Implications for preserving neural stem cells in whole brain radiotherapy and prophylactic cranial irradiation: a review of 2270 metastases in 488 patients

This study delineated the incidence of metastatic involvement of neural stem cell (NSC) regions and further aimed to explore the feasibility of selectively sparing the NSC compartments during whole brain radiotherapy (WBRT) and prophylactic cranial irradiation (PCI). A total of 2270 intracranial metastases in 488 patients were identified. Lesions were classified according to locations, including lesions in the NSC compartments (subventricular zone, SVZ, or hippocampus) and those in the rest of the brain/brainstem. The incidence of involvement of NSC regions was compared between oligometastatic patients (those with 1-4 lesions) and non-oligometastatic patients (those with 5 or more lesions) using a chi-square test. The volume of the NSC regions accounted for 2.23% of the whole brain, and the overall rate of metastatic lesions in NSC regions was 1.1% in 2270 metastases (25/2270), and 4.7% in 488 patients (23/488). Of the NSC region metastases, 7 (0.3%) involved the hippocampus and 18 (0.8%) occurred in the SVZ. Among the 7 hippocampal metastases identified in this study, 1/7 (14.3%) were found in oligometastatic patients, while 6/7 (85.7%) metastases were in non-oligometastatic patients. For metastases in the SVZ, all lesions occurred in non-oligometastatic patients with none in oligometastatic patients. Metastatic involvement of the NSC compartments was significantly lower in oligometastatic patients (0.15%, 1/670) than in non-oligometastatic patients (1.5%, 24/1600) (P < 0.001). Our retrospective review of 2270 metastases in 488 patients is that the volume of the compartments of NSC regions was 2.23% relative to the whole brain, but the incidence of involvement of the NSC compartments was 1.1%, and the vast majority of NSC lesions were found in non-oligometastatic patients. We believe our data supports selective reduction of doses for these aforementioned structures, when treating oligometastatic patients with WBRT and locally advanced-stage small-cell lung cancer patients with PCI.