Congenital myofibroblastic skin tumours in a newborn piglet resembling the multicentric form of infantile myofibromatosis

A 2-day-old female piglet was submitted with multiple congenital, nodular skin masses located on the head, neck, trunk and legs. Histopathological examination revealed the presence of nodular, cutaneous tumours with a biphasic growth pattern and comprising a population of undifferentiated, oval or slightly polygonal, frequently perivascularly located cells and a population of spindle-shaped, fibroblast-like cells arranged in bundles. Multifocally, tumour cells infiltrated subcutaneous adipose and muscular tissue. Immunohistochemically, the undifferentiated tumour cells expressed vimentin and calponin, whereas the spindle-shaped tumour cells were positive for vimentin, α-smooth muscle actin and calponin. Based on these findings, the diagnosis was myofibroblastic tumours closely resembling the multicentric form of human infantile myofibromatosis.