Individualized risk-adapted cranial border for nodal clinical target volume in nasopharyngeal carcinoma: challenging the universal skull base rule.

Abstract

Current international guidelines mandate routine skull base coverage for the nodal clinical target volume (CTVn) in nasopharyngeal carcinoma (NPC), however, this universal approach may lead to unnecessary radiation exposure. This study aimed to validate the feasibility of our optimized CTVn delineation strategy that challenges this conventional paradigm. This retrospective study included non-metastatic NPC patients who received radical IMRT at our institution between 2014 and 2018. We employed an individualized CTVn delineation strategy: the cranial border was primarily set at the caudal edge of the lateral process of atlas (C1), with a minimal 1-cm margin above involved nodes. Skull base coverage was selectively applied only when level IIb nodes extended above the C1 landmark. A total of 627 patients were included. Applying this risk-adapted strategy, only 80 patients (12.8%) required skull base coverage due to nodal extension beyond C1 (Group A), while the majority (547 patients, 87.2%) were successfully treated with the C1-based border (Group B). With a median follow-up of 73 months, the 5-year regional control reached 97.2%. Notably, only one recurrence occurred in the region between the C1 border and skull base, which was an in-situ GTVn failure in Group A. Dosimetric analysis confirmed significant parotid gland sparing with this selective approach. Our optimized CTVn delineation strategy demonstrates both safe and effective, achieving excellent regional control while substantially reducing radiation exposure. This risk-adapted approach provides a superior therapeutic ratio compared to the universal skull base coverage recommended by current guidelines, offering a practical alternative for precision radiotherapy in NPC.