Differences in the Patterns of Failure Between IROC Lung and Spine Phantom Irradiations

Is the Imaging Radiation Oncology Core Head and Neck Credentialing Phantom an Effective Surrogate for Different Anatomic Sites?

PURPOSE

The Imaging Radiation Oncology Core (IROC) head and neck (H&N) phantom is used to credential institutions for intensity modulated radiation therapy delivery for all anatomic sites where delivery of modulated therapy is a primary challenge. This study evaluated how appropriate the use of this phantom is for varied clinical anatomy by evaluating how closely the IROC H&N phantom described clinical dose errors from beam modeling compared with various anatomic sites.

METHODS AND MATERIALS

The multileaf collimator (MLC) offset, transmission, percent depth dose, and 7 additional beam modeling parameters for a Varian accelerator were modified in RayStation to match community data at the 2.5th, 25th, 50th, 75th, and 97.5th percentile levels. Modifications were evaluated on 25 H&N phantom cases and 25 clinical cases (H&N, prostate, lung, mesothelioma, and brain), generating 2000 plan perturbations. Differences in mean dose delivered to clinical target volumes and maximum dose to organs at risk were compared between phantom and clinical plans to assess the relationship between dose deviations in phantom versus clinical target volumes and as a function of 18 different complexity metrics.

RESULTS

Perturbations to MLC offset and transmission parameters demonstrated the greatest impact on dose accuracy for phantom and clinical plans (for all anatomic sites). The phantom demonstrated equivalent or greater sensitivity to these parameter perturbations compared with clinical sites, largely aligning with treatment complexity. The mean MLC gap best described the impact of errors in treatment planning system beam modeling parameters in phantom plans and clinical plans from various anatomic sites.

CONCLUSIONS

When compared across various anatomic sites, the IROC H&N credentialing phantom exhibited similar or greater sensitivity to errors in the treatment planning system. As such, it is a suitable surrogate device for assessing institutional performance across various anatomic sites. If an institution successfully irradiates the phantom, that result confers confidence that intensity modulated radiation therapy to a wide range of anatomic sites can be successfully delivered by the institution.

IROC phantoms accurately detect MLC delivery errors

PURPOSE

We evaluated the impact of random and whole-bank multileaf collimator (MLC) delivery errors on dosimetric delivery accuracy in the Imaging and Radiation Oncology Core (IROC) phantom audits, as well as differences in delivery accuracy between the IROC phantom prescription and typical clinical fraction sizes.

METHODS AND MATERIALS

Plans were created for the IROC IMRT head and neck (H&N) and SBRT spine phantoms. MLC leaf errors were introduced into the plans: random shifts between -2 and 2 mm, and whole bank shifts of 0.5, 1, and 2 mm. Plans were recalculated and delivered on a Varian Truebeam, and the log files were analyzed using Mobius Fx software. A second study examined the impact of fraction size on MLC position accuracy and corresponding dose delivery accuracy. The standard IROC phantom prescriptions (∼6 Gy) were scaled to the extremes of 2 Gy for H&N and 27 Gy for spine. All plans (original and scaled) were delivered on a Varian Truebeam and 21EX machine.

RESULTS

Random MLC positioning errors produced small average dose deviations in the PTV of up to -2.8% for H&N and 0.7% for spine. Whole-bank MLC shifts resulted in larger average PTV dose deviations up to 8% for H&N and 7.1% for spine. The Varian 21EX irradiations had greater MLC root mean square (RMS) error than Truebeam plans. Plans with smaller prescriptions (and faster leaf motion) had greater MLC RMS errors, but plan accuracy was not affected dosimetrically - all results remained within 1% regardless of fraction size.

CONCLUSIONS

Both random and whole bank MLC shifts caused dose deviations in the IROC phantoms that were comparable to clinical results previously found in the literature. Deviations measured with ion chambers were well matched with delivery log file analysis. Smaller dose-per-fraction prescriptions caused larger MLC RMS errors that were detected with log files, but were clinically insignificant compared to the dosimetric accuracy of the plan.

