A new study has reported that breast cancer diagnoses occur in a seasonal pattern worldwide. The authors studied data concerning almost three million breast cancer cases in 64 global regions. Breast cancer was found to be consistently diagnosed more often in spring and fall in both the Northern and Southern Hemispheres, regardless of menopausal status (assuming menopause at age 50). Breast cancer diagnoses peaked near both equinoxes (March and September) and were at low points near both solstices (December and June). This seasonality was also found to be increasingly more pronounced as distance from the equator increased, and this latitude dependence was most pronounced among women living in rural areas, indicating a relationship with hours of exposure to sunlight.

Based on 2005-2006 data, the authors also found that the overall annual rate of breast cancer per 100,000 population increased with the latitude of residence. In other words, populations closest to the poles had the highest rates of breast cancer whereas those living near the equator had the lowest rates. The authors conclude that breast cancer seasonality is clearly related to predictable local day/night length changes which occur seasonally. In addition, its mechanism may depend upon seasonal generation of vitamin D from sunlight and seasonal nocturnal melatonin peak level and duration.

What about long-term outcome based on season of diagnosis?

Few studies have looked at the question of whether season of diagnosis affects breast cancer outcome, however there appears to be a relationship:

• A Swedish study reported that women with breast cancer diagnosed during the summer have worse survival than those diagnosed during other seasons. This difference in outcome coincided with a lower average number of cases and a higher fraction of advanced cases diagnosed in the summer, a pattern of presentation that has been more pronounced in recent years. Differences in cancer stage distribution were found to explain the seasonal variation in outcome seen in this study, in particular, a strong tradition of vacationing from mid-June to mid-August
• A U.K. study found evidence of seasonality acting on mortality from breast cancer during two distinct periods following diagnosis. Diagnosis during the summer was found to be associated with substantially decreased mortality within the first month of diagnosis. However, death from any cause is also less common during the summer months and this underlying pattern accounted for most of the observed short-term seasonal effect on breast cancer mortality. However, fall breast cancer diagnoses were found to be associated with decreased mortality after five years or more
• A study involving 22 experiments over 14 years using mice implanted with mammary cancer cells and raised under identical light conditions (12 hours of light per day) found a significantly higher probability of metastasis when surgery to remove the primary tumor was performed in summer or winter compared to spring and fall, regardless of tumor size at time of surgery. The experimental design appears to rule out light-related vitamin D and melatonin production as the primary causes of the observed seasonality
• Two Norwegian studies reported that the prognosis was best for breast cancer cases diagnosed during summer and fall, a finding that both sets of authors attributed to higher vitamin D levels during these seasons.

The population studies above indicate that fall is the most favorable time of year for a breast cancer diagnosis (based on long-term prognosis) whereas the study using mice identified this season as an unfavorable time of year. However, this information is not particularly useful to breast cancer patients, survivors or those at high risk for breast cancer because the underlying reason for the seasonality has not been convincingly explained.

Note that many studies have reported that delaying breast cancer diagnosis and treatment can have a negative impact on survival. While breast cancer can take many years to develop, once it is symptomatic, it is vitally important to obtain a diagnosis and treatment as quickly as possible. This is part of the message of the first study described above. Many Swedes take extended summer vacations. Putting off having a lump or other symptoms checked until after the vacation could be very costly for some of these women.