Penile and preputial squamous cell carcinoma in the horse: a retrospective study of treatment of 77 affected horses.

Reasons for performing study: The most common penile and preputial neoplasm in the horse is the squamous cell carcinoma (SCC), but no large surveys of treatment and effects of the grade of the tumour, based on the degree of differentiation, on outcome of affected horses are available. Objectives: Analysis of treatment of male horses affected with SCC of the external genitalia and long-term results of treatment. Methods: Seventy-seven cases of SCC were evaluated. Data recorded included treatment, outcome, post operative histopathology and retrospective tumour grading. Results: Treatments included: cryosurgery, excision, partial phallectomy, partial phallectomy and sheath ablation, and en bloc penile and preputial resection with penile retroversion and removal of inguinal lymph nodes. The incidence of recurrence after partial phallectomy was 25.6% (10/39) and following incomplete removal was 17.9% (7/39). The incidence of recurrence after en bloc resection with retroversion was 12.5% (1/8). In horses with confirmed inguinal lymph node metastasis, the incidence of recurrence was 25.0% (1/4). Poorly differentiated SCCs were more likely to metastasise than well differentiated SCCs, and there was a greater chance that the treatment would be unsuccessful. The success of treatment, complete removal and in preventing recurrence of the tumour, of male horses with SCC of the external genitalia was 55.7%. Conclusions: Horses that receive only partial phallectomy for treatment for SCC of the external genitalia have a high incidence of recurrence in contrast to horses that receive an en bloc resection. Tumour grading of SCC can help predict prognosis and guide selection of treatment.