Measuring out-of-field dose to the hippocampus in common radiotherapy indications

Abstract

Background The high susceptibility of the hippocampus region to radiation injury is likely the causal factor of neurocognitive dysfunctions after exposure to ionizing radiation. Repetitive exposures with even low doses have been shown to impact adult neurogenesis and induce neuroinfammation. We address the question whether the out-offeld doses during radiotherapy of common tumour entities may pose a risk for the neuronal stem cell compartment in the hippocampus. Methods The dose to the hippocampus was determined for a single fraction according to diferent treatment plans for the selected tumor entities: Point dose measurements were performed in an anthropomorphic Alderson phantom and the out-of-feld dose to the hippocampus was measured using thermoluminescence dosimeters. Results For carcinomas in the head and neck region the dose exposure to the hippocampal region for a single fraction ranged from to 37.4 to 154.8 mGy. The hippocampal dose was clearly diferent for naso-, oro- and hypopharynx, with maximal values for nasopharynx carcinoma. In contrast, hippocampal dose levels for breast and prostate cancer ranged between 2.7 and 4.1 mGy, and therefore signifcantly exceeded the background irradiation level. Conclusion The mean dose to hippocampus for treatment of carcinomas in the head and neck region is high enough to reduce neurocognitive functions. In addition, care must be taken regarding the out of feld doses. The mean dose is mainly related to scattering efects, as is confrmed by the data from breast or prostate treatments, with a very diferent geometrical set-up but similar dosimetric results.