Examination of a horse’s external genitalia for evidence of neoplasms should begin with visual inspection and palpation of the skin of the prepuce and scrotum. Assessment of an apparent primary tumor should include recording its size, mobility, invasiveness, and location (i.e., whether it involves the urethra, corpus cavernosum penis, corpus spongiosum penis, or a combination). Examination of the penis itself and internal fold of the prepuce may be difficult in the nonsedated horse because the horse tends to keep these structures retracted within the preputial cavity. Partial penile protrusion usually occurs during urination and can be provoked by placing the horse in a bedded stall or by administering a diuretic. Adequate inspection and palpation of the penis can be accomplished after injection of acepromazine (0.04 to 0.1 mg/kg, IV), detomidine (0.01 mg/kg, IV), or xylazine (0.5 to 1 mg/kg, IV). For horses that resist penile protrusion and examination, administration of a combination of 0.02 to 0.05 mg/kg acepromazine and 0.2 to 0.3 mg/kg xylazine has been reported to be effective, with penile relaxation being more profound and longer lasting than from administration of xylazine alone. Acepromazine induces more profound relaxation of the retractor penis muscle, and although detomidine may be slightly less effective than acepromazine in promoting penile protrusion, it induces placidity of the horse more quickly. It should be emphasized that phenothiazine tranquilizers have been reported to cause paraphimosis, priapism, and penile paralysis. None of these adverse effects have been associated with use of detomidine.

Ultrasonographic examination of the penis and prepuce may be useful for grossly determining the extent of the tumor and the degree of involvement of various structures. Ultrasonographic examination is easily performed, and the soft tissue structures are well suited for evaluation by this diagnostic technique.

Preoperative histologic evaluation of the primary tumor is required to classify the tumor and design a treatment plan. A tissue sample can be obtained by fine-needle aspiration biopsy (FNAB) or by punch or excisional biopsy. Fine-needle aspiration biopsy can be used to identify cells with malignant features, but it is not a very reliable means of assessing SCCs because early neoplastic, hyperplastic, or dysplastic keratinocytes can appear similar cytologically. Because tumor architecture and depth of invasion can only be assessed with a larger sample, full-thickness biopsy of the lesion is more reliable diagnostically than FNAB. Independent of the type of tumor, the lesion can be so severe that complete excision is necessary in any case. In such cases, surgical removal or debulking can be combined with harvesting material for histologic examination.

Metastasis is a major concern in evaluation of horses with SCCs of the external genitalia. Tumor metastasis to the inguinal lymph nodes has been reported in 12.5% to 16.9% of male horses with genital SCC. Regional lymph nodes should therefore be palpated, but false-positive results can occur because penile neoplasms can become secondarily infected and lead to lymphadenopathy. Inguinal lymph node enlargement is detected in a relatively high proportion (34.1%) of horses with various penile and preputial tumor types, whereas metastases can be confirmed in only 32.1% of horses with enlarged inguinal lymph nodes. Palpation of the inguinal lymph nodes can be difficult in horses because of the extensive fat deposition in the inguinal region.

When a penile tumor metastasizes, the first lymph nodes affected are usually the superficial inguinal lymph nodes, followed by the medial iliac lymph nodes, which can be evaluated per rectum by palpation and by ultrasound. If enlarged lymph nodes are detected, samples collected by ultrasound-guided FNAB should be examined cytologically. Although identification of neoplastic cells is a reliable indicator of metastasis, failure to identify metastatic cells is less conclusive, and the absence of lymph node enlargement does not rule out metastasis. In 36% of cases in which lymph node enlargement could not be discerned, so-called occult metastasis was found after en bloc penile and preputial resection with penile retroversion or at postmor­tem histologic examination of the lymph nodes. Distant metastases, although rare, can sometimes be detected on lung radiographs.

Tumor Classification

Ideally, grading and staging of a tumor should permit accurate prediction of tumor behavior.