RADT-13. A BIOLOGIC EQUIVALENT DOSE COMPARISON BETWEEN RADIOSURGERY AND COLLAGEN TILE BRACHYTHERAPY FOR POST-OPERATIVE TREATMENT OF A BRAIN METASTASIS RESECTION CAVITY

Abstract

Abstract INTRODUCTION Recurrence of a previously irradiated brain metastasis represents a challenging clinical scenario. Repeat radiosurgery results in sub-optimal local control (LC). Two independently published prospective cohort studies demonstrated durable LC in this setting with collagen tile brachytherapy (CTBT), a novel Cesium-131 carrier. Here we compare biologically equivalent dosimetric parameters of CTBT to radiosurgery to understand dose metrics that may underlie durable LC from CTBT. METHODS A patient was selected from the multi-institutional prospective CTBT registry (NCT04427384) with a brain metastasis resection cavity near brainstem. Planning was performed for radiosurgery (30Gy in 5 fractions, BED of 48Gy) and CTBT (Cs-131, activity of 3.5U). To directly compare dosimetric parameters between radiosurgery and CTBT, biologic equivalent doses (BED) were calculated for brainstem(α/β of 3, BED3) and oncologically at-risk tissue(α/β of 10, BED10). TG101 brainstem dose-constraints were referenced, and respective BED were calculated. RESULTS The resection cavity was 4mm away from brainstem and contained 7 CTBT tiles (28 Cs131 seeds). Gradient index (GI) (24Gy BED10 volume/48Gy BED10 volume) was 3.8 for radiosurgery and 2.2 for CTBT. Conformality index (CI) (48Gy BED10 volume/CTV volume) was 1.2 for radiosurgery and 2.6 for CTBT. BED10 to 90% of the cavity wall (2mm rim around the resection cavity) was 49.6Gy for radiosurgery and 92.6Gy for CTBT. BED3 dose limit goals (max dose 31Gy in 5 fractions, BED3 95.1Gy and threshold dose 23Gy in 5 fractions, BED3 of 58.3Gy to 0.5cc) were met for both radiosurgery and CTBT. CONCLUSION BED comparisons demonstrate that CTBT leads to higher CI, lower GI, and higher dose to the cavity wall than radiosurgery while meeting BED3 dose limit goals to an adjacent organ at risk. These dose characteristics might underlie durable LC seen in the recurrent brain metastasis setting, though larger scale analyses are being performed with with clinical outcomes on NCT04427384.