ESTRO 38 Abstract book

S82 ESTRO 38

shows the change in cortical thickness per dose in the vertex with the greatest association between these two variables.

Conclusion For melanoma patients with brain metastases treated with concurrent targeted- or immunotherapy and SRT without concurrent WBRT, brain metastases volume impacts OS, but not the number of treated brain metastases. We propose adapting the molGPA score for melanoma patients treated with SRT and concurrent targeted- or immunotherapy. OC-0167 Identifying No Fly Zones to prevent long- term thinning of the cerebral cortex in glioma after RT S. Nagtegaal 1 , S. David 2 , H. Mesri 2 , M. Philippens 1 , A. Leemans 2 , J. Verhoeff 1 1 UMC Utrecht, Radiation Oncology, Utrecht, The Netherlands ; 2 UMC Utrecht, Imaging Science Institute, Utrecht, The Netherlands Purpose or Objective Radiation induced brain injury consists of both anatomical and functional deficits in irradiated healthy brain tissue. Especially cognitive and executional impairments lead to a marked decrease in the patient’s quality of life after radiation therapy (RT), and have been linked to thinning of the healthy cerebral cortex. With the rise of more precise radiation techniques, like image-guided RT and proton RT, knowledge of the long-term effects of radiation on the cortex is important for optimal treatment planning. In this work, we present the results of structural morphometry of the cerebral cortex at different time points in glioma patients treated with RT, and compare the changes to the applied dose. Material and Methods We selected 31 patients with adequate follow-up scans who were treated with RT for glioma (grade II-IV) in our institution. Late follow-up was defined as >9 months after start of RT, and the cortical thickness was compared to baseline. The CAT12 (Computational Anatomy Toolbox) was used for the automated preprocessing, segmentation and surface estimation of clinical 3T T1 scans. All scans were bias-field inhomogeneity and noise corrected, then segmented into grey matter, white matter and cerebrospinal fluid. Surface calculation was achieved via projection-based thickness estimation and reparametrized to the Freesurfer surface template, consisting of a 152k mesh per hemisphere. RT dose was 3D mapped to the surface. Change in cortical thickness was associated with locally applied dose via non-parametric permutations tests. Age and sex were included as covariates of no-interest, and correction for multiple comparisons with family wise error-rate adjustment was applied. The significance of a test was determined at p < 0.05. For each test the Cohen's d was also calculated. Results Figure 1 shows the regions where a statistically significant dose-dependent decrease in cortical thickness was found. Data on the two regions are shown in Table 1. Figure 2

Conclusion This study shows that the cerebral cortex is susceptible to long-term radiation-induced cortical changes, and that the rate of change increases with the applied dose. This effect can be considered rapid aging of the healthy brain, which may accompany similar age-related cognitive impairments. Despite this dose-dependent cortical thinning, changes in the cortex can already be seen at lower doses, suggesting