Analysis of dose comparison techniques for patient-specific quality assurance in radiation therapy

Validation of clearcalc for efficient patient specific QA

Uganda's only radiotherapy center is a very busy facility treating about 210 patients daily on three linear accelerators making it sometimes hard to have machine time for pretreatment QAs. This study was aimed at validating an independent calculation software, ClearCalc (ICS) for second checks of the treatment planning system (TPS) calculations. The validation of ICS started with simple phantom test plans consisting of square, irregular, open and wedged fields designed in the TPS and measured in phantoms. Doses and monitor units (MUs) calculated by ICS were compared with TPS calculated doses and with measured doses. ICS was then validated on clinically approved treatment plans: comparison with TPS calculations and with pretreatment QA measurements performed with electronic portal imaging devices (EPIDs) and analyzed using Gamma passing criteria of 3%/3 mm and 3%/2 mm. Results for test plans were within the passing level of 3.0% except for 2 outliers (-3.1% and 3.1%). As for the clinically approved treatment plans, they show good agreement between MUs (0.2 ± 1.8%), reference point doses (0.2 ±1.5%) and mean PTV doses (0.5 ± 1.4%). ICS calculated (3D) mean gamma pass rates were 98.1±1.6% and 98.4±1.0% for 3%/2 mm and 3%/3 mm criteria. No correlation was seen between gamma analysis results from ICS and EPID. This study validated ClearCalc on phantom and clinically approved plans. The result show that ICS based patients specific QA is quick, promising and potentially allows significant time saving that can be utilized for patient treatments.

Evaluation of magnetic resonance imaging derived synthetic computed tomography for proton therapy planning in prostate cancer

Background and purpose

Magnetic resonance imaging (MRI)-only workflow is used in photon radiotherapy (RT) today, but not yet for protons. To bring MRI-only proton RT into clinical use, proton dose calculation on MRI-derived synthetic CT (sCT) must be validated. We evaluated proton dose calculation accuracy of prostate cancer proton plans using a commercially available sCT generator already validated for photon planning.

Materials and methods

The retrospective planning study included 10 prostate cancer patients who underwent MRI and planning CT (pCT) before RT. sCT were generated from the MRI with MRI Planner v2.3, and compared to pCT using structural mean absolute error (MAE). The pCT was used to create one-arc volumetric modulated arc therapy (VMAT) photon plan and two-field intensity modulated proton therapy (IMPT) proton plan. Each plan was recalculated on the sCT and compared to pCT doses. Dose volume histogram parameters, gamma analyses and range differences were evaluated.

Results

Median MAE for the body contour was 71 HU. Dose differences between pCT and sCT were small and similar for VMAT and IMPT plans. Median (range) gamma pass rates were lower for IMPT plans with 95.8 (89.3-98.7) % compared to VMAT plans with 99.4 (91.2-99.6) %. The proton range difference was 1.0 (interquartile range -0.1 - 0.2) mm deeper for sCT compared to the reference.

Conclusion

MRI-only IMPT planning for prostate cancer seems feasible in a clinical setting for the evaluated beam arrangement and sCT generator. More patients and evaluation of other beam arrangements are needed for a more general conclusion.

Using the gamma-index analysis for inter-fractional comparison of in-beam PET images for head-and-neck treatment monitoring in proton therapy: A Monte Carlo simulation study

GOAL

In-beam Positron Emission Tomography (PET) is a technique for in-vivo non-invasive treatment monitoring for proton therapy. To detect anatomical changes in patients with PET, various analysis methods exist, but their clinical interpretation is problematic. The goal of this work is to investigate whether the gamma-index analysis, widely used for dose comparisons, is an appropriate tool for comparing in-beam PET distributions. Focusing on a head-and-neck patient, we investigate whether the gamma-index map and the passing rate are sensitive to progressive anatomical changes.

METHODS/MATERIALS

We simulated a treatment course of a proton therapy patient using FLUKA Monte Carlo simulations. Gradual emptying of the sinonasal cavity was modeled through a series of artificially modified CT scans. The in-beam PET activity distributions from three fields were evaluated, simulating a planar dual head geometry. We applied the 3D-gamma evaluation method to compare the PET images with a reference image without changes. Various tolerance criteria and parameters were tested, and results were compared to the CT-scans.

RESULTS

Based on 210 MC simulations we identified appropriate parameters for the gamma-index analysis. Tolerance values of 3 mm/3% and 2 mm/2% were suited for comparison of simulated in-beam PET distributions. The gamma passing rate decreased with increasing volume change for all fields.

