ESTRO 2021 Abstract Book

S1620

ESTRO 2021

with limited compromise to volumes outside the high-risk CTV.

PO-1899 Hippocampal Dose Sparing in Nasopharyngeal Carcinoma Patients C. Abraham 1 , O. Nicholas 2 , R. Lewis 1 , A. Selby 1 , H. Wong 1 , R. Hugtenburg 1,3 1 Singleton Hospital, Radiotherapy, Swansea, United Kingdom; 2 Singleton Hospital, Clinical Oncology, Swansea, United Kingdom; 3 Swansea University, College of Medicine, Swansea, United Kingdom Purpose or Objective Hippocampal irradiation is shown to result in the impairment of hippocampus-dependent functions such as learning and memory. For brain cancers, hippocampal sparing is well established and included in international guidelines. In nasopharyngeal carcinoma (NPC), the planning target volume (PTV) often lies in close proximity to the hippocampus but is often not considered during radiotherapy planning. This study assesses the dose delivered to the hippocampus in NPC without any plan optimisation constraints and assesses if additional hippocampal optimisation constraints would successfully reduce hippocampal dose, whilst maintaining clinically acceptable PTV coverage. Materials and Methods Ten anonymised NPC patient treatment plans were utilised, treated using standard dual-arc VMAT radiotherapy; primary dose 66 Gy, secondary dose 54 Gy. Mirada RTx v1.8 was used for contouring. Treatment planning and dose analysis was conducted using Philips Pinnacle v16.2. T1-weighted axial MRI scans were fused with CT scans in Mirada to provide greater soft tissue contrast. The contouring of the hippocampus followed RTOG guidance as used when planning brain cancers [1] . All contours and treatment plans were peer-reviewed by a clinical oncologist. The hippocampus optimisation goal was set to a mean dose of 25 Gy, based on the current departmental target for brain cancer patients. A null hypothesis using a paired two sample t-test (a, 0.05) was set that including the hippocampus within the treatment plan optimisation, on average, does not affect the dose delivered to the hippocampus. Results A dosimetric comparison reported hippocampal dose reductions in the new treatment plans of between 30% and 60% compared to the original plans. For the null hypothesis set, p = 0.00003 therefore showing that optimising the hippocampus as part of the treatment plan, on average, significantly reduces the dose delivered to these structures, while still maintaining overall plan quality. There is no reported dose increase in the other NPC clinical structures, however, the isodose distributions cover a larger area of the brain, namely the parietal and occipital lobe regions.

Conclusion This project investigated the dose of both the original treatment and the new, redesigned, treatment plan which accounted for the hippocampus within NPC patients. Significant dose reductions were reported to the hippocampus, while satisfactory PTV coverage was achieved. The level of reduction seen in this study is comparable to studies focused on the cranial irradiation of brain metastases patients, which reported hippocampal sparing reductions of 87% [2] . As NPC has a relatively good prognosis and commonly affects younger patients, hippocampal sparing may offer benefits to the patient’s long-term quality of life. This study shows that the hippocampus can be feasibly outlined and included in NPC treatment planning and should be considered for future clinical plans.