Distinguishing pseudoprogression from true tumor growth after stereotactic surgery in vestibular schwannoma: a volumetric and clinical trajectory analysis.

introductionVestibular schwannoma (VS) often shows transient post-radiosurgery swelling ("pseudoprogression," PP) that mimics true progression (TP). We validated volumetric rules to separate PP from TP and linked growth patterns to clinical outcome.

methodWe retrospectively reviewed 259 sporadic VS cases treated with single-fraction stereotactic radiosurgery (2012-2024). SRS was delivered with a median margin dose of 12 Gy, prescribed to the institutional peripheral isodose line, using a median of 5 isocentres. Tumours were manually segmented on serial MRIs ≥ 24 months. PP was defined as > 20% volume rise ≤ 12 months followed by ≥ 10% fall ≤ 24 months; TP as > 15% rise > 36 months with persistent growth. Two alternative thresholds (> 25% rise any time; >10% rise > 24 months without regression) were tested. Longitudinal trajectories were clustered with Gaussian-mixture models. Outcomes included hearing, facial and trigeminal function, and Dizziness Handicap Index (DHI).

resultsFour growth clusters emerged: early PP (35.5%), late PP (11.2%), stable (41.3%) and TP (12%). The PP rule yielded 86% sensitivity and 93% specificity (AUC 0.92); the TP rule 77%/95% (AUC 0.90). Alternative thresholds performed worse (AUC 0.81-0.85). Serviceable hearing was preserved in ≥ 86% of PP or stable tumours but only 61% in TP (p < 0.01).

conclusionTime-anchored volumetric rules accurately distinguish transient post-SRS swelling from genuine progression. Incorporating these criteria into routine surveillance can prevent premature salvage therapy while ensuring timely intervention for the minority of tumours that truly grow.