Dosimetric evaluation of hippocampus incidental radiation dose in nasopharyngeal cancer patients treated with intensity-modulated radiotherapy

Background: The incidental radiation exposure to hippocampus during radiation therapy for nasopharyngeal cancers may contribute to short-term toxicity like disequilibrium and lack of inhibition and also to long-term memory loss. Objective: To diametrically evaluate the dose received by hippocampus in patients with nasopharyngeal cancer undergoing intensity-modulated radiotherapy (IMRT). Materials and Methods: Eleven patients with histologically proven locally advanced nasopharyngeal squamous cell carcinoma were retrospectively analyzed in this study. The total prescribed dose to the planning target volume was 70 Gy (D95%) delivered in 2 Gy daily fractions using IMRT technique. Employing the anatomical guidelines and magnetic resonance imaging coregistration, the hippocampi were delineated on axial imaging from the simulation computed tomography scan for each patient. IMRT treatment plans were generated without applying dose–volume constraints to the hippocampus. Maximum hippocampus dose, mean hippocampus dose, minimum hippocampus dose, and hippocampus volume receiving 3 Gray dose (V3Gy) were analyzed. Results: The mean hippocampus volume was 4.7 cm3. The average minimum dose to the entire hippocampus was 5.276 Gy (range, 0.072–18.609 Gy); the average maximum point dose to the hippocampus was 21.405 Gy (range, 0.595–59.832 Gy); and the average mean dose to the entire hippocampus volume was 10.922 Gy (range, 0.194–34.706 Gy) and V3Gy was 79.15 Gy (range, 0%–100%). Conclusion: The dosimetric analysis suggests that patients who underwent IMRT for nasopharyngeal cancer received significantly high incidental dose to the hippocampus. The study creates awareness regarding the need to routinely delineate hippocampus as an organ at risk in the radiotherapy for nasopharyngeal cancers.