An uncharacteristically aggressive subcutaneous mast cell tumour with intra-articular invasion in a dog.

The present report describes a highly aggressive subcutaneous mast cell tumour (SCMCT) with intra-articular invasion in an 8-year-old female spayed Australian silky terrier. The dog presented for staging and surgical excision of a rapidly growing tumour on the craniomedial aspect of the right stifle. Ultrasonography revealed the tumour had obliterated normal anatomy within the stifle joint, and staging tests revealed metastasis to the right medial iliac node with neoplastic cells exhibiting multiple features of malignancy. Histopathological evaluation revealed a SCMCT with infiltrative morphological pattern, presence of multinucleation and mitotic count > 4, in addition to other features of malignancy. Local recurrence occurred within 120 days of surgery and survival time from date of diagnosis was approximately 160 days. This is a rarely encountered variant of SCMCT demonstrating its ability to behave much more aggressively than previously reported. [ABSTRACT FROM AUTHOR]