Lymphangiosarcoma is an extremely rare tumour in dogs with only 16 cases reported in the literature. Lymphoedema, whichmaybe primary due to defects in the lymphatic system, or secondary to various other pathologies, often precedes malignancy. Of the 16 canine reports, only 1 dog was confirmed as having had prior primary lymphoedema due to aplasia of the popliteal lymph nodes. A case of lymphangiosarcoma is described in a 3.5-year-old purebred, Bullmastiff bitch which presented with vaginal blood 'spotting' for 3 weeks after cessation of oestrus, during which intromission by the male had been unsuccessful. During ovariohysterectomy a large multicystic, proliferative, spongy, fluid-filled, brownish-red mass surrounding the cervix and projecting into the abdominal space was removed with the cervix, and a diagnosis of lymphangiosarcoma made on histological and electron microscopic examination of the tissue. Ultrastructurally, no basement membrane or pericytes were found, only some of the neoplastic endothelial cells were linked by tight junctions while there were gaps between others, and neither micropinocytotic vesicles nor Weibel-Palade bodies occurred in the cells examined.Very few of the endothelial cells lining the many interlinking, tortuous maze of channels, stained slightly positive immunohistochemically for factor VIII-related antigen. The channels were filled mostly with serous fluid, and occasionally mixed leucocytes and some erythrocytes. The endothelium was often associated with underlying blocks of collagenous material, as well as looselyarranged aggregates of lymphocytes, other mononuclear cells and occasional neutrophils in the connective tissue septae and more prominently perivascularly. The bitch was discharged on antibiotic treatment but returned 2 weeks later with apparent prolapsed vagina which failed to reduce over the next week. Laparotomy revealed the tumour to have spread extensively in the caudal abdomen to involve the broad ligament and the ventral rectal serosa, and the 'prolapsed' tissue was found to be expanded vaginal wall. The bitch was euthanased and necropsied, Histological examination confirmed lymphangiosarcomatous invasion of the submucosal and muscular layers of the retroperitoneal, traumatised, prolapsed part of the vagina, the urethra and the ventral rectal wall. The broad ligament was diffusely invaded with tumour which had proliferated into the caudal abdominal space, and 3 small intra-trabecular foci of tumour were found in the right popliteal lymph node near the hilus. Mitotic figures were generally scarce. There was mild subcutaneous oedema of the ventral trunk extending from the axillae to the inner proximal thighs, which had not been evident clinically, and the lymph nodes (peripheral more so than internal) microscopically showed marked trabecular and perivascular fibrosis especially in hilar regions. Other congenital defects were hepatic capsular and central venous fibrosis with lymphatic duplication and dilatation in all areas of connective tissue, ventrally-incongruou...

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