Evaluation of the ability of three commercially available dosimeters to detect systematic delivery errors in step-and-shoot IMRT plans.

BACKGROUND: There is limited data on error detectability for step-and-shoot intensity modulated radiotherapy (sIMRT) plans, despite significant work on dynamic methods. However, sIMRT treatments have an ongoing role in clinical practice. This study aimed to evaluate variations in the sensitivity of three patient-specific quality assurance (QA) devices to systematic delivery errors in sIMRT plans. MATERIALS AND METHODS: Four clinical sIMRT plans (prostate and head and neck) were edited to introduce errors in: Multi-Leaf Collimator (MLC) position (increasing field size, leaf pairs offset (1-3 mm) in opposite directions; and field shift, all leaves offset (1-3 mm) in one direction); collimator rotation (1-3 degrees) and gantry rotation (0.5-2 degrees). The total dose for each plan was measured using an ArcCHECK diode array. Each field, excluding those with gantry offsets, was also measured using an Electronic Portal Imager and a MatriXX Evolution 2D ionisation chamber array. 132 plans (858 fields) were delivered, producing 572 measured dose distributions. Measured doses were compared to calculated doses for the no-error plan using Gamma analysis with 3%/3 mm, 3%/2 mm, and 2%/2 mm criteria (1716 analyses). RESULTS: Generally, pass rates decreased with increasing errors and/or stricter gamma criteria. Pass rate variations with detector and plan type were also observed. For a 3%/3 mm gamma criteria, none of the devices could reliably detect 1 mm MLC position errors or 1 degree collimator rotation errors. CONCLUSIONS: This work has highlighted the need to adapt QA based on treatment plan type and the need for detector specific assessment criteria to detect clinically significant errors.

Remote dosimetric auditing of clinical trials: The need for vendor specific models to convert images to dose.

INTRODUCTION: A previous pilot study has demonstrated the feasibility of a novel image-based approach for remote dosimetric auditing of clinical trials. The approach uses a model to convert in-air acquired intensity modulated radiotherapy (IMRT) images to delivered dose inside a virtual phantom. The model was developed using images from an electronic portal imaging device (EPID) on a Varian linear accelerator. It was tuned using beam profiles and field size factors (FSFs) of a series of square fields measured in water tank. This work investigates the need for vendor specific conversion models for image-based auditing. The EPID measured profile and FSF data for Varian (vendor 1) and Elekta (vendor 2) systems are compared along with the performance of the existing Varian model (VM) and a new Elekta model (EM) for a series of audit IMRT fields measured on vendor 2 systems. MATERIALS AND METHODS: The EPID measured beam profile and FSF data were studied for the two vendors to quantify and understand their relevant dosimetric differences. Then, an EM was developed converting EPID to dose in the virtual water phantom using a vendor 2 water tank data and images from corresponding EPID. The VM and EM were compared for predicting vendor 2 measured dose in water tank. Then, the performance of the new EM was compared to the VM for auditing of 54 IMRT fields from four vendor 2 facilities. Statistical significance of using vendor specific models was determined. RESULTS: Observed dosimetry differences between the two vendors suggested developing an EM would be beneficial. The EM performed better than VM for vendor 2 square and IMRT fields. The IMRT audit gamma pass rates were (99.8 ± 0.5)%, (98.6 ± 2.3)% and (97.0 ± 3.0)% at respectively 3%/3 mm, 3%/2 mm and 2%/2 mm with improvements at most fields compared with using the VM. For the pilot audit, the difference between gamma results of the two vendors was reduced when using vendor specific models (VM: P < 0.0001, vendor specific models: P = 0.0025). CONCLUSION: A new model was derived to convert images from vendor 2 EPIDs to dose for remote auditing vendor 2 deliveries. Using vendor specific models is recommended to remotely audit systems from different vendors, however, the improvements found were not major.

Dose calibration of EPIDs for segmented IMRT dosimetry.