Which failures do patient-specific quality assurance systems need to catch?

BACKGROUND

The Joint AAPM-ESTRO TG-360 is developing a quantitative framework to evaluate treatment verification systems used for patient-specific quality assurance (PSQA). A subgroup was commissioned to determine which potential failure modes had the greatest risk to treatment quality and safety, and therefore should be evaluated as part of the PSQA verification.

PURPOSE

To create an extensive database of potential radiotherapy failure modes that should be detected by PSQA and to determine their relative importance for maximizing treatment quality.

METHODS

The subgroup consisted of eight physicists from seven countries, including representatives from three international quality assurance groups. We collected error reports from RO-ILS, SAFRON, AAPM TG publications, and other literature, including international audits. We focused on the subset of failure modes that impact whether the planned dose matches the dose received by the patient. We performed a failure-mode-and-effects analysis (FMEA), estimating the severity (S), occurrence (O), and detectability (D) of each failure mode. Detectability was scored assuming that PSQA was not done but other routine clinical QA was performed, which allowed us to see the importance of PSQA for detecting each specific failure mode. We analyzed the risk priority number (RPN = O*S*D), O*S, and severity rankings to determine the priority of each failure mode.

RESULTS

We collected 394 error reports, which we categorized into 33 failure modes that underwent FMEA. Five failure modes were in the top ranks for both RPN and O*S analysis: four involving treatment planning system (TPS) commissioning and one regarding patient model errors. The highest-ranking RPN failure modes were: TPS algorithm limitations, TPS commissioning errors [multileaf collimator (MLC) modeling, output factor, percent-depth-dose/tissue-maximum-ratio (PDD/TMR), off-axis factor], and patient weight variation. The highest O*S failure modes were similar, with the addition of external patient position variation and incorrect linear accelerator isocenter and cGy/monitor units calibration. RPN and O*S analyses prioritized failure modes that impacted multiple patients with high occurrence and detectability scores, while severity analysis gave higher priority to single-patient modes with high severity scores. The highest-ranking severity modes were MLC sequence deletion, collision, and TPS isocenter incorrect.

CONCLUSION

We have developed a list of failure modes critical to be detected during PSQA and ranked them in order of importance. The top failure modes emphasize the importance of utilizing a variety of treatment verification systems for PSQA, from secondary dose calculation through in-vivo dosimetry, in order to detect all possible errors. For failure modes in the top quartile, PSQA is critical. Without adequate PSQA, these errors may go undetected unless caught by an external audit. This analysis can be useful for optimizing PSQA workflows and for designing evaluations of treatment verification systems, and will be used by the Joint AAPM-ESTRO TG-360 to determine an appropriate validation strategy.

Radiation Therapy Quality Assurance Analysis of Alliance A021501: Preoperative mFOLFIRINOX or mFOLFIRINOX Plus Hypofractionated Radiation Therapy for Borderline Resectable Adenocarcinoma of the Pancreas

PURPOSE

Alliance A021501 is the first randomized trial to evaluate stereotactic body radiation therapy (SBRT) for borderline resectable pancreatic ductal adenocarcinoma (PDAC) after neoadjuvant chemotherapy. In this post hoc study, we reviewed the quality of radiation therapy (RT) delivered.

METHODS AND MATERIALS

SBRT (6.6 Gy × 5) was intended but hypofractionated RT (5 Gy × 5) was permitted if SBRT specifications could not be met. Institutional credentialing through the National Cancer Institute-funded Imaging and Radiation Oncology Core (IROC) was required. Rigorous RT quality assurance (RT QA) was mandated, including pretreatment review by a radiation oncologist. Revisions were required for unacceptable deviations. Additionally, we performed a post hoc RT QA analysis in which contours and plans were reviewed by 3 radiation oncologists and assigned a score (1, 2, or 3) based on adequacy. A score of 1 indicated no deviation, 2 indicated minor deviation, and 3 indicated a major deviation that could be clinically significant. Clinical outcomes were compared by treatment modality and by case score.

