RADT-21. VOLUMETRIC AND RADIOGRAPHIC CHANGES OF VESTIBULAR SCHWANNOMA TREATED WITH LINEAR ACCELERATOR-BASED RADIOSURGERY

Abstract BACKGROUND This retrospective study assessed volumetric and radiographic changes including tumor enlargement (TE), cystic formation (CF), and loss of central enhancement (LCE) after linear accelerator-based radiosurgery (RS) for vestibular schwannoma (VS). METHODS Patients with sporadic unilateral VS treated with single or multifraction RS in definitive or postoperative settings were included. Gross tumor volumes (GTV) were manually delineated on baseline and follow-up MRIs. Volumetric change was defined as a ≥20% volume difference, with an added 2 mm threshold for tumors <0.5 cm³. Patterns of change were categorized: (1) continuous regression; (2) stable; (3) TE followed by regression or stabilization; (4) continuous enlargement/progression. RESULTS Among 68 patients (76.4% postoperative RS with median dose of 13 Gy in 1 fraction), the median follow-up time was 63 months. Stable or regressive tumors were observed in 51 patients (75%), while 16 (23.5%) had TE and 1 (1.5%) showed progression, yielding a 5-year radiographic control rate of 97.2%. LCE and CF occurred in 8 (12%) and 4 (6%) patients. TE patients had median time to initial and peak enlargement of 11 (5–39) and 24 (5–68) months, with a median peak volume increase of 67% (20–256). Larger tumors (GTV >0.47 cm³) tended to show delayed (12 vs 7 months) and prolonged TE (36 vs 7 months), and higher risk of TE (OR 4.01, 95% CI, 0.93–17.28; p=0.063). An GTV >0.69 cm³ could predict TE with 81% sensitivity and 73% specificity. TE resolved spontaneously in 6 patients (37.5%) within 3 years. Two patients (12.5%) with volume increase ≥100% developed complications-hydrocephalus requiring shunting, and facial paresthesia with hearing loss requiring surgery. CONCLUSION Most VS tumors stabilize or regress after RS, but TE is common in larger tumors and may cause complications. Close monitoring in these patients may be beneficial.