CONCLUSION

The gamma-index analysis was found to be a useful tool for comparing simulated in-beam PET images, sensitive to sinonasal cavity emptying. Monitoring the gamma passing rate behavior over the treatment course is useful to detect anatomical changes occurring during the treatment course.

2-Dimensional IMRT dose audit: An Indonesian multicenter study

Intensity modulated radiation therapy (IMRT) is an advanced technique in radiation therapy delivery. IMRT depends on the accuracy of the multileaf collimator during treatment. Hence, the actual dose distribution can deviate from the treatment planning system's calculation. This study aimed to perform a multicentre planar dosimetry audit of radiotherapy centres in Indonesia, using the structure sets from AAPM TG-119. The gamma index used to evaluate the dose distribution was 3%/3 mm and 3%/2 mm. We observed 100% gamma index passing rates mostly in the 3%/3 mm evaluations. The gamma index passing rates dropped in the 3%/2 mm analysis. Most of the radiotherapy centres participating in this audit satisfied each criterion's tolerance limit of the action level. This study may become a first result for the next multicenter IMRT audit by using a standardized protocol.

Optimizing the Region for Evaluation of Global Gamma Analysis for Nasopharyngeal Cancer (NPC) Pretreatment IMRT QA by COMPASS: A Retrospective Study

Background

The global gamma passing rate is the most commonly used metric for patient-specific pretreatment quality assurance in radiation therapy. However, the optimal region for evaluation and specific action limits (ALs) need to be explored. Therefore, this study was carried out to explore the optimal region for evaluation of the global gamma passing rate and define ALs by using the COMPASS software.

Methods

A total of 93 intensity-modulated radiation therapy (IMRT) plans for nasopharyngeal cancer (NPC) patients, including 61 original plans and 32 multileaf collimator (MLC) error-introduced test plans, were selected for retrospective analysis. Firstly, the dose distribution was divided into six isodose regions (“≥10%”, “≥20%”, “≥30%”, “≥40%”, “≥50%”, and “≥60%”) based on the prescribed dose and one clinically oriented region for evaluation (“whole”) to perform the three-dimensional (3D) global gamma reanalysis. Meanwhile, the percentage gamma passing rate (%GP), mean gamma index (μGI) based on 3%/2 mm criteria, and percentage dose error (%DE) of the dose–volume histogram (DVH) metrics were recorded by COMPASS application. Secondly, the Pearson’s correlation coefficient was used to analyze the correlation between %GP and %DE and between μGI and %DE in different regions. Additionally, receiver operating characteristic (ROC) methodology was applied to quantify the fraction of patient-specific plans evaluated as “fail” and “pass”. In order to improve the correlation between gamma analysis result and clinical criteria, ROC analysis was carried out in accordance with hybridization analysis criteria (%DE ≤3%, %GP ≥90% and %DE ≤3%, μGI ≤0.6). ROC was performed for two purposes: 1) to analyze the sensitivity and specificity of %GP and μGI in different regions for evaluation and 2) to define the ALs of %GP and μGI in the optimal region for evaluation. Finally, the plans introduced with MLC errors were prepared for validation. Moreover, we also compared the positive rate of ALs of both %GP and μGI in detecting MLC error-introduced plans in different regions.

Results

1) In our study, a number of DVH-based metrics were found to be correlated with the evaluation parameters. The corresponding number was 4, 2, 1, 1, 1, 1, and 3 in γ_whole, γ_10%, γ_20%, γ_30%, γ_40%, γ_50%, and γ_60%, respectively, and 5, 3, 0, 1, 1, 4, and 2 in μGI_whole, μGI_10%, μGI_20%, μGI_30%, μGI_40%, μGI_50%, and μGI_60%, respectively. The results by COMPASS have revealed that the %DE of specific structures has a slightly higher correlation with both %GP and μGI of the “whole” region compared with that of any other region. However, it is a moderate correlation (0.5 ≤ |r| < 0.8). 2) The areas under the curves (AUCs) of γ_whole, μGI_whole, μGI_40%, μGI_50%, and μGI_60% were >0.7 based on 3%/2 mm criteria. According to the Youden coefficient, we defined the ALs of γ_whole ≥92%, μGI_{whole ≤}0.36, μGI_40% ≤0.43, and μGI_60% ≤0.40 based on 3%/2 mm criteria. 3) In the validation, for original plans, the accuracy of AL_γwhole, AL_γ10%, AL_μGIwhole, AL_μGI40%, AL_μGI50%, and AL_μGI60% was 23%, 9.8%, 90%, 80.3%, 9.8%, and 88.5%, respectively. For test plans with systematic MLC errors smaller than 0.8 mm, the positive rates of AL_γwhole, AL_γ10%, AL_μGIwhole, AL_μGI40%, AL_μGI50%, and AL_μGI60% were 25%, 58%, 92%, 92%, 42%, and 100%, respectively. For the plans with systematic MLC errors higher than 0.8 mm, the positive rates of all AL_%GP and AL_μGI were 100%. From the COMPASS validation results, the accuracy of γ_whole, μGI_whole, μGI_40%, and μGI_60% was higher than that of the conventional γ_10% and commonly used μGI_50%.

