Room: AAPM ePoster Library
Purpose: HyperArc (HA) VMAT treatment to multiple brain metastases (m-bm) is an efficient and tolerable stereotactic radiosurgery (SRS) option. However, current clinical practice of HA-SRS does not account for patient setup uncertainty–degrading treatment accuracy. This study quantifies loss of target coverage and dose to organs-at-risk (OAR) due to isocenter misalignment and presents two correction strategies that minimize setup uncertainty.
Methods: Nine patients (2-16 tumor/patient, total 61 tumors) who underwent gamma-knife radiosurgery were replanned in Eclipse using 10MV-FFF beam (2400 MU/min) and a single-isocenter (placed at geometric center of all tumors) VMAT plan mimicking HA-style treatment geometry. 20 Gy to each tumor was prescribed. Average GTV and PTV were 1.1 cc (range:0.02-11.5cc) and 1.9 cc (range:0.11-18.8cc). Isocenter to tumor distance was 5.50 cm, on average (range:1.6-10.1 cm). Six degrees-of-freedom patient setup uncertainty was simulated [±2mm and ±2°] using an in-house script, including randomly generated setup errors. Loss of target coverage, collateral damages to OAR and treatment delivery efficiency were evaluated. Two clinically promising correction strategies: 1) risk-adopted prescriptions and 2) dual-isocenter HA treatment were introduced.
Results: Utilizing 10MV-FFF beam, HA-SRS for m-bm can be delivered within 15 min (<5 min beam-on-time) treatment slot. However, average loss of target coverage for systematically induced errors and random uncertainties were greater than 34.2% (up to 99%) and 22.5% (up to 97%) for GTV and 44.7% (up to 96%) and 38.7% (up to 92%) for PTV. Due to isocenter-misalignment hippocampus, brainstem and optic apparatus received 1.3Gy, 1.0Gy and 0.5Gy higher doses than original plan. With the corrections strategies, potentially improved target localization accuracy and reduced OAR doses including hippocampi by 2Gy.
Conclusion: Utilizing either of these corrections strategies, HR-SRS can become an efficient and more accurate treatment option for m-bm patients–improving patient compliance and sparing OAR. Further clinical validation of these correction strategies is underway.