3757 - Individualized CTV Delineation Method for Nasopharyngeal Carcinoma Based on Safe Distance Expansion from Primary Tumor: A Preliminary Efficacy Report from a Prospective Observational Study

Z. Wu, Q. He, Y. Li, J. Xiao, S. B. Wang, X. Luo, H. Wang, and Y. Q. Han; Department of Radiation Oncology, Hunan Cancer Hospital & the Affiliated Cancer Hospital of Xiangya School of Medicine, Central South University, Changsha, Hunan, China

Purpose/Objective(s): The nasopharyngeal carcinoma (NPC) target delineation guidelines recommend that traditional high risk structures, such as parapharyngeal space and skull base foramina on both sides, should be included in clinical target volume (CTV) and require prophylactic radiation. This study aimed to evaluate the efficacy and feasibility of our individualized CTV delineation strategy based on safe distance expansion from primary tumor. Materials/

Methods: Data on 126 newly diagnosed, biopsy-proven, non-metastatic NPC patients treated at the Hunan Cancer Hospital using intensity modulated radiation therapy (IMRT) between Jan 2021 and Dec 2022 were analyzed. The gross tumor volume (GTVnx/nd) included the primary tumor in the nasopharynx (GTVnx) and the involved cervical lymph nodes (GTVnd) according to clinical and imaging findings; the high-risk clinical target volume (CTVnx-1) was defined as the GTVnx plus a margin of 5-10 mm, the CTVnd-1 was defined as the GTVnd plus a margin of 5 mm; The low-risk clinical target volume (CTV2) was defined by adding a margin of 5 mm to CTVnx/nd-1 and the lymphatic drainage area which need to be prophylactically irradiated. The CTVs would be modified based on anatomic barrier after expansion, and the traditional high-risk structures outside of CTVs is no longer included in CTV after safety distance expansion. The prescribed dose was 6996 cGy to the PGTVnx/nd, 6006 cGy to the PCTVnx/nd-1, and 5445 cGy to PCTV2 in 33 fractions.
Results: Follow-up was updated in Jan 2024, with the median follow-up time of 25 months (range, 12-38 months). The median age was 52 years (range, 19-83 years). The proportions of patients grouped as T3-4 and N2-3 were both 68.3% (77/126), and 85.7% (108/126) patients had stage III-IVa disease. The most frequently used treatment modalities were induction chemotherapy (IC) plus concurrent chemoradiotherapy (CCRT) and IC plus CCRT with adjuvant chemotherapy (77.8%, 99/126). 3 patients had local recurrence, 4 had lymph node recurrence, 14 patients developed distant metastasis, and 1 patient died. All the relapses occurred within high dose area and considered as in-field failures. No recurrences were observed in the shrunk radiation areas. The 2-year local relapse-free survival, nodal relapse-free survival, distant metastasis-free survival, and overall survival were 97.4%, 97.2%, 89.7% and 98.7%, respectively.
Conclusion: Individualized CTV delineation based on safe distance expansion from primary tumor in NPC could yield excellent local control, which is worthy of further exploration.