Irradiated tumor volume as a predictor of local recurrence and radionecrosis in lung cancer with brain metastases treated with stereotactic radiosurgery

Background Stereotactic radiosurgery (SRS) is a standard treatment of brain metastases (BM), but it may lead to radionecrosis (RN) or local recurrence (LR). This study evaluates irradiated tumor volume as a predictor of LR and RN in SRS-treated lung cancer patients with BM. Methods We retrospectively analyzed 431 lung cancer patients with BM who underwent SRS at Karolinska University Hospital (all-comers, 2009-2020). Associations among irradiated tumor volume and risks of RN, symptomatic RN, and LR at 6 and 12 months were assessed using Cox regression models. Furthermore, we evaluated the diagnostic performance of methionine positron emission tomography–computed tomography (PET-CT) in differentiating RN from LR. Results Forty patients (9.3%) developed asymptomatic RN, 36 (8.4%) symptomatic RN, and 67 (15.5%) LR. Larger tumor volumes significantly increased RN and LR risks. At 6 months, a volume of 4.75 cm 3 corresponded to an RN risk reaching up to 20%. By 12 months, substantially smaller volumes were correlated with the same risk levels. Symptomatic RN followed a similar trend, at 6 months and 12 months. A 20% risk of LR was observed with volumes of 5.66 cm 3 and 3.28 cm 3 at 6 months and 12 months, respectively. Methionine PET-CT had a sensitivity of 0.909 and specificity of 0.600 in detecting RN or LR, with no significant diagnostic advantage compared with magnetic resonance imaging (MRI). Conclusion Larger irradiated tumor volumes were correlated with increased RN or LR risk. Smaller volumes can lead to RN and LR, not evident at 6 months, but emerging by 12 months after SRS. Methionine PET-CT showed high sensitivity but modest specificity and offered no significant benefit over MRI in differentiating RN from LR.