The purpose of this study was to investigate the dose response of amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) under different acquisi- tion settings for both open jaw defined fields and segmented intensity-modulated radiation therapy (IMRT) fields. Four different EPIDs were used. Two Siemens and one Elekta plus a standalone Perkin Elmer research EPID. Each was operated with different acquisition systems and settings. Dose response linearity was measured for open static jaw defined fields and 'simple' segmented IMRT fields for a range of equipment and system settings. Six 'simple' segmented IMRT fields were used. The segments of each IMRT field were fixed at 10 × 10 cm2 field size with equal MU per segment, each field having a total of 20 MU. Simultaneous measurements with an ionization chamber array (ICA) and EPID were performed to separate beam and detector response characteristics. Three different pixel calibration meth- ods were demonstrated and compared for an example 'clinical IMRT field'. The dose response with the Elekta EPID for 'simple' segmented IMRT fields versus static fields agreed to within 2.5% for monitor unit (MU) ≥ 2. The dose response for the Siemens systems was difficult to interpret due to the poor reproducibility for segmented delivery, at MU ≤ 5, which was not observed with the standalone research EPID nor ICA on the same machine. The dose response measured under different acquisition settings and different linac/EPID combinations matched closely (≤ 1%), except for the Siemens EPID. Clinical IMRT EPID dosimetry implemented with the different pixel-to-dose calibration methods indicated that calibration at 20 MU provides equivalent results to implementing a ghosting correction model. The nonlinear dose response was consistent across both clinical EPIDs and the standalone research EPID, with the exception of the poor reproducibility seen with Siemens EPID images of IMRT fields. The nonlinear dose response was relatively insensitive to acquisition settings and appears to be primarily due to gain ghosting effects. No additional ghosting correction factor is necessary when the pixel-to- dose calibration factor at small MU calibration method is used. 

Direct dose to water dosimetry for pretreatment IMRT verification using a modified EPID.

PURPOSE: Electronic portal imaging devices (EPIDs) are high resolution systems that produce electronic dose maps with minimal time required for equipment setup, and therefore potentially present a time-saving alternative for intensity modulated radiation therapy (IMRT) pretreatment verification. A modified commercial EPID was investigated operated with an opaque sheet blocking the optical signal produced in the phosphor layer as a precursor to a switched mode dual dosimetry-imaging EPID system. The purpose of this study was to investigate the feasibility of using this system for direct dose to water dosimetry for pretreatment IMRT verification. METHODS: A Varian amorphous silicon EPID was modified by placing an opaque sheet between the Gd(2)S(2)O:Tb phosphor layer and the photodiode array to block the optical photons. The EPID was thus converted to a direct-detecting system (dEPID), in which the high energy radiation deposits energy directly in the photodiode array. The copper build-up was replaced with d(max) solid water. Sixty-one IMRT beams of varying complexity were delivered to the EPID, to EDR2 dosimetric film and to a 2D ion chamber array (MapCheck). EPID data was compared to film and MapCheck data using gamma analysis with 3%, 3mm pass criteria. RESULTS: The fraction of points that passed the gamma test was on average 98.1% and 98.6%, for the EPID versus film and EPID versus MapCheck comparisons, respectively. In the case of comparison with film, the majority of observed discrepancies were associated with problems related to film sensitivity or processing. CONCLUSIONS: The very close agreement between EPID and both film and MapCheck data demonstrates that the modified EPID is suitable for direct dose to water measurement for pretreatment IMRT verification. These results suggest a reconfigured EPID could be an efficient and accurate dosimeter. Alternatively, optical switching methods could be developed to produce a dual-mode EPID with both dosimetry and imaging capabilities.

Lung organ-at-risk volumes: A survey of practice and the need for a consistent definition in the 4DCT era.

INTRODUCTION: In the 4DCT era, the definition of lung organ-at-risk (OAR) volumes for dose-volume histogram (DVH) calculation is unclear, introducing potential for variability in practice. We aimed to identify definitions used clinically and evaluate the magnitude of DVH differences between these. METHODS: We surveyed Australian & New Zealand departments about lung radiotherapy protocols including lung OAR volume definition. We used these definitions to calculate lung DVHs on 10 patients prescribed lung IMRT (60-66 Gy/30-33 fractions). We calculated mean lung dose (MLD), V20 and V30 for 'Lungs - PTV', 'Lungs - CTV', 'Lungs - iGTV' (internal GTV in all respiratory phases) and 'Lungs - GTV_EX' (expiratory phase). RESULTS: The response rate was 39% (34/88). 14% and 29% of departments did not have a departmental protocol for OAR and tumour volume delineation, respectively. All permutations for lung OAR volumes were used with no clear preference. For conventional radiotherapy (n = 27), this included Lungs alone (n = 1), Lungs - PTV (n = 6), Lungs - CTV (n = 2), Lungs - iGTV (n = 6), Lungs - GTV in single phase (n = 5) and individual clinician preference (n = 7). The different lung OAR volumes resulted in MLD difference ranging from 0.9 to 4.15 Gy, V20 from 1.5% to 6.6% and V30 from 1.34% to 7.11%. The largest differences between subtraction of GTV_EX and iGTV were 0.32 Gy, 0.43% and 0.46% for MLD, V20 and V30, respectively. CONCLUSION: A significant number of departments lacked lung cancer radiotherapy contouring protocols. Lung OAR volume definition was variable between and within departments. Potentially clinically significant differences in lung DVH parameters were seen according to the volume used.

