We encountered a case of cutaneous squamous cell carcinoma (SCC) in a 17-year-old female
koala at a zoo. A fragile, papillary, elevated mass was found on the third digit of the
right hind limb. SCC was identified histopathologically: squamous cell-like polygonal
tumor cells showed a nest-like growth pattern with epidermal down growth, central
keratinization and necrotic foci, and invaded dermal connective tissues. Metastatic
lesions were observed in various organs, including the lung and axillary lymph node: in
the lung, multiple metastatic foci similar to the primary lesion, and in the axillary
lymph node, individual polygonal tumor cells infiltrated the sinusoids.
Immunohistochemistry revealed that the tumor cells were positive for proliferating cell
nuclear antigen, which exhibited 32–33% of labeling indices in the tumor cells. To our
knowledge, this is the first report of a case of SCC in a digit of a koala.