Spontaneous massive infarction of mammary gland tumors has been reported to occur
infrequently in humans. A subcutaneous mass (18 × 17 × 10 mm) was observed in the right
axilla extending to the chest region of a 110-week-old female Wistar Hannover GALAS rat.
Histopathologically, a well-circumscribed mass with lobular structures was present in the
subcutis. Most of the mass was occupied by extensive coagulative necrosis of neoplastic
cells with relatively uniform acinar and ductal structures. Although each necrotic acinar
structure was separated by reticular fibers, periacinar stromal collagen fibers were not
abundant. Considering the site of occurrence and histological features, the necrotic
tissue was diagnosed as adenoma of the mammary gland. The necrotic region lacked
hemorrhage and obvious inflammatory cell infiltration, indicating the necrosis was caused
by infarction. Although multiple necrosis and focal infarction are occasionally observed
in large-sized tumors in rodents, especially in adenocarcinomas, the present case was
characteristic, with the massive infarction involving most parts of the tumor despite the
relatively small size and low atypia of neoplastic cells. This is a rare case of
spontaneous infarcted adenoma of the mammary gland in rats histologically resembling human
cases.