Leukemia after radiotherapy for first primary cancers of various anatomic sites

The authors conducted a case-control study designed to evaluate the association between radiotherapy for a first primary cancer and subsequent leukemia risk. Study subjects were selected from four tumor registries (California, Connecticut, Kansas, and Massachusetts). Cases were subjects with two primary cancers; the second cancer was a leukemia diagnosed one year or more after the diagnosis of the first primary cancer. Control subjects were patients with single cancers. Two control subjects were chosen for each case matched for tumor registry, site of the first primary cancer, date of diagnosis of the first primary cancer, age, sex, and duration of survival after the first primary cancer. Separate analyses were carried out for chronic lymphatic leukemia (166 cases) and all other leukemias (232 cases). The relative risk of chronic lymphatic leukemia after radiotherapy was 0.7 and did not differ significantly from unity. For all other leukemias, relative risks were 1.6 after radiotherapy for first primary cancers of all sites, 2.4 after radiotherapy to trunk sites, 2.2 after radiotherapy for breast cancer, and 10.3 after radiotherapy for cancer of the corpus uteri. These relative risks for leukemias other than chronic lymphatic leukemia all differed significantly from unity.