7.1What Is Cancer?
The transition in the causes of death among the Japanese in the postwar period shows a consistent rise in the number of cancer cases*1: it has increased from 77.4 people per 100 thousand population in 1950 to 260.9 people (330 thousand in the actual count) in 2006 (approximate figure). Since the number of deaths from all causes amounted to 859.7, almost 30% had died due to cancer. Cancer became the leading cause of death in 1981, overcoming cerebrovascular diseases such as cerebral stroke that had been the most predominant cause at that time, and has remained at the top since. While there has been an increasing tendency in the cases of colorectal cancer, breast cancer, and prostate cancer, incidence rates of stomach cancer and uterine cancer case have been attaining constancy. This trend is believed to have derived from aging of society and westernization of lifestyle.
Cancer is a disease in which cells in a certain tissue, independent of the surroundings, continue growing autonomously and end up not only expanding the lesion but also spreading the carcinoma tissue to adjacent normal parts (infiltration) or proliferating in distant sites (metastasis). In general, cancer is classified depending on the organ of origin as "lung cancer," "stomach cancer," etc.
A cancerous tumor as a lesion is considered to originate from a single cell proliferating as a result of carcinogenesis (Fig. 7-1). Cellular carcinogenesis is known to stem from diverse factors, which have been discussed later. In the light of intrinsic natures of cells, the occurrence of mutations in their genes is inevitable during a long period of time. Moreover, the effects of carcinogenic substances accumulate in the course of one's lifetime, and therefore, ever-increasing longevity owing to advances in medicine is ineluctably forcing humans to face cancer eventually.
*1 All malignant tumors are treated as "cancer" here. Cancer in a narrow sense indicates only ones derived from specific tissues. Others are classified as sarcoma, leukemia, malignant lymphoma, etc. Incidentally, cancers discussed here are premised on those of humans, unless specified otherwise. Tumors signify cell masses proliferating autonomously, of which those exhibiting properties such as unchecked proliferation, infiltration, and metastasis are malignant tumors.