Quality management in radiation therapy: A 15 year review of incident reporting in two integrated cancer centres.

Fifteen years of reported incidents were reviewed to provide insight into the effectiveness of an Incident Learning System (ISL). The actual error rate over the 15 years was 1.3 reported errors per 1000 treatment attendances. Incidents were reviewed using a regression model. The average number of incidents per year and the number of incidents per thousand attendances declined over time. Two seven-year periods were considered for analysis and the average for the first period (2005-2011) was 6 reported incidents per 1000 attendances compared to 2 incidents for the later period (2012-2018), p < 0.05. SAC 1 and SAC 2 errors have reduced over time and the reduction could be attributed to the quality assurance aspect of IGRT where the incident is identified prior to treatment delivery rather than after, reducing the severity of any potential incidents. The reasoning behind overall reduction in incident reporting over time is unclear but may be associated to quality and technology initiatives, issues with the ISL itself or a change in the staff reporting culture.

EPID dosimetry: effect of different layers of materials on absorbed dose response.

PURPOSE: Commercial EPIDs are normally used in indirect detection mode (iEPID) where incident x-ray photons are converted to optical photons in a phosphor scintillator, which are then detected by a photodiode array. The EPIDs are constructed from a number of nonwater equivalent materials which affect the dose response of the detector. The so-called direct detection EPIDs (dEPIDs), operating without the phosphor layer, have been reported to display dose response close to in-water data. In this study, the effect that different layers of materials in the EPID have on the dose response was experimentally investigated and evaluated with respect to changes in field size response and beam profiles. METHODS: An iEPID was disassembled and the different layers of materials were removed or replaced with other materials. Data were also obtained on and off the support arm and with a sheet of opaque paper blocking the optical photons from the gadolinium oxysulfide (Gd2S2O:Tb) phosphor layer. Field size response was measured for field sizes ranging from 2 x 2 to 25 x 25 cm2, and profiles for the 25 x 25 cm2 beams were extracted from the data. RESULTS: The iEPID configuration was found to be very sensitive to backscatter. The increases in output with solid water backscatter compared to the no backscatter case were 14.7% and 6.6% at the largest field size investigated for the 6 and 18 MV beams, respectively. The Gd2S2O:Tb phosphor layer had a large influence on field size response as well as beam profiles for 6 MV photons, while no major effects were observed for the 18 MV beam. For 18 MV large differences in dose response were found when the standard 1 mm Cu buildup was changed for dmax equivalent Cu or solid water buildup, indicating that head scatter largely influences dose response for this energy. When the optical photons originating in the Gd2S2O:Tb layer were blocked from reaching the photodiodes, both field size output data and beam profiles corresponded well with data obtained in the dEPID configuration as well as reference ion chamber data for both energies. CONCLUSIONS: As expected, changing the layers of material in the EPID had a dramatic effect on dose response, which was often quite complex. For 6 MV, the complex dose response is mainly caused by the optical photons from the Gd2S2O:Tb layer, while insufficient filtering of scattered radiation largely affects the dose response for the 18 MV beam. The iEPID was also found to be very sensitive to backscatter for both energies. Blocking the optical photons created in the Gd2S2O:Tb layer essentially changed the iEPID configuration into the dEPID configuration, thus demonstrating great potential for a system that can be optimized for both imaging and dosimetry.

Adapting automated treatment planning configurations across international centres for prostate radiotherapy.

