Cutaneous melanoma--season and invasion? A preliminary report.

Histological diagnosis of superficial spreading melanoma (SSM) of invasive type was found to vary significantly with season, such cases being most numerous in April-September. Cases of in situ type were evenly distributed throughout the year. This distinction has not been made previously. It suggests that different mechanisms may be involved in the development of these two lesions. The results as a whole support the theory that the development of invasive melanoma is related to periodic sun exposure, and are in keeping with the proposed role of ultraviolet radiation as a late-stage promoter. The possibility of virus-related initiation is also discussed.