Enhanced lymphoma incidence in BALB/c mice after ultraviolet light treatment

Following UV irradiation of BALB/c mice three times a week for 9 months, lymphatic leukemia or reticulosarcoma developed in 44 (12%) of 378 mice whose backs were shaved and whose ears were intact. Only 11 (5%) of 224 UV-irradiated mice whose backs were shaved and whose ears were removed developed lymphomas. Among 311 unirradiated mice whose backs were shaved, 9 lymphomas (3%) occurred. The diagnosis of leukemia or reticulosarcoma was confirmed by microscopy and transplantation. Among irradiated mice, ear tumors developed in 24% of the mice with intact ear, whereas skin tumors developed on the shaved backs in 4% of the mice whose ears were removed and also in 4% of the mice with intact ears.