BACKGROUND AND PURPOSE: Automated configurations are increasingly utilised for radiotherapy treatment planning. This study investigates whether automated treatment planning configurations are adaptable across clinics with different treatment planning protocols for prostate radiotherapy. MATERIAL AND METHODS: The study comprised three participating centres, each with pre-existing locally developed prostate AutoPlanning configurations using the Pinnacle3® treatment planning system. Using a three-patient training dataset circulated from each centre, centres modified local prostate configurations to generate protocol compliant treatment plans for the other two centres. Each centre applied modified configurations on validation datasets distributed from each centre (10 patients from 3 centres). Plan quality was assessed through DVH analysis and protocol compliance. RESULTS: All treatment plans were clinically acceptable, based off relevant treatment protocol. Automated planning configurations from Centre's A and B recorded 2 and 18 constraint and high priority deviations respectively. Centre C configurations recorded no high priority deviations. Centre A configurations produced treatment plans with superior dose conformity across all patient PTVs (mean = 1.14) compared with Centre's B and C (mean = 1.24 and 1.22). Dose homogeneity was consistent between all centre's configurations (mean = 0.083, 0.077, and 0.083 respectively). CONCLUSIONS: This study demonstrates that automated treatment planning configurations can be shared and implemented across multiple centres with simple adaptations to local protocols.

Initial evaluation of a commercial EPID modified to a novel direct-detection configuration for radiotherapy dosimetry.

Electronic portal imaging devices (EPIDs) integrated with medical linear accelerators utilize an indirect-detection EPID configuration (ID-EPID). Amorphous silicon ID-EPIDs provide high quality low dose images for verification of radiotherapy treatments but they have limitations as dosimeters. The standard ID-EPID configuration includes a high atomic number phosphor scintillator screen, a 1 mm copper layer, and other nonwater equivalent materials covering the detector. This configuration leads to marked differences in the response of an ID-EPID compared to standard radiotherapy dosimeters such as ion chambers in water. In this study the phosphor and copper were removed from a standard commercial EPID to modify the configuration to a direct-detection EPID (DD-EPID). Using solid water as the buildup and backscatter for the detector, dosimetric measurements were performed on the DD-EPID and compared to standard dose-in-water data for 6 and 18 MV photons. The sensitivity of the DD-EPID was approximately eight times less than the ID-EPID but the signal was sufficient to produce accurate and reproducible beam profile measurements for open beams and an intensity-modulated beam. Due to the lower signal levels it was found necessary to ensure that the dark field correction (no radiation) DD-EPID signal was stable or updated frequently. The linearity of dose response was comparable to the ID-EPID but with a greater under-response at low doses. DD-EPID measurements of field size output factors and beam profiles at the depth of maximum dose (dmax), and tissue-maximum ratios between the depths of 0.5 and 10 cm, were in close agreement with dose in water measurements. At depths beyond dmax the DD-EPID showed a greater change in response to field size than ionisation chamber measurements and the beam penumbrae were broader compared to diode scans. The modified DD-EPID configuration studied here has the potential to improve the performance of EPIDs for dose verification of radiotherapy treatments.

The impact of MLC transmitted radiation on EPID dosimetry for dynamic MLC beams.

The purpose of this study was to experimentally quantify the change in response of an amorphous silicon (a-Si) electronic portal imaging device (EPID) to dynamic multileaf collimator (dMLC) beams with varying MLC-transmitted dose components and incorporate the response into a commercial treatment planning system (TPS) EPID prediction model. A combination of uniform intensity dMLC beams and static beams were designed to quantify the effect of MLC transmission on EPID response at the central axis of 10 x 10 cm2 beams, at off-axis positions using wide dMLC beam profiles, and at different field sizes. The EPID response to MLC transmitted radiation was 0.79 +/- 0.02 of the response to open beam radiation at the central axis of a 10 x 10 cm2 field. The EPID response to MLC transmitted radiation was further reduced relative to the open beam response with off-axis distance. The EPID response was more sensitive to field size changes for MLC transmitted radiation compared to open beam radiation by a factor of up to 1.17 at large field sizes. The results were used to create EPID response correction factors as a function of the fraction of MLC transmitted radiation, off-axis distance, and field size. Software was developed to apply the correction factors to each pixel in the TPS predicted EPID image. The corrected images agreed more closely with the measured EPID images in areas of intensity modulated fields with a large fraction of MLC transmission and, as a result the accuracy of portal dosimetry with a-Si EPIDs can be improved. Further investigation into the detector response function and the radiation source model are required to achieve improvements in accuracy for the general case.

A high DQE water-equivalent EPID employing an array of plastic-scintillating fibers for simultaneous imaging and dosimetry in radiotherapy.

