Nimg-02. adverse radiation effect versus tumor progression following stereotactic radiosurgery for brain metastases: implications of radiographic uncertainty

Despite efforts from RANO to standardize treatment response reporting, adverse radiation effect (ARE) following stereotactic radiosurgery (SRS) for brain metastases (BM) remains challenging to distinguish from tumor progression (TP) due to radiographic uncertainty (RU), often necessitating serial imaging until definitive diagnosis (DD) is resolved. We characterized management and implications of RU after BM irradiation for multiple primary tumor histologies. We identified 138 BM cases reviewed at our multi-disciplinary SRS Tumor Board (TB) for RU of ARE versus TP on MRI following SRS between 2015-2022. Target diameter (mm), histology, location, and SRS dose (Gy) on initial treatment and lesion diameter, associated symptoms, and supportive medications at the time of TB presentation were extracted from the medical record. Final lesion etiology, use of any intervention, and neurologic deficits upon DD were documented. Time from RU to DD was calculated. Median time from SRS to initial RU was 11.3 months (IQR 5.5-18.2). Median RU lesion diameter was 14.0 (IQR 8.0-20.0). At the time of RU, 43% of patients were symptomatic, 28% and 4% requiring steroids and bevacizumab, respectively. Time to DD following RU was > 6 and 12 months in 28% and 8% of patients, respectively (maximum 47 months). Median lesion diameter at DD was 16.0 mm (IQR 10.3-24.0). Most (54%) had a DD of ARE, while 41% had TP; 23% and 77% were diagnosed pathologically and radiographically, respectively. Upon DD, 41% underwent local intervention with surgery and/or re-irradiation and 25% had a new neurologic deficit, including weakness (7%), ataxia (5%), and aphasia (4%). In an interdisciplinary setting, it took > 6 months to reach a DD after RU in > 25% of patients previously irradiated for BM. Diagnostic uncertainty and delay may lead to sub-optimal outcomes and symptom burden. Improved characterization of post-SRS imaging findings is needed in this population.