Cancer and laterality: a study of the five major paired organs (UK).

OBJECTIVE: The human body displays marked asymmetry: paired organs differ bilaterally exerting effects upon cancer incidence and progression. However the factors involved remain contentious. In this large study involving over a quarter of a million cancer patients, we examine the epidemiological correlates of cancer laterality including incidence, stage at diagnosis and survival in the five major paired organs: the breasts, lungs, kidneys, testes and ovaries. METHODS: Cancer patients were selected from the Thames Cancer Registry database and age-standardised incidence rates (ASRs), stage distribution at diagnosis and survival rates computed, stratifying appropriately. RESULTS: Cancer incidence differed significantly by laterality at all sites studied (p < 0.01) but substantially in the lung (left-right incidence-rate ratio [IRR] 0.87), breast (IRR 1.07), testis (IRR 0.87) and in ovarian cancer (IRR 0.86). Autopsy data showed strongly coincident left-right organ size ratios (0.87 in the lungs and 0.87 in the testes). Patients with left testicular cancer, right lung cancer and left ovarian cancer showed significantly better survival than those with contralateral disease (p < 0.05). CONCLUSIONS: In the lungs and testes, asymmetries in cancer incidence closely coincided with asymmetries in organ size. Our results suggest that tissue mass in these organs is an important contributor to asymmetry in cancer incidence.