Increasing demands on commissioning and quality assurance for cone-based multiple brain metastases robotic radiosurgery: evaluation in a custom phantom.

CyberKnife stereotactic radiosurgery (SRS) treatment of multiple brain metastases (MBM) using fixed cone collimation is well established. This study aims to rigorously evaluate the commissioning of MBM SRS using feasible complex clinical scenarios. Sixteen gross tumour volumes (GTVs) were contoured on the CT of an in-house developed phantom. Six treatment plans, testing different levels of complexity, were generated using the Ray Tracing (RT) dose calculation algorithm and recalculated with the Monte Carlo (MC) algorithm. Respective quality assurance (QA) plans were generated on the same phantom and delivered on a CyberKnife M6 system. QA was performed with EBT4 film inserted in the GTVs under investigation. The films were scanned and analysed using in-house developed software for comparison with RT and MC calculated dose distributions in the film planes. For simple cases, RT and MC performed equally well. As the number and proximity of GTVs increased, with complex GTV geometrical distribution and multiple prescription dose levels (MPDLs), the plan gamma pass rate at 3%/1 mm was 36.2% for RT compared to 96.4% for MC. Corresponding RT and MC calculated dose planes were also compared for the failing cases, and similar differences were observed. In conclusion, commissioning of the algorithm with simple and few brain metastases should not be assumed to hold for MBM SRS. The RT algorithm was found to be unsuitable for complex MBM SRS. When commissioning SRS techniques for treating multiple GTVs, the increased demands of proximity and geometrical distribution of GTVs, and the use of MPDLs, should be carefully considered.