2221 - Rapid Delivery of HyperArc Stereotactic Radiosurgery/Radiotherapy is Safe and Effective Treatment Option for Patients with Single- and Multiple Brain Metastatic Lesions

Purpose/Objective(s): To examine the safety and treatment effectiveness of single-isocenter/single-lesion (SISL) and single-isocenter/multi-lesion (SIML) brain stereotactic radiosurgery/radiotherapy (SRS/SRT) via fully automated Linac-based HyperArc delivery.

Materials/Methods: In this IRB approved study, 112 metastatic brain cancers patients (1–11 lesions; total 231 treated lesions) were simulated via Encompass support system. Patients were treated with highly conformal SISL or SIML brain SRS/SRT plans via fully automated non-coplanar HyperArc module, AcurosXB engine and 6MV FFF beam. Each planning target volume (PTV) was 1-2 mm margin around gross tumor volume (GTV) delineated via contrast enhanced high resolution MPRAGE MRI scan co-registered to planning CT. Mean GTV and PTV volumes were 11.4 (0.25–64.6) cc and 18.23 (0.95–84.6) cc. Common prescriptions were 18–24 Gy, 24–27 Gy and 30 Gy in 1, 3 and 5 fractions prescribed to 70-80% isodose line. For SIML plans, mean tumor distance to isocenter was 5.6 cm, maximum up to 7 cm. Target conformity and organs at risk (OAR) dose limits including normal brain doses were evaluated. Treatment was delivered every other day with CBCT guidance and 6dof couch shifts on TrueBeam equipped with 120Millinium MLCs in a 15-minute slot. Dosimetric factors, tumor local control (TLC) rates and toxicity data based on CTCAE v5 criteria for brain radionecrosis, optic neuropathy, and brainstem dysfunction were evaluated.

Results: All brain SRS/SRT plans met Alliance A071801 brain trial’s criteria for each tumor dose, target conformity (1–1.2), steep dose gradient (<4) and dose to OAR including optic apparatus and brainstem. Normal brain volume receiving V_12Gy, V_19.6Gy, and V_24.4Gy for 1, 3 and 5 fractions were 4.6 ± 1.9 cc, 7.3 ± 4.4 cc and 8.0 ± 7.3 cc respectively; compliance with HyTEC recommendations. For 2%/2mm gamma passing criteria, average EPID-based patient specific QA result was 97.2%. Independent inhouse Monte Carlo second check results were within ±3.5%. 80/112 patients (174 lesions) had posttreatment MRI scans to assess TLC and toxicity with a median follow up interval of 13.3 ± 9.5 (3–36.6) months. TLC was achieved in 160/174 (92%) of the followed-up lesions; 14 lesions in 12 patients had locoregional failure. CTCAE grade 2 brain radionecrosis was seen in 6 patients (larger tumors treated in 3 & 5 fractions) and were managed with dexamethasone. No patient reported optic neuropathy, brainstem dysfunction or grade 3+ events of brain radionecrosis after HyperArc SRS/SRT delivery.

Conclusion: HyperArc brain SRS/SRT for single and multiple brain tumors via SISL or SIML treatment gave an excellent TLC rate and low normal brain toxicity. SIML method reduced treatment time significantly as compared to traditional multi-isocenter brain SRS/SRT approach or sequential SRS courses, thus improving patient compliance, comfort, and clinic efficiency. Longer median follow up periods on larger patient cohort via HyperArc SRS/SRT with Kaplan-Meier curves will be presented.