Background Elective radiation of lower neck is controversial for nasopharyngeal carcinoma (NPC) without lymph node metastasis (N0 disease).Tumor volume is an important prognostic indicator.The objective of this study is to explore the potential impact of tumor volume on the indication of the lower neck irradiation for N0-NPC,by a qualitative evaluation of the relationship between tumor volume and nodal metastasis.Methods Magnetic resonance (MR) images of 99 consecutive patients with NPC who underwent treatment were retrospectively reviewed.Primary tumor volumes of NPC were semi-automatically measured,nodal metastases were N-classified and neck level involvements were examined.Distributions of tumor volumes among N-category-based groups and distributions of N-categories among tumor volume-based groups were analyzed,respectively.Results The numbers of patients with N0 to N3 disease were 12,39,32,and 16,respectively.The volumes of primary tumor were from 3.3 to 89.6 ml,with a median of 17.1 ml.For patients with nodal metastasis,tumor volume did not increase significantly with the advancing of N-category (P ＞0.05).No significant difference was found for the distribution of N1,N2,and N3 categories among tumor volume-based groups (P ＞0.05).Nevertheless patients with nodal metastasis had significantly larger tumor volumes than those without metastasis (P ＜0.05).Patients with larger tumor volumes were associated with an increased incidence of nodal metastasis.Conclusions Certain positive correlations existed between tumor volume and the presence of nodal metastasis.The tumor volume (＞10 ml) is a potential indicator for the lower neck irradiation for N0-NPC.