RESULTS

Forty patients were registered to receive RT (1 planned but not treated) at 27 centers (18 academic and 9 community). Twenty-three centers were appropriately credentialed for moving lung/liver targets and 4 for static head and neck only. Thirty-two of 39 patients (82.1%) were treated with SBRT and 7 (17.9%) with hypofractionated RT. Five cases (13%) required revision before treatment. On post hoc review, 23 patients (59.0%) were noted to have suboptimal contours or plan coverage, 12 (30.8%) were scored a 2, and 11 (28.2%) were scored a 3. There were no apparent differences in failure patterns or surgical outcomes based on treatment technique or post hoc case score. Details related to on-treatment imaging were not recorded.

CONCLUSIONS

Despite rigorous QA, we encountered variability in simulation, contouring, plan coverage, and dose on trial. Although clinical outcomes did not appear to have been affected, findings from this analysis serve to inform subsequent PDAC SBRT trial designs and QA requirements.

Multi-institution single geometry plan complexity characteristics based on IROC phantoms

BACKGROUND

Clinical intensity modulated radiation therapy plans have been described using various complexity metrics to help identify problematic radiotherapy plans. Most previous studies related to the quantification of plan complexity and their utility have relied on institution-specific plans which can be highly variable depending on the machines, planning techniques, delivery modalities, and measurement devices used. In this work, 1723 plans treating one of only four standardized geometries were simultaneously analyzed to investigate how radiation plan complexity metrics vary across four different sets of common objectives.

PURPOSE

To assess the treatment plan complexity characteristics of plans developed for Imaging and Radiation Oncology Core (IROC) phantoms. Specifically, to understand the variability in plan complexity between institutions for a common plan objective, and to evaluate how various complexity metrics differentiate relevant groups of plans.

METHODS

1723 plans treating one of four standardized IROC phantom geometries representing four different anatomical sites of treatment were analyzed. For each plan, 22 MLC-descriptive plan complexity metrics were calculated, and principal component analysis (PCA) was applied to the 22 metrics in order to evaluate differences in plan complexity between groups. Across all metrics, pairwise comparisons of the IROC phantom data were made for the following classifications of the data: anatomical phantom treated, treatment planning system (TPS), and the combination of MLC model and treatment planning system. An objective k-means clustering algorithm was also applied to the data to determine if any meaningful distinctions could be made between different subgroups. The IROC phantom database was also compared to a clinical database from the University of Wisconsin-Madison (UW) which included plans treating the same four anatomical sites as the IROC phantoms using a TrueBeam™ STx and Pinnacle3 TPS.

RESULTS

The IROC head and neck and spine plans were distinct from the prostate and lung plans based on comparison of the 22 metrics. All IROC phantom plan group complexity metric distributions were highly variable despite all plans being designed for identical geometries and plan objectives. The clusters determined by the k-means algorithm further supported that the IROC head and neck and spine plans involved similar amounts of complexity and were largely distinct from the prostate and lung plans, but no further distinctions could be made. Plan complexity in the head and neck and spine IROC phantom plans were similar to the complexity encountered in the UW clinical plans.

CONCLUSIONS

There is substantial variability in plan complexity between institutions when planning for the same objective. For each IROC anatomical phantom treated, the magnitude of variability in plan complexity between institutions is similar to the variability in plan complexity encountered within a single institution database containing several hundred unique clinical plans treating corresponding anatomies in actual patients.

