Primary Peritoneal Perivesical Hemangiosarcoma in 16 Dogs (2020–2024)

Background Canine hemangiosarcoma (HSA) commonly arises in the spleen, heart, and skin, whereas primary peritoneal presentations are rare and poorly characterized. A perivesical peritoneal location without involvement of adjacent organs has not been previously well described. Objective This study aims to characterize the clinical presentation, diagnostic features, treatment, and outcomes of dogs with primary peritoneal perivesical HSA. Animals This study included 16 client‐owned dogs diagnosed with peritoneal perivesical HSA at 2 referral centres between 2020 and 2024. Methods Methods consisted of retrospective multicentre case series. Medical records were reviewed for clinical signs, laboratory abnormalities, imaging findings, diagnostic procedures, treatment, metastatic status, and outcome measures, including progression‐free survival (PFS) and disease‐specific survival time (DSST). Results Common presenting signs included lethargy, hyporexia, pale mucous membranes, and abdominal distension. Hemoperitoneum was identified in 50% of the dogs. Frequent laboratory abnormalities included anaemia, thrombocytopenia, neutrophilia, and azotaemia. All dogs had a mass adjacent to the urinary bladder without evidence of origin from surrounding organs. Diagnosis was achieved by cytology or histopathology. Metastatic disease was suspected at diagnosis in 56.25% of the dogs, most commonly affecting the lungs, and the overall metastatic rate was 87.5%. Treatment protocols included chemotherapy alone ( n = 8), surgery with adjuvant chemotherapy ( n = 7), and neoadjuvant chemotherapy followed by surgery ( n = 1). All surgical procedures consisted of debulking surgery. Chemotherapy alone resulted in a 55.55% partial response rate. Mean PFS and DSST were both 125 days. Combination therapy, response to chemotherapy, and rescue treatment were significantly associated with improved DSST. Conclusions Primary peritoneal perivesical HSA may represent a distinct anatomical presentation of canine HSA. Multimodal treatment strategies may improve clinical outcomes and survival in affected dogs.