ESTRO 2025 - Abstract Book

S926

Clinical – Head & neck

ESTRO 2025

185

Poster Discussion Recurrent patterns in NPC patients and risks leading to inaccurate delineation in marginal failure in the era of intensity-modulated radiotherapy Shu Zhang 1,2 , Ni Zeng 1 , Di Yan 3,4 , Nian yong Chen 1,2 1 Department of Head and Neck Oncology, Cancer Center, West China Hospital, Sichuan University, Chengdu, China. 2 Radiation Oncology, Cancer Center, West China Hospital, Sichuan University, Chengdu, China. 3 Adaptive Radiotherapy Research Laboratory, Cancer Center, West China Hospital, Sichuan University, Chengdu, China. 4 Radiation Oncology, Oakland University/Beaumont School of Medicine, Michigan, USA Purpose/Objective: To investigate the failure patterns of recurrent nasopharyngeal carcinoma (NPC), especially to identify the relationship between the recurrent-prone anatomic structures and the tumor regression sites after the introduction chemotherapy (IC). Material/Methods: 1121 non-metastatic NPC patients were retrospectively enrolled. The main clinical characteristics were extracted, and pretreatment and recurrent images of each patient were registered to the planning CT for delineation or dosimetric analysis. Failures were classified as: “in-field”, “marginal”, or “outfield” 1 . The recurrent tumor were defined as rGTV. The relationship between marginal rGTV and tumor extension area before induction chemotherapy (GTV0) was investigated by calculating the percentage volume of rGTV inside GTV0 (rGTV0%). The regression sites from GTV0 to the planning GTV was defined as Vicr. V0 below60 and V0 below70 were considered as Vicr uncovered by 60Gy or 70Gy isodose curve, respectively. According to the tumor biological infiltration behavior, the extension of local disease of nasopharyngeal carcinoma were risk-stratified as high-risk, medium-risk and low-risk 2 . Results: Out of 1121 patients, 126 (11.24 %) experienced tumor recurrence. The 3-year and 5-year locoregional recurrent free survival rate (LRRFS) from 1121 NPC patients was 95.3%and 89.2%, respectively. Distribution of failure patterns was 61% “in-field”, 29% “marginal”, and 10% “outfield”. The volumes of rGTV within 70Gy, 66Gy, and the 60Gy isodose curves were notably smaller in marginal failures than those of in-field failures. Upon re-evaluation of 12 patients with local marginal recurrence, who have received IC showed that the GTVnx regressed from GTV0. The rGTV within Vicr predominantly located at sites such as, choanae, sphenoidal sinus, clivus, cavernous sinus, foramen lacerum, and great wing of sphenoid bone in which are considered to be recurrent-prone as well as easily penetrating anatomical structures for NPC tumor. Further dosimetric analysis shows that the regression sites post IC did not receive the full 70Gy dose but over half receiving with 60Gy.