Prognostic efficacy of extensive invasion of primary tumor volume for T3-4 nasopharyngeal carcinoma receiving intensity-modulated radiotherapy.

•First prognostic value analysis of different invasion volume in T3-4 NPC.•Total or confined invasion volume was not associated with treatment failure.•Extensive invasion volume (GTV-E) was predictive for poorer prognosis.•The optimum cutoff point of GTV-E for local control prediction was 22.0 mL.