Photocarcinogenesis by near-ultraviolet (UVA) radiation in Sencar mice

The carcinogenic effect of long-term exposure to UVA (315-400 nm) radiation was examined in Cr:ORL Sencar mice. Daily exposure to FR40T12 PUVA bulbs, filtered with Mylar to eliminate wavelengths below 315 nm, induced skin tumors with 50% probability of tumor development (T50) occurring after 68 weeks of irradiation. Tumors developed primarily on the dorsa of mice and included squamous cell carcinomas, poorly differentiated (spindle cell) tumors, and benign squamous papillomas. By comparison, thrice-weekly exposure of Sencar mice to unfiltered FS40 sunlamps containing both UVB (280-315 nm) and UVA radiation, induced skin tumors with T50 occurring after 23 weeks of irradiation. Tumors developed primarily on the ears and included squamous cell carcinomas and spindle-cell tumors. A small number of spontaneous mammary adenocarcinomas occurred in mice (both irradiated and unirradiated controls) that were older than 50 weeks. This study demonstrates that UVA radiation, which is the major UV waveband in solar radiation, is carcinogenic in a haired mouse strain, although far less carcinogenic than combined UVB/UVA radiation.