Dosimetric impact of induction chemotherapy on intensity-modulated radiotherapy for locally advanced nasopharyngeal carcinoma and dosimetric characteristics of replanning

Objective To investigate the benefits of replanning after induction chemotherapy (IC) by analyzing the dosimetric impact of IC on intensity-modulated radiotherapy (IMRT) for locally advanced nasopharyngeal carcinoma (NPC) and the dosimetric characteristics of replanning after IC, and to provide data for the rational design of clinical radiotherapy plans. Methods 16 NPC patients underwent contrast-enhanced CT scan once before and after IC. Target volumes were delineated and the chemotherapy plans were created, defined as Plan-1 and Plan-2, respectively. Then the target structure after IC was copied to Plan-1, generating the third plan, defined as Plan-1-2. The paired t-test was used to compare the dosimetric parameters between Plan-1 and Plan-1-2 and between Plan-2 and Plan-1-2. Results Plan-1 vs. Plan-1-2: Plan-1-2 showed significantly reduced Dmean of target volume compared with Plan-1(P<0.05). Plan-1-2 significantly increased Dmean and Dmax of the spinal cord (P<0.05), although significantly reduced Dmean of the brain stem and Dmax of the temporal lobes compared with Plan-1. Plan-1-2 also had significantly reduced conformity index (CI) and significantly increased homogeneity index (HI) for the target volume compared with Plan-1(P<0.05). Plan-2 vs. Plan-1-2: Compared with Plan-1-2, Plan-2 significantly increased Dmean and Dmin of gross tumor volume (GTV) and primary GTV (P<0.05) and significantly reduced Dmean of the temporal lobes and Dmax and Dmean of the spinal cord (P<0.05), with Dmax decreased to 430.48 cGy; Plan-2 had significantly increased CI and significantly reduced HI for the target volume compared with Plan-1-2(all P<0.05). Conclusions IMRT plan-1 after IC has worse dosimetric distribution, while replanning after IC has more dosimetric benefits. Key words: Nasopharyngeal neoplasms/induction chemotherapy; Nasopharyngeal neoplasms/intensity modulated radiotherapy; Target delineation; Dosimetry