The impact of dosimetric inadequacy on treatment outcome of nasopharyngeal carcinoma with IMRT.

BACKGROUND AND PURPOSE: This study aims to address the relationship between tumor size and dosimetric inadequacy in treating nasopharyngeal carcinoma (NPC), and how it subsequently affects the local control. MATERIAL AND METHODS: 444 NPC patients treated with IMRT from 2005 to 2010 were included in the study. The planning aim was to deliver at least 66.5 Gy (i.e. 95% of 70 Gy) to 95% of the primary gross tumor volume (GTV_P) while keeping all the critical neurological organs at risk (OAR) within dose tolerance. Treatment outcome were analyzed according to T stage, GTV_P volume and the degree of under-dosing. RESULTS: Disease outcome was related to T stage, GTV_P volume and the degree of under-dosing. The 5-year local failure free survival (LFFS), disease free survival (DFS) and overall survival (OS) for T4 disease were 74%, 50.4% and 63.6% respectively. 48 cm(3) was identified as the critical cut-off GTV_P volume, the large volume group (GTV_P ≥ 48 cm(3)) had lower 5-year DFS (50.4% vs. 76.6%) and OS (65.2% vs. 86.3%, p < 0.001). Most T4 diseases (and some T3) were under-dosed (<66.5 Gy) and an under-dosed GTV_P volume of 3.4 cm(3) was found to be prognostically important. Multivariate analyses showed that the effect of GTV_P volume on LFFR and DFS was outweighed by the degree of under-dosing. CONCLUSIONS: Treatment outcome of locally advanced NPC was significantly affected by the volume of under-dosed (<66.5 Gy) GTV_P due to the neighboring neurological structures. A new set of OAR dose constraint and specification is proposed.