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.

Overview of 3-year experience with large-scale electronic portal imaging device-based 3-dimensional transit dosimetry.

PURPOSE: To assess the usefulness of electronic portal imaging device (EPID)-based 3-dimensional (3D) transit dosimetry in a radiation therapy department by analyzing a large set of dose verification results. METHODS AND MATERIALS: In our institution, routine in vivo dose verification of all treatments is performed by means of 3D transit dosimetry using amorphous silicon EPIDs. The total 3D dose distribution is reconstructed using a back-projection algorithm and compared with the planned dose distribution using 3D gamma evaluation. Dose reconstruction and gamma evaluation software runs automatically in our clinic, and analysis results are (almost) immediately available. If a deviation exceeds our alert criteria, manual inspection is required. If necessary, additional phantom measurements are performed to separate patient-related errors from planning or delivery errors. Three-dimensional transit dosimetry results were analyzed per treatment site between 2012 and 2014 and the origin of the deviations was assessed. RESULTS: In total, 4689 of 15,076 plans (31%) exceeded the alert criteria between 2012 and 2014. These alerts were patient-related and attributable to limitations of our back-projection and dose calculation algorithm or to external sources. Clinically relevant deviations were detected for approximately 1 of 430 patient treatments. Most of these errors were because of anatomical changes or deviations from the routine clinical procedure and would not have been detected by pretreatment verification. Although cone beam computed tomography scans yielded information about anatomical changes, their effect on the dose delivery was assessed quantitatively by means of 3D in vivo dosimetry. CONCLUSIONS: EPID-based transit dosimetry is a fast and efficient dose verification technique. It provides more useful information and is less time-consuming than pretreatment verification measurements of intensity modulated radiation therapy and volumetric modulated arc therapy. Large-scale implementation of 3D transit dosimetry is therefore a powerful method to guarantee safe dose delivery during radiation therapy.

Impact of daily anatomical changes on EPID-based in vivo dosimetry of VMAT treatments of head-and-neck cancer.

BACKGROUND AND PURPOSE: Target dose verification for VMAT treatments of head-and-neck (H&N) cancer using 3D in vivo EPID dosimetry is expected to be affected by daily anatomical changes. By including these anatomical changes through cone-beam CT (CBCT) information, the magnitude of this effect is investigated. MATERIALS AND METHODS: For 20 VMAT-treated H&N cancer patients, all plan-CTs (pCTs), 633 CBCTs and 1266 EPID movies were used to compare four dose distributions per fraction: treatment planning system (TPS) calculated dose and EPID reconstructed in vivo dose, both determined using the pCT and using the CBCT. D2, D50 and D98 of the planning target volume (PTV) were determined per dose distribution. RESULTS: When including daily anatomical information, D2, D50 and D98 of the PTV change on average by 0.0±0.4% according to TPS calculations; the standard deviation of the difference between EPID and TPS target dose changes from 2.5% (pCT) to 2.1% (CBCT). Small time trends are seen for both TPS and EPID dose distributions when using the pCT, which disappear when including CBCT information. CONCLUSIONS: Daily anatomical changes hardly influence the target dose distribution for H&N VMAT treatments according to TPS recalculations. Including CBCT information in EPID dose reconstructions slightly improves the agreement with TPS calculations.

In vivo portal dosimetry for head-and-neck VMAT and lung IMRT: linking γ-analysis with differences in dose-volume histograms of the PTV.

PURPOSE: To relate the results of γ-analysis and dose-volume histogram (DVH) analysis of the PTV for detecting dose deviations with in vivo dosimetry for two treatment sites. METHODS AND MATERIALS: In vivo 3D dose distributions were reconstructed for 722 fractions of 200 head-and-neck (H&N) VMAT treatments and 183 fractions of 61 lung IMRT plans. The reconstructed and planned dose distributions in the PTV were compared using (a) the γ-distribution and (b) the differences in D2, D50 and D98 between the two dose distributions. Using pre-defined tolerance levels, all fractions were classified as deviating or not deviating by both methods. The mutual agreement, the sensitivity and the specificity of the two methods were compared. RESULTS: For lung IMRT, the classification of the fractions was nearly identical for γ- and DVH-analyses of the PTV (94% agreement) and the sensitivity and specificity were comparable for both methods. Less agreement (80%) was found for H&N VMAT, while γ-analysis was both less sensitive and less specific. CONCLUSIONS: DVH- and γ-analyses perform nearly equal in finding dose deviations in the PTV for lung IMRT treatments; for H&N VMAT treatments, DVH-analysis is preferable. As a result of this study, a smooth transition to using DVH-analysis clinically for detecting in vivo dose deviations in the PTV is within reach.

