Changes of healthy brain tissue after salvage radiotherapy of glioblastoma

BACKGROUND: Salvage radiotherapy (SRT) with photons is a valid treatment option for patients suffering from recurrent glioblastoma (GBM). However, the tolerance of healthy brain to ionizing radiation (IR) is limited. The aim of this study was to determine to what extent brain structures in the radiographically tumor-free hemisphere change after repeated radiotherapy. METHODS: Five of 26 patients treated with SRT for local recurrence of GBM were found to have magnetic resonance imaging (MRI) studies available for complete volumetric analysis before and after primary chemo-radiation and after SRT. Manual segmentation and joint segmentation (JS) based on a convolutional neural network were used for the segmentation of the gray matter, the white matter and the ventricles in T1 MRIs. RESULTS: Qualitative results of manual segmentation and JS were comparable. After primary chemo-radiation and SRT, the volume of the contralateral ventricles increased steadily by 1.3–4.75% (SD ± 2.8 %, R(2) = 0.82; P = <.01) with a manual segmentation and by 1.4–7.4% (SD 2.1%, R(2) = 0.48; P = .025) with JS. The volume of the cortex decreased by 3.4–7.3% except in one patient, the cortex volume increased by 2.5% (SD ± 2.9%, R(2) = 0.18; P = .19) when measured manually. When measured with JS GM decreased by 1.0–7.4%, in one case it increased by 3.0% (SD = 3.2%, P = .22, R(2) = 0.18). The white matter remained stable when assessed with manual segmentation (P = .84, R(2) = 0.004) or JS (P = .44, R(2) = 0.07). CONCLUSION: SRT of relapsed GBM leads to continuous changes of the tumor-free contralateral brain by means of manual segmentation or JS. The cortex seems more susceptible to repeated RT compared to the white matter. Larger cohort studies and complementary functional analysis are encouraged.