Radiotherapy quality assurance of SBRT for patients with centrally located lung tumours within the multicentre phase II EORTC Lungtech trial: Benchmark case results.

PURPOSE: To report on the benchmark case (BC) study performed in the context of the European Organisation for Research and Treatment of Cancer prospective multicentre Lungtech trial of SBRT for patients with inoperable centrally located lung tumours. METHODS AND MATERIALS: Target volume and organs at risk (OARs) delineations first needed to be acceptable before the treatment plan was reviewed. Retrospectively, Dice similarity coefficients of the OARs and the target volumes were calculated and a set of gold standard contours adapted for each institution margins was applied on the accepted dose submissions to evaluate the influence of acceptable delineation variations on dosimetry. RESULTS: Twenty-five institutions participated. Five BCs were accepted at the first attempt. Twenty institutions had to revise their delineation at least once and seven had to revise their planning once. The V60 Gy dose coverage improved significantly (p = 0.05) between the first and final submissions from median (range) 94.8% (22.5-97.8) to 95.3% (70.5-99.3). The median Dice coefficient varied significantly between OARs: The lowest values were found for the brachial plexus 0.25 (0.01-0.54) and the highest for the spinal cord 0.89 (0.71-0.95). The mean PTV Dice coefficient was 0.82 (0.48-0.92). Applying the gold standard contours, only one institution remained compliant with the dose coverage criteria with V60 Gy median (range) of 83.4% (54.2-93.9). CONCLUSIONS: Clinical guidelines and radiotherapy protocols are not a substitute for timely radiotherapy quality assurance procedures, which improve dose coverage significantly. Delineation remains the main source of BC rejection and plan review without first reviewing delineation may not be efficient. Our results show that delineation variations seem to have a larger influence on PTV coverage than variations in planning and irradiation techniques and thus suggest that dose tolerance criteria should preferably take into account the accuracy of delineation.

Results of a multicentre dosimetry audit using a respiratory phantom within the EORTC LungTech trial.

INTRODUCTION: The EORTC 22113-08113 LungTech trial assesses the safety and efficacy of SBRT for centrally located NSCLC. To insure protocol compliance an extensive RTQA procedure was implemented. METHODS: Twelve centres were audited using a CIRS008A phantom. The phantom was scanned using target inserts of 7.5 mm and 12.5 mm radius in static condition. For the 7.5 mm insert a 4DCT was acquired while moving according to a cos6 function. Treatment plans were measured using film and an ionization chamber. Wilcoxon's signed-rank tests were performed to compare the three plans across institutions. A Spearman correlation was calculated to evaluate the influence of factors such as PTV, slice thickness and total number of monitor units on the dosimetric results. RESULTS: The reference output dose median [min, max] variation was 0.5% [-1.1, +1.5]. The median deviations between chamber doses and point-planned doses were 1.8% [-0.1; 6.7] for the 7.5 mm and 1.1% [-2.8; 5.0] for the 12.5 mm sphere in static situation and 3.2% [-3.2; 15.7] for the dynamic situation. Film gamma median pass rates were 92.0% [68.0, 99.0] for 7.5 mm static, 96.2% [73.0, 99.0] for 12.5 mm static and 71.0% [40.0, 99.0] for 7.5 mm dynamic. Wilcoxon's signed-rank tests showed that the dynamic irradiations resulted in significantly lower gamma pass rates compared to the 12.5 mm static plan (p = 0.001). The total number of MUs per plan was correlated to both film and IC results. CONCLUSION: An end-to-end audit was successfully performed, revealing important variations between institutions especially in dynamic irradiations. This shows the importance of dosimetry audits and the potentials for further technique and methodology improvements.

Quality assurance of four-dimensional computed tomography in a multicentre trial of stereotactic body radiotherapy of centrally located lung tumours.

BACKGROUND AND PURPOSE: Extensive radiation therapy quality assurance (RTQA) programs are needed when advanced radiotherapy treatments are used. As part of the RTQA four dimensional computed tomography (4DCT) imaging performance needs to be assessed. Here we present the RTQA data related to 4DCT procedures used within the context of stereotactic body radiotherapy (SBRT) of centrally located lung tumours. It provides an overview of the 4DCT acquisition methods and achievable accuracy of imaging lung tumour volumes. MATERIALS AND METHODS: 3DCT and 4DCT images were acquired from a CIRS phantom with spheres of 7.5 and 12.5 mm radius using the institutional scan protocols. Regular asymmetric tumour motion was simulated with varying amplitudes and periods. Target volumes were reconstructed using auto-contouring with scanner specific thresholds. Volume and amplitudes deviations were assessed. RESULTS: Although acquisition parameters were rather homogeneous over the eleven institutions analysed, volume deviations were observed. Average volume deviations for the 12.5 mm sphere were 15% (-4% to 69%) at end of inspiration, 2% (-2% to 9.0%) at end of expiration and 12% (0% to 36%) at mid-ventilation. For the 7.5 mm sphere deviations were 13% (-99% to 65%), 16% (-34% to 66%) and 1% (-13% to 20%), respectively. The amplitude deviation was generally within 2 mm although underestimations up to 6 mm were observed. CONCLUSIONS: The expiration phase was the most accurate phase to define the tumour volume and should be preferred for GTV delineation of tumours exhibiting large motion causing motion artefacts when using mid-ventilation or tracking techniques. The large variation found among the institutions indicated that further improvements in 4DCT imaging were possible. Recommendations for 4DCT QA have been formulated.

Lungtech, a phase II EORTC trial of SBRT for centrally located lung tumours - a clinical physics perspective.

BACKGROUND: The EORTC has launched a phase II trial to assess safety and efficacy of SBRT for centrally located NSCLC: The EORTC 22113-08113-Lungtech trial. Due to neighbouring critical structures, these tumours remain challenging to treat. To guarantee accordance to protocol and treatment safety, an RTQA procedure has been implemented within the frame of the EORTC RTQA levels. These levels are here expanded to include innovative tools beyond protocol compliance verification: the actual dose delivered to each patient will be estimated and linked to trial outcomes to enable better understanding of dose related response and toxicity. METHOD: For trial participation, institutions must provide a completed facility questionnaire and beam output audit results. To insure ability to comply with protocol specifications a benchmark case is sent to all centres. After approval, institutions are allowed to recruit patients. Nonetheless, each treatment plan will be prospectively reviewed insuring trial compliance consistency over time. As new features, patient's CBCT images and applied positioning corrections will be saved for dose recalculation on patient's daily geometry. To assess RTQA along the treatment chain, institutions will be visited once during the time of the trial. Over the course of this visit, end-to-end tests will be performed using the 008ACIRS-breathing platform with two separate bodies. The first body carries EBT3 films and an ionization chamber. The other body newly developed for PET- CT evaluation is fillable with a solution of high activity. 3D or 4D PET-CT and 4D-CT scanning techniques will be evaluated to assess the impact of motion artefacts on target volume accuracy. Finally, a dosimetric evaluation in static and dynamic conditions will be performed. DISCUSSION: Previous data on mediastinal toxicity are scarce and source of cautiousness for setting-up SBRT treatments for centrally located NSCLC. Thanks to the combination of documented patient related outcomes and CBCT based dose recalculation we expect to provide improved models for dose response and dose related toxicity. CONCLUSION: We have developed a comprehensive RTQA model for trials involving modern radiotherapy. These procedures could also serve as examples of extended RTQA for future radiotherapy trials involving quantitative use of PET and tumour motion.