Functional cranio-spinal irradiation: A hippocampal and hypothalamic-pituitary axis sparing radiation technique using two IMRT modalities.

Cranio-spinal irradiation (CSI) treatment of embryonal tumors is associated with long-term endocrine and neuro-cognitive sequelae. As an example, the radiation regiment for standard risk medulloblastoma is 23.4 Grays (Gy) CSI followed by a boost of 30.6Gy to the tumor bed. We hypothesize that a novel CSI technique, which we named "Functional" CSI (F-CSI) can reduce the dose to the hypothalamic-pituitary axis (HPA) and hippocampi compared to standard CSI (S-CSI) without sacrificing coverage. In this study, we compared the efficacy of Volumetric Modulated Arc Therapy (VMAT) and Helical Tomotherapy (HT) in delivering this novel CSI technique. Plans were constructed from 10 patients with embryonal tumors previously treated at our institution. Target volumes and organs at risk were delineated as per our local protocol and the ACNS0331 Atlas. The HPA and hippocampi contours were verified by an experienced neuro-radiologist. Primary objective was to achieve a D95% to the prescribed dose of 23.4Gy for CSI and 30.6Gy for the boost. Dmean ≤18Gy was assigned to the HPA and hippocampi. A two-sided t-test was used for comparison. F-CSI in both modalities were able to achieve the D95% target coverage. Hot spots (D2%) were lower with HT for both the CSI component (p = 0.03) and boost component (p < 0.01). VMAT was able to achieve better conformality (p < 0.01). Compared to S-CSI, both F-CSI modalities were able to achieve a significant decrease in dose to the HPA and Hippocampi. The average S-CSI HPA and Hippocampi Dmean were 23.9Gy and 23.8Gy. In contrast, respective F-CSI Dmean were 13.9Gy and 17.2Gy in VMAT and 15Gy and 15.9Gy in HT. The average composite (F-CSI plus boost) Dmean to the HPA and hippocampi often exceeded 18Gy. Compared to S-CSI, F-CSI with VMAT and HT were capable of achieving acceptable coverage while sparing the HPA and hippocampi. However, the addition of the boost component often exceeded the mean dose of 18Gy. This may be overcome with more conformal modalities for the boost phase such as stereotactic radiotherapy or proton therapy.