Level Ib CTV delineation in nasopharyngeal carcinoma based on lymph node distribution and topographic anatomy

To analyze the distribution pattern of lymph nodes (LNs) metastasis of level Ib in nasopharyngeal cancer (NPC) and propose shrinkage of clinical target volume (CTV) boundaries to avoid unnecessary radiation for some space with very low-risk of involvement.Pretreatment images of pathologically proven NPC patients were reviewed and those with positive level Ib LN metastasis was enrolled. The geometric center of each level Ib LN in the neck was marked on a template CT. The spatial relationship of nodes with key structures in level Ib was analyzed. Modified level Ib CTV according to the 2013 International CTV consensus was proposed based on the LN distribution pattern. A PlanIQ Feasibility DVH module was implemented to evaluate the feasibility analysis of the best possible sparing of organs at risk (OAR) with modified Ib CTV.A total of 1518 NPC patients were reviewed and 54 with positive level Ib nodes were enrolled. Four sub-level anatomical regions were defined within the gross area of level Ib. Of 106 positive nodes identified, none, one, 88, and 17 were found in the intraglandular (IG), medial mandibular (MM), supra perivascular (SP), and infra perivascular (IP) sub-level, respectively. This study proposes sparing the IG and MM sub-level and including the area within a specified distance from the submandibular gland (11 mm for SP, 17 mm for IP) for CTV coverage. Compared with planning based on CTV-consensus, planning based on CTV-proposed results in a significantly reduced CTV volume, and mean dose (Dmean) of both the ipsilateral SMG and bilateral SLG.Based on detailed analysis of the relationship between positive node distribution and several important anatomical structures, modified level Ib CTV for prophylactic irradiation was proposed to reduce the dose of OAR irradiation.