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.

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.

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.

In vivo dosimetry with a linear MOSFET array to evaluate the urethra dose during permanent implant brachytherapy using iodine-125.

PURPOSE: To develop a technique to monitor the dose rate in the urethra during permanent implant brachytherapy using a linear MOSFET array, with sufficient accuracy and without significantly extending the implantation time. METHODS AND MATERIALS: Phantom measurements were performed to determine the optimal conditions for clinical measurements. In vivo measurements were performed in 5 patients during the (125)I brachytherapy implant procedure. To evaluate if the urethra dose obtained in the operating room with the ultrasound transducer in the rectum and the patient in treatment position is a reference for the total accumulated dose; additional measurements were performed after the implantation procedure, in the recovery room. RESULTS: In vivo measurements during and after the implantation procedure agree very well, illustrating that the ultrasound transducer in the rectum and patient positioning do not influence the measured dose in the urethra. In vivo dose values obtained during the implantation are therefore representative for the total accumulated dose in the urethra. In 5 patients, the dose rates during and after the implantation were below the maximum dose rate of the urethra, using the planned seed distribution. CONCLUSION: In vivo dosimetry during the implantation, using a MOSFET array, is a feasible technique to evaluate the dose in the urethra during the implantation of (125)I seeds for prostate brachytherapy.

In vivo dosimetry using a linear Mosfet-array dosimeter to determine the urethra dose in 125I permanent prostate implants.

PURPOSE: In vivo dosimetry during brachytherapy of the prostate with (125)I seeds is challenging because of the high dose gradients and low photon energies involved. We present the results of a study using metal-oxide-semiconductor field-effect transistor (MOSFET) dosimeters to evaluate the dose in the urethra after a permanent prostate implantation procedure. METHODS AND MATERIALS: Phantom measurements were made to validate the measurement technique, determine the measurement accuracy, and define action levels for clinical measurements. Patient measurements were performed with a MOSFET array in the urinary catheter immediately after the implantation procedure. A CT scan was performed, and dose values, calculated by the treatment planning system, were compared to in vivo dose values measured with MOSFET dosimeters. RESULTS: Corrections for temperature dependence of the MOSFET array response and photon attenuation in the catheter on the in vivo dose values are necessary. The overall uncertainty in the measurement procedure, determined in a simulation experiment, is 8.0% (1 SD). In vivo dose values were obtained for 17 patients. In the high-dose region (> 100 Gy), calculated and measured dose values agreed within 1.7% +/- 10.7% (1 SD). In the low-dose region outside the prostate (< 100 Gy), larger deviations occurred. CONCLUSIONS: MOSFET detectors are suitable for in vivo dosimetry during (125)I brachytherapy of prostate cancer. An action level of +/- 16% (2 SD) for detection of errors in the implantation procedure is achievable after validation of the detector system and measurement conditions.

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.

Total body irradiation, toward optimal individual delivery: dose evaluation with metal oxide field effect transistors, thermoluminescence detectors, and a treatment planning system.

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.

Treatment verification in the presence of inhomogeneities using EPID-based three-dimensional dose reconstruction.

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.

A fast algorithm for gamma evaluation in 3D.

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%.

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.

Routine individualised patient dosimetry using electronic portal imaging devices.

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.

A Monte Carlo based three-dimensional dose reconstruction method derived from portal dose images.

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.

Clinical implementation of MOSFET detectors for dosimetry in electron beams.

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.

Impact of geometrical uncertainties on 3D CRT and IMRT dose distributions for lung cancer treatment.

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.

Intensity-modulated radiotherapy of breast cancer using direct aperture optimization.

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.

Anatomy changes in radiotherapy detected using portal imaging.

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.