Gross tumor volume increase and need for adaptive radiotherapy in pediatric-type diffuse high-grade glioma of the midline structures

Introduction: Current pediatric-type diffuse high-grade glioma radiotherapy protocols apply a 1.0 cm clinical target volume (CTV) margin around the gross tumor volume (GTV). However, in adults with glioblastoma, large variations in GTV are observed during radiotherapy. The study aimed to map the GTV variation during a 6-week course of radiotherapy using repeated MR-imaging and to evaluate the need for plan adaptation. Also, the relation between GTV increase and time to disease progression (TTP) was assessed. Material and methods: Patients with newly diagnosed diffuse midline glioma or diffuse pediatric-type high-grade glioma of the midline structures undergoing a 6-week radiotherapy course, were eligible for inclusion. MRI scans were performed in the pre-treatment phase (MRI0), and at fraction 10 + 20 (rMRI10/rMRI20). On all scans, GTV was delineated. An increase was defined as a >5 % increase of GTV between scans. The need for treatment plan adaptation was based on dosimetric and visual criteria. GTV increase was compared to TTP. Results: Twenty patients were eligible. In 12/20 patients, a GTV increase was observed at rMR10/rMR20, more specifically in 6/11 pontine and 6/9 non-pontine tumors. Combining dosimetric criteria and visual inspection, 20 plan adaptations in 14 patients were required. The TTP (range: 1.6–17.6 months) was not significantly different between the group with (median 8.1 months) versus without a GTV increase (median 7.6 months; p = 0.66). Conclusion: Repeated imaging demonstrated a GTV increase in 60 % of patients and plan adaptation in 70 %. When applying CTV margins of 1.0 cm, plan adaptation is recommended to ensure adequate radiotherapy treatment.