neutrons than with photons.

Clinical dosimetry of an epithermal neutron beam for neutron capture therapy: dose distributions under reference conditions.

PURPOSE: The aim of this study was to asses the dose distribution under reference conditions for the various dose components of the Petten clinical epithermal neutron beam for boron neutron capture therapy (BNCT). METHODS AND MATERIALS: Activation foils and a silicon alpha-particle detector with a 6Li converter plate have been used for the determination of the thermal neutron fluence rate. The gamma-ray dose rate and the fast neutron dose rate have been determined using paired ionization chambers. Circular beam apertures of 8, 12 and 15 cm diameters have been investigated using a 15 x 15 x 15 cm3 solid polymethyl-methacrylate phantom, a water phantom of the same dimensions and a 30 x 30 x 30 cm3 water phantom at various phantom to beam-exit distances. RESULTS: The effect of phantom to beam-exit distance could be modeled using an inverse square law with a virtual source to beam-exit distance of 3.0 m. At a reference phantom to beam-exit distance of 30 cm, three-dimensional dose and fluence distributions of the various dose components have been determined in the phantoms. The absolute thermal neutron fluence rate at a reference depth of 2 cm in the 15 cm water phantom increased by 43% when the field size was increased from 8 to 15 cm. Simultaneously the gamma-ray dose rate increased by 46% while the fast neutron dose rate increased by only 5%. CONCLUSION: A reference treatment position at 30 cm from the beam exit allows convenient patient positioning with a relatively small increase in irradiation time compared to positions very close to the beam-exit. A more homogeneous distribution of thermal neutrons over a target volume, a higher absolute thermal neutron fluence rate and a lower contribution of the fast neutron dose to the total dose will result in improved treatment plans when using a 12 cm or 15 cm field compared to a 8 cm field. The dose distributions will be used as benchmark data for treatment planning systems for BNCT.

New method to obtain the midplane dose using portal in vivo dosimetry.

PURPOSE: The aim of this study was to develop a method to derive the midplane dose [i.e., the two-dimensional (2D) dose distribution in the middle of a patient irradiated with high-energy photon beams] from transmission dose data measured with an electronic portal imaging device (EPID). A prerequisite for this method was that it could be used without additional patient information (i.e., independent of a treatment-planning system). Second, we compared the new method with several existing (conventional) methods that derive the midline dose from entrance and exit dose measurements. METHODS AND MATERIALS: The proposed method first calculates the 2D contribution of the primary and scattered dose component at the exit side of the patient or phantom from the measured transmission dose. Then, a correction is applied for the difference in contribution for both dose components between exit side and midplane, yielding the midplane dose. To test the method, we performed EPID transmission dose measurements and entrance, midplane, and exit dose measurements using an ionization chamber in homogeneous and symmetrical inhomogeneous phantoms. The various methods to derive the midplane dose were also tested for asymmetrical inhomogeneous phantoms applying two opposing fields. A number of combinations of inhomogeneities (air, cork, and aluminum), phantom thicknesses, field sizes, and a few irregularly shaped fields were investigated, while each experiment was performed in 4-, 8-, and 18-MV open and wedged beams. RESULTS: Our new method can be used to assess the midplane dose for most clinical situations within 2% relative to ionization chamber measurements. Similar results were found with other methods. In the presence of large asymmetrical inhomogeneities (e.g., lungs), discrepancies of about 8% have been found (for small field sizes) using our transmission dose method, owing to the absence of lateral electron equilibrium. Applying the other methods, differences between predicted and measured midplane doses were even larger, up to 10%. For large field sizes, the agreement between measured and predicted midplane dose was within 3% using our transmission dose method. CONCLUSIONS: Using our new method, midplane doses were estimated with a similar or higher accuracy compared with existing conventional methods for in vivo dosimetry. The advantage of our new method is that the midplane dose can be determined in the entire (2D) field. With our method, portal in vivo dosimetry is an accurate alternative for conventional in vivo dosimetry.

Transmission dosimetry with a liquid-filled electronic portal imaging device.