Optimized scoring of end-to-end dosimetry audits for passive motion management - A simulation study using the IROC thorax phantom

Dosimetry audits for passive motion management require dynamically-acquired measurements in a moving phantom to be compared to statically calculated planned doses. This study aimed to characterise the relationship between planning and delivery errors, and the measured dose in the Imaging and Radiation Oncology Core (IROC) thorax phantom, to assess different audit scoring approaches. Treatment plans were created using a 4DCT scan of the IROC phantom, equipped with film and thermoluminescent dosimeters (TLDs). Plans were created on the average intensity projection from all bins. Three levels of aperture complexity were explored: dynamic conformal arcs (DCAT), low-, and high-complexity volumetric modulated arcs (VMATLo, VMATHi). Simulated-measured doses were generated by modelling motion using isocenter shifts. Various errors were introduced including incorrect setup position and target delineation. Simulated-measured film doses were scored using gamma analysis and compared within specific regions of interest (ROIs) as well as the entire film plane. Positional offsets were estimated based on isodoses on the film planes, and point doses within TLD contours were compared. Motion-induced differences between planned and simulated-measured doses were evident even without introduced errors Gamma passing rates within target-centred ROIs correlated well with error-induced dose differences, while whole film passing rates did not. Isodose-based setup position measurements demonstrated high sensitivity to errors. Simulated point doses at TLD locations yielded erratic responses to introduced errors. ROI gamma analysis demonstrated enhanced sensitivity to simulated errors compared to whole film analysis. Gamma results may be further contextualized by other metrics such as setup position or maximum gamma.

Early results of a remote dosimetry audit program for lung stereotactic body radiation therapy

Background and purpose

A dosimetry audit program based on alanine electron paramagnetic resonance (EPR) and radiochromic film dosimetry, may be a valuable tool for monitoring and improving the quality of lung stereotactic body radiotherapy (SBRT). The aim of this study was to report the initial, independent assessment of the dosimetric accuracy for lung SBRT practice using these dosimeters in combination with a novel phantom design.

Materials and Methods

The audit service was a remote audit program performed on a commercial lung phantom preloaded with film and alanine detectors. An alanine pellet was placed in the centre of the target simulated using silicone in a 3D-printed mould. Large film detectors were placed coronally through the target and the lung/tissue interface and analysed using gamma analysis. The beam output was always checked on the same day with alanine dosimetry in water. We audited 29 plans from 14 centres up to now.

Results

For the alanine results 28/29 plans were within 5 % with 19/29 plans being within 3 %. The passing rates were > 95 % for the film through the target for 27/29 plans and 17/29 plans for the film at the lung/tissue interface. For three plans the passing rate was < 90 % for the film on top of the lungs.

Conclusions

The preliminary results were very satisfactory for both detectors. The high passing rates for the film in the interface region indicate good performance of the treatment planning systems. The phantom design was robust and performed well on several treatment systems.

Measuring dose in lung identifies peripheral tumour dose inaccuracy in SBRT audit

PURPOSE

Stereotactic Body Radiotherapy (SBRT) for lung tumours has become a mainstay of clinical practice worldwide. Measurements in anthropomorphic phantoms enable verification of patient dose in clinically realistic scenarios. Correction factors for reporting dose to the tissue equivalent materials in a lung phantom are presented in the context of a national dosimetry audit for SBRT. Analysis of dosimetry audit results is performed showing inaccuracies of common dose calculation algorithms in soft tissue lung target, inhale lung material and at tissue interfaces.

METHODS

Monte Carlo based simulation of correction factors for detectors in non-water tissue was performed for the soft tissue lung target and inhale lung materials of a modified CIRS SBRT thorax phantom. The corrections were determined for Gafchromic EBT3 Film and PTW 60019 microDiamond detectors used for measurements of 168 SBRT lung plans in an end-to-end dosimetry audit. Corrections were derived for dose to medium (Dm,m) and dose to water (Dw,w) scenarios.

RESULTS

Correction factors were up to -3.4% and 9.2% for in field and out of field lung respectively. Overall, application of the correction factors improved the measurement-to-plan dose discrepancy. For the soft tissue lung target, agreement between planned and measured dose was within average of 3% for both film and microDiamond measurements.

