Prognosticators and risk grouping in patients with lung metastasis from nasopharyngeal carcinoma: a more accurate and appropriate assessment of prognosis. - Cao, Xun; Luo, Rong-Zhen; He, Li-Ru; Li, Yong; Lin, Wen-Qian; Chen, You-Fang; Wen, Zhe-Sheng (Radiation Oncology, 2011)