我们知道,癌症的发生和发展,通常要经历细胞突变、癌前病变和癌的形成这么几个阶段。整个癌变的过程,往往要经历十几年乃至几十年的时间。因此,许多癌症,如食管癌、胃癌、肺癌、肝癌、子宫癌、乳腺癌等大都发生在中年以后的人身上,尤其是食管癌、胃癌、肺癌等,往往与年龄的增长成正比。至于癌的形成,到目前为止,科学家们尚未找到确切的病因。但总的来讲,癌症的发生不外乎内因和外因两个方面。外因方面,主要是与外界接触致癌物质有关。人的一生中,时 We know that the occurrence and development of cancer usually go through such stages as cell mutations, precancerous lesions, and the formation of cancer. The entire process of cancerization often takes more than ten years or even decades. Therefore, many cancers, such as esophageal cancer, stomach cancer, lung cancer, liver cancer, uterine cancer, and breast cancer, mostly occur in people after middle age, especially esophageal cancer, stomach cancer, and lung cancer, and are often proportional to the increase in age. As for the formation of cancer, so far scientists have not yet found the exact cause. But in general, the occurrence of cancer is no more than two aspects of internal and external factors. Externalities are mainly related to exposure to carcinogenic substances. During a person’s life,