Radiotherapy is necessary for 70％ of malignant tumor patients. Local recurrence and metastasis are primary failure models, where radioresistance is one of important factors. It is critical to establish radioresistant tumor cell lines for understanding the mechanism of radioresistance. According to single radiation, fractioned radiation and the compound radiation method, four representative radiation models are classified: conventional radiation, repeated radiation, gradient radiation and other radiation. These different radiation models have difference in total dose and radiation model as well as the biological characteristics. Superior to other three models, the gradient fractioned irradiation model increase fractioned doses gradually along with the enhancement of radioresistance, which favorably balances the fractioned doses and the time of irradiated cells approaching to exponential growth phase. Clinically relevant radioresistant cell line ( CRR) with a genotype in consistent with its parental cells is an important research direction on tumor radioresistance.

Objective: To study the clinical characteristics, strategy of treatment and prognosis of multiple primary cancers(MPC) diagnosed of digestive system malignant tumor firstly. Methods: From January, 2000 to December, 2015, the clinical, follow-up and prognostic data of 138 MPC patients diagnosed of digestive system malignant tumor firstly were retrospectively analyzed. Results: 138 cases were found in 10 580 cases with malignant tumors, and the incidence was 1.30%. There were 129 cases of duplex primary cancers, 8 cases of triple primary cancers and 1 case of quintuple primary cancers. The repetitive primary cancer was occurred in digestive system (61cases, 44.2%) most frequently, with the next in respiratory system (46 cases, 33.3%). 52.2% (72 cases) suffered second primary cancer in 2 years after first primary cancer diagnosed, and 75.4% (104 cases) in 5 years. The median overall survival in patients with all cancer lesions radically treated was 168 months, better than any other treatment (68 months, P<0.05). Conclusions The second primary cancers of MPC cases initially diagnosed of digestive system malignant tumor most frequently occurred in the digestive system and respiratory system. More concern should be attracted in follow-up, especially in the first 5 years. The key to improve patient′ prognosis was radical treatment to every primary cancer.