CONCLUSIONS

The correction factors developed for this work are provided for clinical users to apply to commissioning measurements using a commercially available thorax phantom where inhomogeneity is present. The end-to-end dosimetry audit demonstrates dose calculation algorithms can underestimate dose at lung tumour/lung tissue interfaces by an average of 2-5%.

Analysis of Performance and Failure Modes of the IROC Proton Liver Phantom

Purpose

To analyze trends in institutional performance and failure modes for the Imaging and Radiation Oncology Core's (IROC's) proton liver phantom.

Materials and Methods

Results of 66 phantom irradiations from 28 institutions between 2015 and 2020 were retrospectively analyzed. Univariate analysis and random forest models were used to associate irradiation conditions with phantom results. Phantom results included pass/fail classification, average thermoluminescent dosimeter (TLD) ratio of both targets, and percentage of pixels passing gamma of both targets. The following categories were evaluated in terms of how they predicted these outcomes: irradiation year, treatment planning system (TPS), TPS algorithm, treatment machine, number of irradiations, treatment technique, motion management technique, number of isocenters, and superior-inferior extent (in cm) of the 90% TPS isodose line for primary target 1 (PTV1) and primary target 2 (PTV2). In addition, failures were categorized by failure mode.

Results

Average pass rate was approximately 52% and average TLD ratio for both targets had slightly improved. As the treatment field increased to cover the target, the pass rate statistically significantly fell. Lower pass rates were observed for Mevion machines, scattered irradiation techniques, and gating and internal target volume (ITV) motion management techniques. Overall, the accuracy of the random forest modeling of the phantom results was approximately 73% ± 14%. The most important predictor was the superior-inferior extent for both targets and irradiation year. Three failure modes dominated the failures of the phantom: (1) systematic underdosing, (2) poor localization in the superior-inferior direction, and (3) range error. Only 44% of failures have similar failure modes between the 2 targets.

Conclusion

Improvement of the proton liver phantom has been observed; however, the pass rate remains the lowest among all IROC phantoms. Through various analysis techniques, range uncertainty, motion management, and underdosing are the main culprits of failures of the proton liver phantom. Clinically, careful consideration of the influences of liver proton therapy is needed to improve phantom performance and patient outcome.

End-to-end test for lung SBRT: An Italian multicentric pilot experience

PURPOSE

Set up a lung SBRT end-to-end (e2e) test and perform a multicentre validation.

MATERIAL AND METHODS

A group of medical physicists from four hospitals and the Italian Institute of Ionizing Radiation Metrology designed the present e2e test. One sub-group set up the test, while another tested its feasibility and ease of use. A satisfaction questionnaire was used to collect user feedback. Each participating centre (PC) received the ADAM breathing phantom, a microDiamond detector and radiochromic films. Following the e2e protocol, each PC performed its standard internal procedure for simulating, planning, and irradiating the phantom. Each PC uploaded its planning and treatment delivery data in a shared Google Drive. A single centre analyzed all the data.

RESULTS

The e2e test was successfully performed by all PCs. Participants' comments indicated that ADAM was well suited to the purpose and the protocol well described. All PCs performed the test in static and dynamic modes. The ratio between measured and planned point dose obtained by PC1, PC2, PC3, PC4 was: 0.99, 0.96, 1.01 and 1.01 (static track) and 0.99, 1.02, 1.01 and 0.94 (dynamic track). The gamma passing rates (3 % global, 3 mm) between planned and measured dose maps were 98.5 %, 94.0 %, 99.1 % and 94.0 % (static track) and 99.5 %, 96.5 %, 86.0 % and 94.5 % (dynamic track) for PC1, PC2, PC3 and PC4, respectively.

CONCLUSIONS

An e2e test for lung SBRT has been proposed and tested in a multicentre framework. The results and user feedback prove the validity of the proposed e2e test.

