Hippocampal Dosimetric Evaluation in Locally Advanced Nasopharyngeal Carcinoma Patients Treated with Intensity Modulated Radiotherapy

Introduction: Treatment of locally advanced nasopharyngeal (Npx) cancer usually requires the inclusion of the sphenoid body or higher in intermediate risk CTV dose regions. This makes the avoidance of intracranial OAR such as temporal lobes, optic apparatus and cochlea challenging. There are different accumulating data confirming the impact of hippocampus (HC) irradiation on neurocognitive functions after whole brain radiotherapy & Npx cases were specifically addressed in several publications. The specified dose constrain in RTOG 0933 was Dmax <9 Gy. Our aim is to retrospectively evaluate the doses delivered to the HC in patients with locally advanced Npx carcinoma. Methods: Our study included twelve patients with locally advanced nasopharyngeal carcinoma (Stage III-IVA) previously treated at NEMROCK by either IMRT or VMAT. The HC was retrospectively delineated without being accounted for in the inverse treatment planning. Bilateral HC were delineated as a single OAR, and a margin of 3 mm was added to create the HC zone. We evaluated the doses delivered to the hippocampus by conducting a dosiemtric analysis on the treatment plan already used. Results: DVH Analysis: The mean volumes (std) for PTV70 primary, PTV70 LN and PTV59.4 were 83.62 cm3 (±33.51), 119.5 cm3 (±122.66) and 1017.58 cm3 (±140.28) respectively. The mean HC volume (std) was 4.17 cm3 (±1.66). The mean HC Dmin, Dmean, Dmedian and Dmax were 6.57 Gy, 25.05 Gy, 24.18 Gy and 52.95 Gy respectively. The mean HC V5, V10, V20, V40, V50 and V60 were 92.83%, 78.95%, 55.91%, 20.37%, 9.9% and 3.38% respectively. There was no difference between IMRT and VMAT in terms of HC volumetric and dosimetric parameters. Conclusion: In locally advanced Npx carcinoma, HC is exposed to significant doses, particularly with T4 disease (intracranial extension). In view of the low radiation tolerance of HC and its close proximity to CTV54-60 Gy in Npx cases, HC contouring and avoidance should be considered.