History

A 10-year-old castrated mixed-breed dog was examined at the University of Minnesota Veterinary Medical Center because of a 2-month history of frequent posturing to urinate; the posturing was maintained for extended durations. The dog roamed a fairly large area, but the owners were unaware of any specific recent episodes of trauma and had not noticed a change in urine color or urinary incontinence. The owners reported that the dog licked his prepuce often; however, the frequency of this behavior had not changed. There had been no tenesmus or change in size or consistency of feces passed.

The dog was current on vaccinations. He received heartworm preventative but was receiving no other medications at the time of admission. The dog had a history of seizures but, to the owners' knowledge, had not had a seizure in years.

Physical examination revealed that the dog weighed 24.5 kg (54 lb) and had a body condition score of 3 (scale of 1 to 5). Rectal temperature, pulse rate, and respiratory rate of the dog were all within the respective reference ranges. There was a small laceration on the left flank. A firm, irregular mass was palpable on the proximal portion of the penile shaft. All other findings were unremarkable.

Results of a CBC were within reference ranges; however, 1+ reactive lymphocytes were detected. Results of serum biochemical analysis were within reference ranges, and results of urinalysis performed on a sample of urine collected via a urinary catheter were also within reference ranges.

Examination of thoracic radiographs revealed that the thorax was normal. However, examination of a radiograph of the caudal portion of the abdomen revealed an abnormality (Figure 1).

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Lateral radiographic view of the caudal portion of the abdomen of a 10-year-old castrated mixed-breed dog examined because of a 2-month history of frequent posturing to urinate.

Citation: Journal of the American Veterinary Medical Association 230, 2; 10.2460/javma.230.2.197

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Question

What are the 3 most likely differential diagnoses for the abnormality visible on the radiograph? Please turn the page.

Answer

Neoplasia of the os penis, fracture of the os penis, and dystrophic mineralization.

Results

Examination of radiographs of the caudal portion of the abdomen revealed evidence of a radiopaque structure in the area of the penile mass (Figure 2). The owners opted for excision of the penile mass in an effort to promote natural urine flow and prevent development of uremia. Penile amputation and urethrostomy in the area cranial to the scrotum were performed. The excised tissues were submitted for histologic assessment. A benign mesenchyoma was diagnosed. Examination of the surgical margins revealed that the entire tumor had been excised.

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Same view as in Figure 1. Notice the radiopaque structure in the penile area (arrow), which corresponded to a firm, irregular penile mass.

Citation: Journal of the American Veterinary Medical Association 230, 2; 10.2460/javma.230.2.197

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Discussion

Neoplasms of the penile soft tissues in dogs include transmissible venereal tumor, squamous cell carcinoma, lymphosarcoma, adenocarcinoma, and mast cell tumor.^1,2 To our knowledge, the only reported neoplasms of the os penis in dogs are mesenchymal chondrosarcoma, fibroma (with ossification), and osteosarcoma.^3–5 All of the dogs in those case reports had a primary clinical sign of dysuria.

Fracture of the os penis is uncommon in dogs, with a reported incidence of 2% in 1 study.¹ There may be a history of trauma, but often the cause of the fracture is unknown. Acute fracture of the os penis is associated with variable signs, depending on the type of fracture and extent of soft tissue injury. Clinical signs include dysuria and hematuria, evidence of pain and crepitus during manipulation of the penis, distention of the urinary bladder, and evidence of abdominal pain.

When there is nonunion of a penile fracture or excessive callus or scar tissue formed at the fracture site, affected dogs may develop clinical signs consistent with urethral obstruction months to years after the initial injury.⁶ Clinical signs may include dysuria, anuria, and evidence of abdominal pain. Physical examination may reveal a palpable mass on the penis and distention of the urinary bladder. Signs of uremia may be evident. It is often not possible to pass a urinary catheter, or passage of a urinary catheter is possible but associated with a grating sensation. Postrenal azotemia often is identified during serum biochemical analysis.

Other rare but possible causes of dysuria and a palpable penile mass exist. This includes mucosal leishmaniasis, which can be detected by use of serologic tests or histologic examination of biopsy specimens from the mass.⁷ In addition, granuloma formation secondary to parasite migration has also been reported⁸ as a cause of a penile mass.

Outcome

The dog developed cystitis several weeks after surgery, which resolved with appropriate antimicrobial treatment. As of 7 months after surgery, cystitis had not recurred. The owners' only complaint was that the dog continued to lick his groin region. However, that behavior was apparent before development of the tumor and had not increased in frequency or duration.