Second Primary Cancers in Patients with Invasive and In Situ Squamous Cell Skin Carcinoma, Kaposi Sarcoma, and Merkel Cell Carcinoma: Role for Immune Mechanisms?

Second primary cancers (SPCs) are becoming a common cancer entity, which may interfere with survival in relatively benign first primary cancers. We examined the hypothesis that immune dysfunction may contribute to SPCs by assessing SPCs associated with known immune responsive skin cancers, invasive and in situ squamous cell carcinoma, Kaposi sarcoma, and Merkel cell carcinoma. Cancers were identified from the Swedish Cancer Registry from the year 1958 to 2015. Standardized relative risks were calculated bidirectionally for any SPC after skin cancer and for skin cancer as SPC. Over 80,000 first primary cancers were identified for each invasive and in situ squamous cell carcinoma of the skin. Bidirectional increased risks were observed for 26 cancers associated with invasive skin cancer; the Spearman rank correlation was 0.72 (P = 4.6 × 10-5). The highest bidirectional relative risks were for invasive and in situ skin cancer as SPCs (14.59 and 16.71, respectively). Remarkably high risks for second in situ squamous cell carcinoma of the skin were found after Kaposi sarcoma (685.68) and Merkel cell carcinoma (117.23). The high systematic bidirectional risks between immune responsive skin cancers and most other cancers suggest that immune suppression is a key mechanism contributing to an increased risk of SPCs.