PURPOSE: First measurements of the imaging performance of a novel prototype water-equivalent electronic portal imaging device (EPID) designed for simultaneous imaging and dose verification in radiotherapy and previously characterized by our group for dosimetry are reported. Experiments were conducted to characterize the prototype's imaging performance relative to a standard commercial EPID and Monte Carlo (MC) simulations were performed to quantify the impact of several detector parameters on image quality and to inform the design of a proposed next-generation prototype. METHODS: The prototype EPID utilizes an array of 3 cm long plastic-scintillating fibers in place of the metal plate/phosphor screen in standard EPIDs. Using a clinical 6 MV photon beam, the prototype's modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) were measured and compared to measurements taken using a standard commercial EPID. A sensitivity analysis was then performed using the MC model by quantifying these metrics while varying the values of several geometrical and optical transport parameters that were unspecified by the prototype manufacturer. Finally, the MC model was used to quantify the imaging performance of a proposed next-generation prototype incorporating 1.5 cm long fibers that is better suited for integration with clinical portal imaging and dosimetry systems. RESULTS: The prototype EPID's zero spatial frequency DQE exceeded 3%, more than doubling that measured with the standard EPID (1.25%). This increased DQE was a consequence of using a prototype array detector with a greater equivalent thickness than the combined copper plate and phosphor screen in a standard EPID. The increased thickness of our prototype decreased spatial resolution relative to the standard EPID; however, the prototype EPID NPS was also lower than that measured with the standard EPID across all spatial frequencies. The sensitivity analysis demonstrated that the NPS was strongly affected by the roughness of the boundaries between fiber core and cladding regions. By comparison, the MTF was most sensitive to beam divergence and the presence of air between the fiber array and underlying photodiode panel. Simulations demonstrated that by optimizing these parameters, DQE(0) >4% may be achievable with the proposed next-generation prototype design. CONCLUSIONS: The first measurements characterizing the imaging performance of a novel water-equivalent EPID for imaging and dosimetry in radiotherapy demonstrated a DQE(0) more than double that of a standard EPID. MC simulations further demonstrated the potential for developing a next-generation prototype better suited for clinical translation with even higher DQE.

A remote EPID-based dosimetric TPS-planned audit of centers for clinical trials: outcomes and analysis of contributing factors.

BACKGROUND: A novel remote method for external dosimetric TPS-planned auditing of intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) for clinical trials using electronic portal imaging device (EPID) has been developed. The audit has been applied to multiple centers across Australia and New Zealand. This work aims to assess the audit outcomes and explores the variables that contributed to the audit results. METHODS: Thirty audits were performed of 21 radiotherapy facilities, 17 facilities underwent IMRT audits and 13 underwent VMAT audits. The assessment was based on comparisons between the delivered doses derived from images acquired with EPIDs and planned doses from the local treatment planning systems (TPS). Gamma pass-rate (GPR) and gamma mean value (GMV) were calculated for each IMRT field and VMAT arc (total 268 comparisons). A multiple variable linear model was applied to the GMV results (3%/3 mm criteria) to assess the influence and significance of explanatory variables. The explanatory variables were Linac-TPS combination, TPS grid resolution, IMRT/VMAT delivery, age of EPID, treatment site, record and verification system (R&V) type and dose-rate. Finally, the audit results were compared with other recent audits by calculating the incidence ratio (IR) as a ratio of the observed mean/median GPRs for the remote audit to the other audits. RESULTS: The average (± 1 SD) of the centers' GPRs were: 99.3 ± 1.9%, 98.6 ± 2.7% & 96.2 ± 5.5% at 3%, 3 mm, 3%, 2 mm and 2%, 2 mm criteria respectively. The most determinative variables on the GMVs were Linac-TPS combination, TPS grid resolution and IMRT/VMAT delivery type. The IR values were 1 for seven comparisons, indicating similar GPRs of the remote audit with the reference audits and > 1 for four comparisons, indicating higher GPRs of the remote audit than the reference audits. CONCLUSION: The remote dosimetry audit method for clinical trials demonstrated high GPRs and provided results comparable to established more resource-intensive audit methods. Several factors were found to influence the results including some effect of Linac-TPS combination.

Experimental investigation of the response of an amorphous silicon EPID to intensity modulated radiotherapy beams.