Adoption of respiratory motion management in radiation therapy

Background and Purpose

A survey on the patterns of practice of respiratory motion management (MM) was distributed to 111 radiation therapy facilities to inform the development of an end-to-end dosimetry audit including respiratory motion.

Materials and methods

The survey (distributed via REDCap) asked facilities to provide information specific to the combinations of MM techniques (breath-hold gating – BHG, internal target volume – ITV, free-breathing gating – FBG, mid-ventilation – MidV, tumour tracking – TT), sites treated (thorax, upper abdomen, lower abdomen), and fractionation regimes (conventional, stereotactic ablative body radiation therapy – SABR) used in their clinic.

Results

The survey was completed by 78% of facilities, with 98% of respondents indicating that they used at least one form of MM. The ITV approach was common to all MM-users, used for thoracic treatments by 89% of respondents, and upper and lower abdominal treatments by 38%. BHG was the next most prevalent (41% of MM users), with applications in upper abdominal and thoracic treatment sites (28% vs 25% respectively), but minimal use in the lower abdomen (9%). FBG and TT were utilised sparingly (17%, 7% respectively), and MidV was not selected at all.

Conclusions

Two distinct treatment workflows (including use of motion limitation, imaging used for motion assessment, dose calculation, and image guidance procedures) were identified for the ITV and BHG MM techniques, to form the basis of the initial audit. Thoracic SABR with the ITV approach was common to nearly all respondents, while upper abdominal SABR using BHG stood out as more technically challenging. Other MM techniques were sparsely used, but may be considered for future audit development.

AAPM MEDICAL PHYSICS PRACTICE GUIDELINE 5.b: Commissioning and QA of treatment planning dose calculations-Megavoltage photon and electron beams

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education, and professional practice of medical physics. The AAPM has more than 8000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized. The following terms are used in the AAPM practice guidelines:

Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline. While must is the term to be used in the guidelines, if an entity that adopts the guideline has shall as the preferred term, the AAPM considers that must and shall have the same meaning.

Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances.

Photon beam modeling variations predict errors in IMRT dosimetry audits

Background & purpose:

To evaluate treatment planning system (TPS) beam modeling parameters as contributing factors to IMRT audit performance.

Materials & methods:

We retrospectively analyzed IROC Houston phantom audit performance and concurrent beam modeling survey responses from 337 irradiations performed between August 2017 and November 2019. Irradiation results were grouped based on the reporting of typical or atypical beam modeling parameter survey responses (<10th or >90th percentile values), and compared for passing versus failing (>7% error) or “poor” (>5% error) irradiation status. Additionally, we assessed the impact on the planned dose distribution from variations in modeling parameter value. Finally, we estimated the overall impact of beam modeling parameter variance on dose calculations, based on reported community variations.

Results:

Use of atypical modeling parameters were more frequently seen with failing phantom audit results (p = 0.01). Most pronounced was for Eclipse AAA users, where phantom irradiations with atypical values of dosimetric leaf gap (DLG) showed a greater incidence of both poor-performing (p = 0.048) and failing phantom audits (p = 0.014); and in general, DLG value was correlated with dose calculation accuracy (r = 0.397, p < 0.001). Manipulating TPS parameters induced systematic changes in planned dose distributions which were consistent with prior observations of how failures manifest. Dose change estimations based on these dose calculations agreed well with true dosimetric errors identified.

Conclusion:

Atypical TPS beam modeling parameters are associated with failing phantom audits. This is identified as an important factor contributing to the observed failing phantom results, and highlights the need for accurate beam modeling.

Calculation algorithms and penumbra: Underestimation of dose in organs at risk in dosimetry audits

PURPOSE

The aim of this study is to investigate overdose to organs at risk (OARs) observed in dosimetry audits in Monte Carlo (MC) algorithms and Linear Boltzmann Transport Equation (LBTE) algorithms. The impact of penumbra modeling on OAR dose was assessed with the adjustment of MC modeling parameters and the clinical relevance of the audit cases was explored with a planning study of spine and head and neck (H&N) patient cases.

