Sparing of the Neural Stem Cell Compartment During Whole-Brain Radiation Therapy: A Dosimetric Study Using Helical Tomotherapy

Purpose To assess the feasibility of dosimetrically sparing the hippocampus and neural stem cell (NSC) compartment during whole-brain radiotherapy (WBRT) and prophylactic cranial irradiation (PCI). Methods and Materials We contoured the brain/brainstem on fused magnetic resonance /computed tomography images as the planning target volume (PTV) in 10 patients, excluding the hippocampus and NSC compartment as organs at risk. PCI and WBRT helical tomotherapy plans were prepared for each patient, with 1.0-cm field width, a pitch of 0.285, and a modulation factor of 2.5. We attempted to maximally spare the hippocampus and NSC compartment while treating the rest of the brain to 30 Gy in 15 fractions (PCI) or 35 Gy in 14 fractions (WBRT) with a V100 of ≥95%. Plan quality was assessed by calculating mean dose, equivalent uniform dose (EUD), and biologically equivalent dose (BED) for organs at risk and the percent volume of the PTV receiving the prescribed dose of V100. Results In the PCI plans, mean doses/EUD/BED for the hippocampus and NSC compartment were 11.5 Gy/13.1 Gy/15.7 Gy2 (BED assuming alpha/beta ratio of 2Gy) and 11.5 Gy/13.1 Gy/12.3 Gy10 (BED assuming alpha/beta ratio of 10Gy), respectively. In the WBRT plans, mean doses/EUD/BED for the hippocampus and NSC compartment were 11.8 Gy/14.8 Gy/16.8 Gy2 and 11.8 Gy/14.8 Gy/12.8 Gy10 , respectively. The mean V95 for the rest of the brain (PTV) was 96.9% for both the PCI and WBRT plans. Mean PCI and WBRT treatment times were 15.93 min (range, 14.28 min–17.50 min) and 20.18 min (range, 18.43 min–22.32 min), respectively. Conclusions It is dosimetrically feasible to spare the hippocampus and NSC compartment using helical tomotherapy during the administration of whole-brain irradiation.