The aim of this work was to experimentally determine the difference in response of an amorphous silicon (a-Si) electronic portal imaging device (EPID) to the open and multileaf collimator (MLC) transmitted beam components of intensity modulated radiation therapy (IMRT) beams. EPID dose response curves were measured for open and MLC transmitted (MLCtr) 10 x 10 cm2 beams at central axis and with off axis distance using a shifting field technique. The EPID signal was obtained by replacing the flood-field correction with a pixel sensitivity variation matrix correction. This signal, which includes energy-dependent response, was then compared to ion-chamber measurements. An EPID calibration method to remove the effect of beam energy variations on EPID response was developed for IMRT beams. This method uses the component of open and MLCtr fluence to an EPID pixel calculated from the MLC delivery file and applies separate radially dependent calibration factors for each component. The calibration procedure does not correct for scatter differences between ion chamber in water measurements and EPID response; these must be accounted for separately with a kernel-based approach or similar method. The EPID response at central axis for the open beam was found to be 1.28 +/- 0.03 of the response for the MLCtr beam, with the ratio increasing to 1.39 at 12.5 cm off axis. The EPID response to MLCtr radiation did not change with off-axis distance. Filtering the beam with copper plates to reduce the beam energy difference between open and MLCtr beams was investigated; however, these were not effective at reducing EPID response differences. The change in EPID response for uniform sliding window IMRT beams with MLCtr dose components from 0.3% to 69% was predicted to within 2.3% using the separate EPID response calibration factors for each dose component. A clinical IMRT image calibrated with this method differed by nearly 30% in high MLCtr regions from an image calibrated with an open beam calibration factor only. Accounting for the difference in EPID response to open and MLCtr radiation should improve IMRT dosimetry with a-Si EPIDs.

Clinical implementation of an exit detector-based dose reconstruction tool for helical tomotherapy delivery quality assurance.

PURPOSE: The aim of this study was to validate the accuracy of an exit detector-based dose reconstruction tool for helical tomotherapy (HT) delivery quality assurance (DQA). METHODS AND MATERIAL: Exit detector-based DQA tool was developed for patient-specific HT treatment verification. The tool performs a dose reconstruction on the planning image using the sinogram measured by the HT exit detector with no objects in the beam (i.e., static couch), and compares the reconstructed dose to the planned dose. Vendor supplied (three "TomoPhant") plans with a cylindrical solid water ("cheese") phantom were used for validation. Each "TomoPhant" plan was modified with intentional multileaf collimator leaf open time (MLC LOT) errors to assess the sensitivity and robustness of this tool. Four scenarios were tested; leaf 32 was "stuck open," leaf 42 was "stuck open," random leaf LOT was closed first by mean values of 2% and then 4%. A static couch DQA procedure was then run five times (once with the unmodified sinogram and four times with modified sinograms) for each of the three "TomoPhant" treatment plans. First, the original optimized delivery plan was compared with the original machine agnostic delivery plan, then the original optimized plans with a known modification applied (intentional MLC LOT error) were compared to the corresponding error plan exit detector measurements. An absolute dose comparison between calculated and ion chamber (A1SL, Standard Imaging, Inc., WI, USA) measured dose was performed for the unmodified "TomoPhant" plans. A 3D gamma evaluation (2%/2 mm global) was performed by comparing the planned dose ("original planned dose" for unmodified plans and "adjusted planned dose" for each intentional error) to exit detector-reconstructed dose for all three "Tomophant" plans. Finally, DQA for 119 clinical (treatment length <25 cm) and three cranio-spinal irradiation (CSI) plans were measured with both the ArcCHECK phantom (Sun Nuclear Corp., Melbourne, FL, USA) and the exit detector DQA tool to assess the time required for DQA and similarity between two methods. RESULTS: The measured ion chamber dose agreed to within 1.5% of the reconstructed dose computed by the exit detector DQA tool on a cheese phantom for all unmodified "Tomophant" plans. Excellent agreement in gamma pass rate (>95%) was observed between the planned and reconstructed dose for all "Tomophant" plans considered using the tool. The gamma pass rate from 119 clinical plan DQA measurements was 94.9% ± 1.5% and 91.9% ± 4.37% for the exit detector DQA tool and ArcCHECK phantom measurements (P = 0.81), respectively. For the clinical plans (treatment length <25 cm), the average time required to perform DQA was 24.7 ± 3.5 and 39.5 ± 4.5 min using the exit detector QA tool and ArcCHECK phantom, respectively, whereas the average time required for the 3 CSI treatments was 35 ± 3.5 and 90 ± 5.2 min, respectively. CONCLUSION: The exit detector tool has been demonstrated to be faster for performing the DQA with equivalent sensitivity for detecting MLC LOT errors relative to a conventional phantom-based QA method. In addition, comprehensive MLC performance evaluation and features of reconstructed dose provide additional insight into understanding DQA failures and the clinical relevance of DQA results.