METHODS

Dosimetric audits performed by the Australian Clinical Dosimetry Service (ACDS) of 43 anthropomorphic spine plans and 1318 C-shaped target plans compared the planned dose to doses measured with ion chamber, microdiamond, film, and ion chamber array. An MC EGSnrc model was created to simulate the C-shape target case. The electron cut-off energy E_cut(kinetic) was set at 500, 200, and 10 keV, and differences between 1 and 3 mm voxel were calculated. A planning study with 10 patient stereotactic body radiotherapy (SBRT) spine plans and 10 patient H&N plans was calculated in both Acuros XB (AXB) v15.6.06 and Anisotropic Analytical Algorithm (AAA) v15.6.06. The patient contour was overridden to water as only the penumbral differences between the two different algorithms were under investigation.

RESULTS

The dosimetry audit results show that for the SBRT spine case, plans calculated in AXB are colder than what is measured in the spinal cord by 5%-10%. This was also observed for other audit cases where a C-shape target is wrapped around an OAR where the plans were colder by 3%-10%. Plans calculated with Monaco MC were colder than measurements by approximately 7% with the OAR surround by a C-shape target, but these differences were not noted in the SBRT spine case. Results from the clinical patient plans showed that the AXB was on average 7.4% colder than AAA when comparing the minimum dose in the spinal cord OAR. This average difference between AXB and AAA reduced to 4.5% when using the more clinically relevant metric of maximum dose in the spinal cord. For the H&N plans, AXB was cooler on average than AAA in the spinal cord OAR (1.1%), left parotid (1.7%), and right parotid (2.3%). The EGSnrc investigation also noted similar, but smaller differences. The beam penumbra modeled by E_cut(kinetic)  = 500 keV was steeper than the beam penumbra modeled by E_cut(kinetic)  = 10 keV as the full scatter is not accounted for, which resulted in less dose being calculated in a central OAR region where the penumbra contributes much of the dose. The dose difference when using 2.5 mm voxels of the center of the OAR between 500 and 10 keV was 3%, reducing to 1% between 200 and 10 keV.

CONCLUSIONS

Lack of full penumbral modeling due to approximations in the algorithms in MC based or LBTE algorithms are a contributing factor as to why these algorithms under-predict the dose to OAR when the treatment volume is wrapped around the OAR. The penumbra modeling approximations also contribute to AXB plans predicting colder doses than AAA in areas that are in the vicinity of beam penumbra. This effect is magnified in regions where there are many beam penumbras, for example in the spinal cord for spine SBRT cases.

Failure Modes in IROC Photon Liver Phantom Irradiations

PURPOSE

Our purpose was to analyze and classify the patterns of failure for irradiations of the Imaging and Radiation Oncology Core photon liver phantom.

METHODS AND MATERIALS

Imaging and Radiation Oncology Core's anthropomorphic liver phantom simulates multitarget liver disease with respiratory motion. Two hundred forty-nine liver phantom results from 2013 to 2019 were analyzed. Phantom irradiations that failed were categorized by the error attributed to the failure. Phantom results were also compared by demographic data, such as machine type, treatment planning system, motion management technique, number of isocenters, and whether the phantom was a first time or repeat irradiation.

RESULTS

The failure rate for the liver phantom was 27%. From the 68 irradiations that did not pass, 5 failure modes were identified. The most common failure mode was localization errors in the direction of motion, with over 50% of failures attributed to this mode. The second-most common failure mode was systematic dose errors. The internal target volume technique performed worse than other motion management techniques. Failure modes were different by the number of isocenters used, with multi-isocenter irradiations having more failure modes in a single phantom irradiation.

