Left-sided Excess in the Laterality of Cutaneous Melanocytic Nevi and Melanomas.

Background and objectives: Asymmetrical distribution of melanomas in favor of the left body half has been repeatedly described. Patients and methods: In a prospective cross-sectional study, we investigated the distribution of melanocytic nevi between the left and right halves of the body in 702 patients. In 2,004 consecutive cases with melanomas, we retrospectively determined left to right (L/R) ratios of primary melanomas, lymph node metastases, and melanocytic nevi in sentinel lymph nodes (SN). Results: The L/R ratios for cutaneous nevi and melanomas were 1.23 (95 % confidence interval [CI] 1.12-1.36) and 1.6 (95 % CI 1.37-1.88), respectively. In both cutaneous nevi and melanomas, the left-sided excess was mainly found in intermittently light-exposed skin. Of the nevus patients, 92.4 % were right-handed. Breslow thickness and rates of ulceration did not differ significantly between the two body halves. Conclusions: Melanocytic nevi and melanomas are more frequently found on the left body half. We assume asymmetric solar radiation as the causative factor. The vast majority of right-handed people might unconsciously align their posture during outdoor activities so that the light falls in from the left. This avoids shadow cast by the dominant arm, shoulder, or head.