Sensitivity evaluation of two commercial dosimeters in detecting Helical TomoTherapy treatment delivery errors.

PURPOSE: To assess the sensitivity of two commercial dosimetry systems in detecting Helical TomoTherapy (HT) delivery errors. METHOD: Two commercial dosimeters i) MatriXXEvolution and ii) ArcCHECK® were considered. Ten retrospective nasopharynx HT patients were analysed. For each patient, error plans were created by independently introducing systematic offsets in: a) Jaw width error ±1, ±1.5 and ±2mm, b) Couch speed error ±2%, ±2.5, ±3% and ±4%, and c) MLC Leaf Open Time (MLCLOT) errors (3 separate MLC errors: either leaf 32 open or leaf 42 remains open during delivery, and 4% random reductions in MLCLOT). All error plans, along with the no error plan for each patient, were measured using both dosimeters in the same session. Gamma evaluation (3%/3mm) was applied to quantitatively compare the measured dose from each dosimeter to the treatment planning system. The error sensitivity was quantified as the rate of decrease in gamma pass rate. RESULTS: The gamma pass rate decreases with increase in error magnitude for both dosimeters. ArcCHECK was insensitive for couch speed error up to 2.5% and jaw width error up to -1.5mm while MatriXXEvolution was found to be insensitive to couch speed error up to 2% and couch speed up to -1mm. Both of the detectors show similar sensitivity to all the MLCLOT errors that were clinically relevant. CONCLUSION: No statistically significant (p>0.05) differences exist in detecting the simulated delivery errors between MatriXXEvolution and ArcCHECK dosimeter systems for HT plans. Both dosimeters were able to pick up clinically relevant delivery errors.

Dose discrepancy between planning system estimation and measurement in spine stereotactic body radiation therapy: A case report.

Stereotactic body radiation therapy (SBRT) to treat spinal metastases has shown excellent clinical outcomes for local control. High dose gradients wrapping around spinal cord make this treatment technically challenging. In this work, we present a spine SBRT case where a dosimetric error was identified during pre-treatment dosimetric quality assurance (QA). A patient with metastasis in T7 vertebral body consented to undergo SBRT. A dual arc volumetric modulated arc therapy plan was generated on the Pinnacle treatment planning system (TPS) with a 6 MV Elekta machine using gantry control point spacing of 4°. Standard pre-treatment QA measurements were performed, including ArcCHECK, ion chamber in CTV and spinal cord (SC) region and film measurements in multiple planes. While the dose measured at CTV region showed good agreement with TPS, the dose measured to the SC was significantly higher than reported by TPS in the original and repeat plans. Acceptable agreement was only achieved when the gantry control point spacing was reduced to 3°. A potentially harmful dose error was identified by pre-treatment QA. TPS parameter settings used safely in conventional treatments should be re-assessed for complex treatments.

Clinical significance of treatment delivery errors for helical TomoTherapy nasopharyngeal plans - A dosimetric simulation study.

PURPOSE: Develop a framework to characterize helical TomoTherapy (HT) machine delivery errors and their clinical significance. METHOD AND MATERIALS: Ten nasopharynx HT plans were edited to introduce errors in Jaw width (JW), couch speed (CS), gantry period (GP), gantry start position (GSP), multi leaf collimator leaf open times (MLC LOT). In case of MLC LOT only, both systematic and random delivery errors were investigated. Each error type was simulated independently for a range of magnitudes. Dose distributions for the clinical reference plans and the error simulated plans were compared to establish the magnitude for each error type which resulted in a change in clinical tolerance, defined as 5% variation in D95 of PTV70, D0.1cc of spinal cord, D0.1cc of brainstem and the smallest value of either a 10% or 3.6Gy dose variation in mean parotid dose. RESULTS: Dose variation from systematic delivery errors in JW ±0.5mm, CS ranges between -1% to 1.5%, GP ±1s, GSP ranges between -20 to 2.50 and MLC LOT random error up to 2% from the planned value relative to the clinical reference plan was within the set tolerance values for all the patient cohorts. GSP errors and the random MLC LOT errors with up to 10% standard deviation were found to be relatively insensitive compared to other delivery errors. CONCLUSION: This work has established a framework to characterize HT machine delivery errors. This framework could be applied to any patient dataset to determine clinically relevant HT QA tolerances.