Conclusions

Compared with the traditional evaluation region (i.e., the criteria with a threshold of 10% or a threshold of 50%, it was the same with the isodose regions of “≥10%”, “≥50%” based on the prescribed dose in our study), the “whole” region is more meaningful to the clinic by COMPASS. The accuracy of μGI_whole is higher than that of the conventional γ_10% and the commonly used μGI_50%.

DIR-based models to predict weekly anatomical changes in head and neck cancer proton therapy

Objective. We proposed two anatomical models for head and neck patients to predict anatomical changes during the course of radiotherapy.Approach. Deformable image registration was used to build two anatomical models: (1) the average model (AM) simulated systematic progressive changes across the patient cohort; (2) the refined individual model (RIM) used a patient's CT images acquired during treatment to update the prediction for each individual patient. Planning CTs and weekly CTs were used from 20 nasopharynx patients. This dataset included 15 training patients and 5 test patients. For each test patient, a spot scanning proton plan was created. Models were evaluated using CT number differences, contours, proton spot location deviations and dose distributions.Main results. If no model was used, the CT number difference between the planning CT and the repeat CT at week 6 of treatment was on average 128.9 Hounsfield Units (HU) over the test population. This can be reduced to 115.5 HU using the AM, and to 110.5 HU using the RIM3(RIM, updated at week (3). When the predicted contours from the models were used, the average mean surface distance of parotid glands can be reduced from 1.98 (no model) to 1.16 mm (AM) and 1.19 mm (RIM3) at week 6. Using the proton spot range, the average anatomical uncertainty over the test population reduced from 4.47 ± 1.23 (no model) to 2.41 ± 1.12 mm (AM), and 1.89 ± 0.96 mm (RIM3). Based on the gamma analysis, the average gamma index over the test patients was improved from 93.87 ± 2.48 % (no model) to 96.16 ± 1.84% (RIM3) at week 6.Significance. The AM and the RIM both demonstrated the ability to predict anatomical changes during the treatment. The RIM can gradually refine the prediction of anatomical changes based on the AM. The proton beam spots provided an accurate and effective way for uncertainty evaluation.

The effect of measurement geometry on patient specific QA pass/fail rates for stereotactic body radiation therapy (SBRT) Plans

Patient quality assurance (QA) is a required part of the treatment care path, and plan failure can lead to increased personnel hours or delay of treatment. The recommendation by the American Association of Physicists in Medicine is that gamma analysis be used to evaluate measured volumetric modulated arc therapy plans. Vendors have developed many different measurement geometries for patient QA devices which could yield varying pass rates when used with the recommended tolerances, normalization, and criterion. For this study, clinically treated stereotactic body radiation therapy plans were used to evaluate differences in gamma dose tolerances and sampled dose distribution complexity for centralized or peripheral measurement geometries on a cylindrical phantom. Random errors were then introduced into a subset of these plans, and the differences in pass rates between the geometries were correlated with differences in the observed mathematical differences. Finally, a single clinically relevant target coverage deviation was introduced to another subset of plans to evaluate whether a particular geometry is measurably better at identifying clinically relevant errors. It was found that centralized geometries resulted in more lenient dose tolerances and less complex sampled dose distributions compared to peripheral geometries. Pass rates were uniformly lower in the peripheral measurement geometry, and the difference in pass rates between the geometries correlated strongly with the difference in dose tolerance and weakly with the difference in the chosen complexity metrics. However, neither of the geometries were sufficiently sensitive enough to detect clinically relevant changes to target coverage when using recommended tolerances and criteria, and no statistically significant difference was found between their pass rates. Given these findings, the authors concluded that stereotactic body radiation therapy plans could fail patient QA when measured in the peripheral geometry but pass in the centralized geometry, with possibly neither having correlation to true clinical deviation.