CONCLUSIONS

Motion management techniques and proper alignment of moving targets play a large role in the successful irradiation of the liver phantom. These errors should be examined to ensure accurate patient treatment for liver disease or other sites where multiple moving targets are present.

Measuring the dose in bone for spine stereotactic body radiotherapy

PURPOSE

Current quality assurance of radiotherapy involving bony regions generally utilises homogeneous phantoms and dose calculations, ignoring the challenges of heterogeneities with dosimetry problems likely occurring around bone. Anthropomorphic phantoms with synthetic bony materials enable realistic end-to-end testing in clinical scenarios. This work reports on measurements and calculated corrections required to directly report dose in bony materials in the context of comprehensive end-to-end dosimetry audit measurements (63 plans, 6 planning systems).

MATERIALS AND METHODS

Radiochromic film and microDiamond measurements were performed in an anthropomorphic spine phantom containing bone equivalent materials. Medium dependent correction factors, k_med, were established using 6 MV and 10 MV Linear Accelerator Monte Carlo simulations to account for the detectors being calibrated in water, but measuring in regions of bony material. Both cortical and trabecular bony material were investigated for verification of dose calculations in dose-to-medium (D_m,m) and dose-to-water (D_w,w) scenarios.

RESULTS

For D_m,m calculations, modelled correction factors for cortical and trabecular bone in film measurements, and for trabecular bone in microDiamond measurements were 0.875(±0.1%), 0.953(±0.3%) and 0.962(±0.4%), respectively. For D_w,w calculations, the corrections were 0.920(±0.1%), 0.982(±0.3%) and 0.993(±0.4%), respectively. In the audit, application of the correction factors improves the mean agreement between treatment plans and measured microDiamond dose from -2.4%(±3.9%) to 0.4%(±3.7%).

CONCLUSION

Monte Carlo simulations provide a method for correcting the dose measured in bony materials allowing more accurate comparison with treatment planning system doses. In verification measurements, algorithm specific correction factors should be applied to account for variations in bony material for calculations based on D_m,m and D_w,w.

Dose calculation errors as a component of failing IROC lung and spine phantom irradiations

PURPOSE

Between July 2013 and August 2019, 22% of the imaging and radiation oncology core (IROC) spine, and 15% of the moving lung phantom irradiations have failed to meet established acceptability criteria. The spine phantom simulates a highly modulated stereotactic body radiation therapy (SBRT) case, whereas the lung phantom represents a low-to-none modulation moving target case. In this study, we assessed the contribution of dose calculation errors to these phantom results and evaluated their effects on failure rates.

METHODS

We evaluated dose calculation errors by comparing the calculation accuracy of various institutions' treatment planning systems (TPSs) vs IROC-Houston's previously established independent dose recalculation system (DRS). Each calculation was compared with the measured dose actually delivered to the phantom; cases in which the recalculation was more accurate were interpreted as a deficiency in the institution's TPS. A total of 258 phantom irradiation plans (172 lung and 86 spine) were recomputed.

RESULTS

Overall, the DRS performed better than the TPSs in 47% of the spine phantom cases. However, the DRS was more accurate in 93% of failing spine phantom cases (with an average improvement of 2.35%), indicating a deficiency in the institution's treatment planning system. Deficiencies in dose calculation accounted for 60% of the overall discrepancy between measured and planned doses among spine phantoms. In contrast, lung phantom DRS calculations were more accurate in only 35% and 42% of all and failing lung phantom cases respectively, indicating that dose calculation errors were not substantially present. These errors accounted for only 30% of the overall discrepancy between measured and planned doses.

CONCLUSIONS

Dose calculation errors are common and substantial in IROC spine phantom irradiations, highlighting a major failure mode in this phantom and in clinical treatment management of these cases. In contrast, dose calculation accuracy had only a minimal contribution to failing lung phantom results, indicating that other failure modes drive problems with this phantom and similar clinical treatments.