Virtual EPID standard phantom audit (VESPA) for remote IMRT and VMAT credentialing.

A virtual EPID standard phantom audit (VESPA) has been implemented for remote auditing in support of facility credentialing for clinical trials using IMRT and VMAT. VESPA is based on published methods and a clinically established IMRT QA procedure, here extended to multi-vendor equipment. Facilities are provided with comprehensive instructions and CT datasets to create treatment plans. They deliver the treatment directly to their EPID without any phantom or couch in the beam. In addition, they deliver a set of simple calibration fields per instructions. Collected EPID images are uploaded electronically. In the analysis, the dose is projected back into a virtual cylindrical phantom. 3D gamma analysis is performed. 2D dose planes and linear dose profiles are provided and can be considered when needed for clarification. In addition, using a virtual flat-phantom, 2D field-by-field or arc-by-arc gamma analyses are performed. Pilot facilities covering a range of planning and delivery systems have performed data acquisition and upload successfully. Advantages of VESPA are (1) fast turnaround mainly driven by the facility's capability of providing the requested EPID images, (2) the possibility for facilities performing the audit in parallel, as there is no need to wait for a phantom, (3) simple and efficient credentialing for international facilities, (4) a large set of data points, and (5) a reduced impact on resources and environment as there is no need to transport heavy phantoms or audit staff. Limitations of the current implementation of VESPA for trials credentialing are that it does not provide absolute dosimetry, therefore a Level I audit is still required, and that it relies on correctly delivered open calibration fields, which are used for system calibration. The implemented EPID based IMRT and VMAT audit system promises to dramatically improve credentialing efficiency for clinical trials and wider applications.

A survey of modulated radiotherapy use in Australia & New Zealand in 2015.

A survey of radiation oncology medical physics departments across Australia and New Zealand was conducted to assess the usage, commissioning and quality assurance of modulated radiotherapy techniques such as IMRT and VMAT. Survey responses were collected in April-May 2015 to snapshot current practice and historical implementation. The survey asked 142 questions, and is the most detailed survey of its kind published to date. Analysis of results at overall department level, as well as sub-analysis for different equipment and techniques in use, was performed. Results show a high prevalence of IMRT and VMAT in use, and demonstrate the large heterogeneity in clinical practice and experience across the region.

An experimental investigation into the radiation field offset of a dynamic multileaf collimator.

In this study we investigate the characteristics of a rounded leaf end multileaf collimator (MLC) that is used for delivering intensity-modulated radiotherapy (IMRT) with a Varian linear accelerator. The rounded leaf end MLC design results in an offset between the radiation field edge (the physical leaf position) and the light field (the geometric leaf position). We call this the radiation field offset (RFO). The leaf position is calibrated to the leaf tip at the mid-leaf plane. There is an additional offset between the geometric leaf position and the projected leaf tip position that varies as a function of distance from the collimator central axis due to the MLC geometry. We call this the leaf position offset (LPO). There is a lack of consistency in the interpretation and implementation of the RFO and the LPO in the literature. We investigated the RFO and the LPO on Varian's 600 C/D and 21 EX linear accelerators. We used a combination of film and ion chamber measurements of static, segmental MLC (SMLC) and dynamic MLC (DMLC) fields to quantify the leaf offsets across the range of leaf positions. We were able to improve the dosimetry at large off-axis positions with minor adjustments to the vendor's LPO file. The RFO was determined to within 0.1 mm accuracy at the collimator central axis. The measured RFO value depends on whether the method is based on the radiation field edge position or on an integral dose measurement. The integral dose method results in an RFO that is approximately 0.2 mm greater than the radiation field edge method. The difference is due to the MLC penumbra shape. We propose a methodology for measuring and implementing MLC leaf offsets that is suitable for both SMLC and DMLC IMRT. In addition, we propose some definitions that more clearly describe the MLC leaf position for accurate IMRT dosimetry.