Validation of a secondary dose check tool against Monte Carlo and analytical clinical dose calculation algorithms in VMAT

PURPOSE

Patient-specific quality assurance (QA) is very important in radiotherapy, especially for patients with highly conformed treatment plans like VMAT plans. Traditional QA protocols for these plans are time-consuming reducing considerably the time available for patient treatments. In this work, a new MC-based secondary dose check software (SciMoCa) is evaluated and benchmarked against well-established TPS (Monaco and Pinnacle3 ) by means of treatment plans and dose measurements.

METHODS

Fifty VMAT plans have been computed using same calculation parameters with SciMoCa and the two primary TPSs. Plans were validated with measurements performed with a 3D diode detector (ArcCHECK) by translating patient plans to phantom geometry. Calculation accuracy was assessed by measuring point dose differences and gamma passing rates (GPR) from a 3D gamma analysis with 3%-2 mm criteria. Comparison between SciMoCa and primary TPS calculations was made using the same estimators and using both patient and phantom geometry plans.

RESULTS

TPS and SciMoCa calculations were found to be in very good agreement with validation measurements with average point dose differences of 0.7 ± 1.7% and -0.2 ± 1.6% for SciMoCa and two TPSs, respectively. Comparison between SciMoCa calculations and the two primary TPS plans did not show any statistically significant difference with average point dose differences compatible with zero within error for both patient and phantom geometry plans and GPR (98.0 ± 3.0% and 99.0 ± 3.0% respectively) well in excess of the typical 95 % clinical tolerance threshold.

CONCLUSION

This work presents results obtained with a significantly larger sample than other similar analyses and, to the authors' knowledge, compares SciMoCa with a MC-based TPS for the first time. Results show that a MC-based secondary patient-specific QA is a clinically viable, reliable, and promising technique, that potentially allows significant time saving that can be used for patient treatment and a per-plan basis QA that effectively complements traditional commissioning and calibration protocols.

Exploring the gamma surface: A new method for visualising modulated radiotherapy quality assurance results

PURPOSE

This work presents a novel method of visualising the results of patient-specific quality assurance (QA) for modulated radiotherapy treatment plans, using a three-dimensional distribution of gamma pass rates, referred to as the "gamma surface". The method was developed to aid in comparing borderline and failing QA plans, and to better compare patient-specific QA results between departments.

METHODS

Gamma surface plots were created for a representative sample of situations encountered during patient-specific QA. To produce a gamma surface plot, for each QA result, gamma pass rates were plotted as a heat map, with dose difference on one axis and distance-to-agreement on the other. This involved the calculation of 100 × 100 gamma pass rates over a dose difference and distance-to-agreement grid. As examples, five 220 × 680 arrays of dose points from radiotherapy treatment plans were compared against measurement data consisting of 21 × 66 arrays of dose points spaced 10 mm apart.

RESULTS

The gamma surface plots facilitated the rapid evaluation of criteria combinations for each plan, clearly highlighting the difference between plans that are modelled and delivered well, and those that are not. Large scale features were also evident in each surface, hinting at potential over-modulation, systematic dose errors, and small or large scale areas of disagreement in the distributions.

CONCLUSIONS

Gamma surface plots are a useful tool for investigating QA failures and borderline results, and have the capacity to grant insights into treatment plan QA performance that may otherwise be missed.

Application of TG-218 action limits to SRS and SBRT pre-treatment patient specific QA

AAPM TG-218 provides recommendations for standard IMRT pre-treatment QA without giving specifics for stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). In light of this, our purpose is to report our experience with applying TG-218 recommendations to a large multicenter clinical SRS and SBRT program for a range of diverse clinical pre-treatment QA systems. Pre-treatment QA systems included Delta4 (Scandidos), Portal Dosimetry (Varian Medical Systems), ArcCHECK (SunNuclear), and SRS MapCHECK (SunNuclear). Plans were stratified by technique for each QA system, and included intracranial and extracranial IMRT and VMAT (total QA cases n=275). Gamma analysis was re-analyzed with spatial/dose criteria combinations ranging from 1 to 3 mm and 1% to 4%, and action and tolerance limits were calculated per plan type and compared to the "universal" TG-218 action limit of 90%. The analysis indicated that spatial tolerance criteria could be tightened to 1 mm while still maintaining an in-control QA process for all QA systems evaluated.