Retroperitoneal T-cell lymphoma with cystitis and megacolon in a young feline leukaemia virus-positive cat.

Retroperitoneal neoplasms are rare in both human and veterinary medicine, particularly in cats, where they account for less than 0.1 % of all neoplasms. This report describes a rare case of retroperitoneal T-cell lymphoma in a 7-month-old, feline leukaemia virus (FeLV)-positive Domestic Shorthair cat, leading to urethral and intestinal obstruction and subsequently cystitis and megacolon, respectively. The cat presented with clinical signs of weight loss, vomiting, tenesmus and urinary retention, and was diagnosed with a retroperitoneal mass compressing the colon and the urinary bladder. Due to the poor prognosis, the cat was euthanized during an exploratory laparoscopy and submitted for post-mortem examination. At necropsy, there was a large retroperitoneal, white, soft-to-firm, irregular mass that encompassed and compressed the rectum, colon, urethra and urinary bladder neck, along with megacolon and haemorrhagic cystitis. Histologically, the mass consisted of sheets of medium-sized neoplastic lymphocytes with mild anisocytosis and anisokaryosis. In the urinary bladder, a necrohaemorrhagic cystitis was observed. The cytoplasm of the neoplastic lymphocytes was positive by immunolabelling for CD3; therefore, the neoplasm was classified as a lymphoblastic T-cell lymphoma. Additionally, multifocal cytoplasmic FeLV immunolabelling was observed within the neoplastic cells, suggesting a possible viral involvement in the case's oncogenesis. This is the first reported case of a possibly FeLV-related retroperitoneal lymphoma in a cat. It had both intestinal and urinary tract obstructions as clinical outcomes. Our findings highlight retroperitoneal lymphoma as a new differential diagnosis for cats with lower urinary tract and gastrointestinal tract obstructions.