2ND PRIMARY CANCERS AFTER CANCERS OF THE COLON AND RECTUM IN NEW-SOUTH-WALES, AUSTRALIA, 1972-1991

Data from the New South Wales Central Cancer Registry for the period 1 972-1991 were examined to determine the risk of second primary cancers after an initial invasive cancer of the colon (ICD-9 153) or rectum ( ICD-9 154). The expected numbers of cancers were obtained by assuming that subjects experienced the same cancer incidence as prevailed in th e corresponding general population and by applying sex-, age-, and cal endar-specific rates to the appropriate person-years at risk. The rela tive risk (RR) of a second primary cancer was taken to be the ratio of observed:expected numbers of second cancers, After colon cancer, ther e was an excess of cancers of the small intestine in both sexes (RRs o f 4.5 and 4.4); prostate (RR = 1.4) and kidney (RR = 1.8) in men; and breast (RR = 1.3), body of uterus (RR = 1.9), ovary (RR = 2.8), and th yroid (RR = 2.7) in women. Lung cancer occurred less frequently in men than expected (RR = 0.7), After rectal cancer, men had increased risk s of cancers of the colon (RR = 1.5) and prostate (RR = 1.3) and a red uced risk of pancreatic cancer (RR = 0.3). A reciprocal relationship o f increased risk was seen between cancers of the proximal (but not the distal) colon and rectum. Shared luminal risk factors for proximal co lon cancer and rectal cancer and three syndromes of hereditary predisp osition to colon cancer seem to be the major contributors to second pr imary cancers in patients with an initial colon cancer. Sources of bia s have been considered.