Introduction:The current metastatic category(M) of nasopharyngeal carcinoma(NPC) is a “catch-all” classification,covering a heterogeneous group of tumors ranging from potentially curable to incurable.The aim of this study was to design an M categorization system that could be applied in planning the treatment of NPC with synchronous metastasis.Methods:A total of 505 NPC patients diagnosed with synchronous metastasis at Sun Yat-sen University Cancer Center between 2000 and 2009 were involved.The associations of clinical variables,metastatic features,and a proposed M categorization system with overall survival(OS) were determined by using Cox regression model.Results:Multivariate analysis showed that Union for International Cancer Control(UICC) N category(N1-3/N0),number of metastatic lesions(multiple/single),liver involvement(yes/no),radiotherapy to primary tumor(yes/no),and cycles of chemotherapy(>4/<4) were independent prognostic factors for OS.We defined the following subcategories based on liver involvement and the number of metastatic lesions:Mia,single lesion confined to an isolated organ or location except the liver;Ml b,single lesion in the liver and/or multiple lesions in any organs or locations except the liver;and M1 c,multiple lesions in the liver.Of the 505 cases,74(14.7%) were classified as Mia,296(58.6%)as M1 b,134(26.5%) as M1 c,and 1 was not specified.The three Ml subcategories showed significant difference in OS[Ml b vs.Mia,hazard ratio(HR) = 1.69,95%confidence interval(CI) = 1.16-2.48,P = 0.007;Ml c vs.Ml a,HR = 2.64,95%CI = 1.75-3.98,P< 0.001],Conclusions:We developed an M categorization system based on the independent factors related to the prognosis of patients with metastatic NPC.This system may be helpful to further optimize individualized care for NPC patients. Introduction: The current metastatic category (M) of nasopharyngeal carcinoma (NPC) is a “catch-all” classification, covering a heterogeneous group of tumors ranging from potentially curable to incurable. The aim of this study was to design an M categorization system that could be applied in planning the treatment of NPC with synchronous metastasis. Methods: A total of 505 NPC patients diagnosed with synchronous metastasis at Sun Yat-sen University Cancer Center between 2000 and 2009 were involved. The associations of clinical variables, metastatic features , and a proposed M categorization system with overall survival (OS) were determined by using Cox regression model. Results: Multivariate analysis showed that Union for International Cancer Control (UICC) N category (N1-3 / NO), number of metastatic lesions multiple / single, liver involvement (yes / no), radiotherapy to primary tumor (yes / no), and cycles of chemotherapy (> 4 / <4) were independent prognostic factors for OS.We defined the following subcategories based on liver involvement and the number of metastatic lesions: Mia, single lesion confined to an isolated organ or location except the liver; Ml b, single lesion in the liver and / or multiple lesions in any organs or locations except the liver; and M1 c, multiple lesions in the liver. Of the 505 cases, 74 (14.7%) were classified as Mia, 296 (58.6%) as M1 b, 134 (26.5%) as M1 c, and 1 was not specified. The three Ml subcategories showed significant difference in OS [Ml b vs. Mia, hazard ratio (HR) = 1.69, 95% confidence interval (CI) = 1.16-2.48, P = 0.007; Ml c vs. Ml a, HR = 2.64, 95% CI = 1.75-3.98, P <0.001], Conclusions: We developed an M categorization system based on the independent factors related to the prognosis of patients with metastatic NPC. This system may be helpful to further optimize individualized care for NPC patients.