PURPOSE: To assess the accuracy of transmission dose rate measurements for various phantom-detector geometries, performed with an electronic portal imaging device (EPID) and to compare these transmission dose rate values with exit dose rate data. METHODS AND MATERIALS: Transmission dose rate values on the central beam axis and beam profiles were measured with an EPID consisting of a matrix of liquid-filled ionization chambers. These data were compared with transmission and exit dose rate values, obtained using air-filled ionization chambers for a number of field sizes, phantom thickness, and phantom-detector distances. Various homogeneous and inhomogeneous phantoms were applied. RESULTS: The increase in dose rate with field size is larger for the EPID than in air, due to the larger amount of side scatter in the EPID. The difference has been taken into account by a deconvolution of the EPID images. An additional build-up layer on top of the commercial device is needed to reach dose maximum at the liquid ionization chambers for photon beam energies higher than about 4 MV. The transmission off-axis ratios (OAR) determined with the EPID and in air agreed within 2% for all tested cases, after deconvolution of the EPID signal. The agreement between the EPID-and exit-OAR decreased with increasing phantom-detector distance and the presence of inhomogeneities. For a phantom-detector distance of about 10 cm, the EPID- and exit-OARs agree within 2.5%. The difference could be up to 8% for an air inhomogeneity and a phantom-detector distance of 30 cm. CONCLUSIONS: The difference between EPID measurements and measurements in air can be explained by side scatter effects in the EPID and lack of adequate buildup, and can easily be taken into account. The loss of scatter compared with the situation at the exit side of the phantom explains the difference between transmission and exit dose values. At short phantom-detector distances, good agreement exists between transmission and exit dose rate. This implies that at this distance, the EPID can be used for simple comparison with exit dose calculations during patient treatments. At larger distances, more sophisticated conversion methods are required.

Benchmark measurements for lung dose corrections for X-ray beams.

A well defined set of clinically relevant reference measurements for photon dose calculations in the presence of the lung have been provided. These benchmark data were mainly obtained in low-density (rho = 0.31 gcm-3) lunglike material as well as in waterlike plastic for 4 and 15 MV X-ray beams. Some additional measurements were performed with materials having a density of 0.015 gcm-3 and 0.18 gcm-3. Phantom geometries included simple layered geometries, finite lung cross section geometries, simulated mediastinum geometries, and simulated tumor in lung geometries. The data are reported as central axis depth doses. A number of parameters were varied, including the field size, the lung geometry, and the distance in and behind the lung.

The influence of air cavities on interface doses for photon beams.

PURPOSE: As the quantification of dose in homogeneous media is now better understood, it is necessary to further quantify effects from heterogeneous media. The most extreme case is related to air cavities. Although dose corrections at large distances beyond a cavity are accountable by attenuation differences, perturbations at air-tissue interfaces are complex to measure or calculate. These measurements helps understand the physical processes that govern these perturbations. METHODS AND MATERIALS: A thin window parallel-plate chamber and a special diode were used for measurements with various air cavity geometries (layer, channel, cubic cavity, triangle) in x-ray beams of 4 and 15 MV. RESULTS: Underdosing effects occur at both the distal and proximal air cavity interfaces. The magnitude depends on geometry, energy, and field sizes. As the cavity thickness increases, the central axis dose at the distal interface decreases. Increasing field size remedied the underdosing, as did the introduction of lateral walls. Following a 2.0 cm wide air channel for a 4 MV, 4 x 4 cm2 field there was an 11% underdose at the distal interface, while a 2.0 cm cubic cavity yielded only a 3% loss. Measurements at the proximal interface showed losses of 5% to 8%. For a 4 MV parallel opposed beam irradiation the losses at the interfaces were 10% for a channel cavity (in comparison with the homogeneous case) and 1% for a cube. The losses were slightly larger for the 15 MV beam. Underdosage at the lateral interface was 4% and 8% for the 4 MV and 15 MV beams, respectively. CONCLUSION: Although reports suggest better clinical results using lower photon energies with the presence of air cavities, there is no reliable dose calculation algorithm to predict interface doses accurately. The measurements reported here can be used to guide the development of new calculation models under nonequilibrium conditions. This situation is of clinical concern when lesions such as larynx carcinoma beyond air cavities are irradiated.