Seasonal modulation of post-resection breast cancer metastasis.

BACKGROUND: Human breast cancer incidence, histopathologic grade, invasiveness, and mortality risk vary significantly throughout each year. In order to better understand this seasonal cancer biology, we investigated the circannual pattern of post-resection breast cancer metastasis, under genetically and environmentally controlled conditions.
METHODS: Over a span of 14 consecutive years, we conducted 22 similar experiments to investigate metastatic biology of breast cancer among 1,214 C3HeB/FeJ female mice. All mice were kept in temperature-controlled environment with 12 h light:12 h dark photoperiod, with food and water freely available, from birth until death. At 10-13 weeks of age, each mouse received 20,000 viable syngeneic mammary cancer cells subcutaneously and the tumor bearing leg was resected 10-12 days after tumor inoculation for potential cure. Once 10% of resected mice were found moribund, due to autopsy proven pulmonary metastases, all remaining mice were sacrificed and metastatic lung nodules were counted.
RESULTS: The incidence of post-resection pulmonary metastasis was not randomly distributed throughout the year, but peaked prominently in Summer and Winter. Although tumor volume at resection was strongly associated with metastatic potential, a significantly higher probability of pulmonary metastasis was observed if surgery was performed in Summer and Winter, regardless of tumor volume at resection, compared to Spring and Fall.
CONCLUSION: These results support the likelihood that human breast cancer seasonality is real and of biological origin. There are implications of this cancer chronobiology for breast cancer prevention, screening, diagnosis, and treatment.