Detection of rotational errors in single-isocenter multiple-target radiosurgery: Is a routine off-axis Winston-Lutz test necessary?

PURPOSE: Recently the use of linear accelerator (linac)-based stereotactic radiosurgery (SRS) has increased, including single-isocenter multiple-target SRS. The workload of medical physicists has grown as a result and so has the necessity of maximizing the efficiency of quality assurance (QA). This study aimed to determine if measurement-based patient-specific QA with a high-spatial-resolution dosimeter is sensitive to rotational errors, potentially reducing the need for routine off-axis Winston-Lutz (WL) testing. METHODS: The impact of rotational errors along gantry, couch, and collimator axes on dose coverage of the gross tumor volume (GTV) and planning target volume (PTV) was determined with a 1-mm GTV/PTV expansion margin. Two techniques, the off-axis WL test using the StereoPHAN MultiMet-WL Cube (Sun Nuclear Corporation, Melbourne, Florida, USA) and patient-specific QA using the SRS MapCHECK (Sun Nuclear Corporation, Melbourne, Florida, USA), were assessed on their ability to detect introduced errors before target coverage was compromised. These findings were also considered in the context of routine machine QA of rotational axis calibrations. RESULTS: Rotational errors significantly impacted PTV dose coverage, especially in the couch angle. GTV dose coverage remained unaffected except for with large couch angle errors (≥1.5°). The off-axis WL test was shown to be sensitive to rotational errors with results consistently exceeding tolerance levels when or before coverage fell below departmentally accepted limits. Although patient-specific QA using the SRS MapCHECK was previously validated for SRS, this study showed inconsistency in detection of rotational errors. CONCLUSIONS: It is recommended that off-axis WL testing be conducted regularly to supplement routine monthly machine QA, as it is sensitive to errors that patient-specific QA may not detect. This frequency should be determined by individual departments, with consideration of GTV-PTV margins used, limitations on target off-axis distances, and routine mechanical QA results for particular linacs.

The use of collimator angle optimization and jaw tracking for VMAT-based single-isocenter multiple-target stereotactic radiosurgery for up to six targets in the Varian Eclipse treatment planning system.

PURPOSE: Island blocking occurs in single-isocenter multiple-target (SIMT) stereotactic radiotherapy (SRS) whenever targets share multi-leaf collimator (MLC) leaf pairs. This study investigated the effect on plan quality and delivery, of reducing island blocking through collimator angle optimization (CAO). In addition, the effect of jaw tracking in this context was also investigated. METHODS: For CAO, an algorithm was created that selects the collimator angle resulting in the lowest level of island blocking, for each beam in any given plan. Then, four volume-modulated arc therapy (VMAT) SIMT SRS plans each were generated for 10 retrospective patients: two CAO plans, with and without jaw tracking, and two plans with manually selected collimator angles, with and without jaw tracking. Plans were then assessed and compared using typical quality assurance procedures. RESULTS: There were no substantial differences between plans with and without CAO. Jaw tracking produced statistically significant reduction in low-dose level parameters; healthy brain V10% and mean dose were reduced by 9.66% and 15.58%, respectively. However, quantitative values (108 cc for V10% and 0.35 Gy for mean dose) were relatively small in relation to clinical relevance. Though there were no statistically significant changes in plan deliverability, there was a notable trend of plans with jaw tracking having lower gamma analysis pass rates. CONCLUSION: These findings suggest that CAO has limited benefit in VMAT SIMT SRS of 2-6 targets when using a low-dose penalty to the healthy brain during plan optimization in Eclipse. As clinical benefits of jaw tracking were found to be minimal and plan deliverability was potentially reduced, a cautious approach would be to exclude jaw tracking in SIMT SRS plans.