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RADT-01. THE EFFICACY AND SAFETY OF TWO-STAGED STEREOTACTIC RADIOSURGERY FOR LARGE POSTERIOR FOSSA METASTASES: POST-TREATMENT VOLUMETRIC CHANGES IN TUMOR SIZE, PERI-TUMORAL EDEMA, AND FOURTH VENTRICLE


 
 
 Two-staged stereotactic radiosurgery (2-SSRS) delivers a high radiation dose to large (≥2 cm) brain metastases to achieve improved tumor control. However, treating large posterior fossa metastases (LPFM) with SRS is challenging because of the potential risk of increased peritumoral edema (PTE) and compression to brain stem and/or obstructive hydrocephalus of the fourth ventricle (4V).
 
 
 
 21 patients (22 LPFM) treated with 2-SSRS were retrospectively reviewed. Volumetric analysis for tumor, PTE, and 4V volume was assessed for objective response. We defined local failure (LF) as ≧ 71.5% of volumetric increase, equivalent to ≧ 20% increase in maximum diameter (RECIST criteria). Kaplan-Meier estimated overall survival (OS), time-to-local failure, and linear mixed model estimated and compared measurements between time points.
 
 
 
 The median age was 61 years (range: 39–79 years), and nine patients (42.9%) were female. The most common primary cancer was NSCLC (47.6%), followed by breast (14.3%) and renal (14.3). Median OS was 13 months (95% CI: 6–24 months) with median follow-up 18 months (range 7–42 months). The median tumor, PTE and 4V volumes at diagnosis were 7.43 cm3, 18.9 cm3, and 0.95 cm3 respectively. At the first follow-up (median 51 days after the second session), mean tumor (–59%, p< 0.0001) and PTE (–47%, p=0.0008) volumes decreased, while mean 4V volume (+35%, p=0.001) increased. Significant volumetric changes were maintained at second (median 179 days after the second session) follow-up for tumor (–43%, p=0.01) and 4V (+44%, p=0.002), but not for PTE (+8%, p=0.87). Tumor LC rate was 83% (95% CI: 64–100%) at 1 year, and no patient developed the post-treatment hydrocephalus requiring CSF diversion or required surgical resection for tumor progression.
 
 
 
 This is the first report documenting detailed time courses and excellent safety/ efficacy of 2-SSRS for LPFM. Additional studies with longer follow-ups are needed.
