Spontaneous basal cell carcinoma in a 7-week-old Sprague-Dawley rat.

Spontaneous basal cell carcinoma (BCC) is very rare in rats, with an incidence rate of only 0.14% reported in aged animals. A spontaneous BCC occurred in a 7-week-old Sprague-Dawley rat housed in a specific-pathogen-free animal facility. The tumor was a single, well-delineated reddish-brown subcutaneous mass measuring 2 x 2 cm and located in the left inguinal region. Microscopically, the tumor consisted of basaloid cells in lobular and cribriform growth patterns and with a high mitotic rate. Immunohistochemically, cytokeratin 14 (an indicator for basal keratinocytes of the epidermis) showed strong reactions throughout the whole tumor, and cytokeratin 18 showed weak but positive reaction in the majority of nested tumor cells. To the authors' knowledge, this is the first report of spontaneous BCC occurrence in young Sprague-Dawley rats.