Urinary Obstruction Secondary to an Ossifying Fibroma of the Os Penis in a Dog

Case Report

A 13-year-old, 25-kg, castrated male border collie was referred to the Western College of Veterinary Medicine for a 4-week history of intermittent dribbling of urine, pollakiuria, stranguria, and dysuria. He was initially presented to the referring veterinarian for decreased appetite and apparent incontinence. Results of a complete blood count (CBC), serum biochemical panel, and urinalysis performed by the referring veterinarian were within reference ranges. The dog did not respond to a 7-day course of enrofloxacin^a (2.5 mg/kg per os q 12 hours) and was referred for further evaluation of his urinary tract signs.

On physical examination, the dog was bright and alert, with a thin body condition. Small amounts of urine dribbled intermittently from the urethra during the initial physical examination. A moderately large, soft, and nonpainful bladder was palpated. When voluntary urination was observed, the dog produced a fine, attenuated stream of urine and exhibited prolonged straining after voiding. After urination, the bladder was still palpably distended, confirming incomplete emptying. A 1.5-cm, firm swelling was palpable at the level of the bulbus glandis. Rectal examination and rectal palpation of the prostate were normal. A urine sample was collected by cystocentesis and submitted for urinalysis, aerobic culture, and susceptibility testing. A 10-French polyvinyl feeding tube^b passed without difficulty through the urethra into the urinary bladder. The urinalysis was within reference ranges, and there was no bacterial growth on aerobic urine culture.

On lateral survey abdominal radiographs, a 1.5-cm diameter, discrete calcified mass was observed at the base of the os penis in the region of the bulbus glandis [Figure 1]. On oblique projections, the mass appeared to be closely associated with the os penis; however, direct involvement of the os penis could not be confirmed. The base of the os penis appeared to be enlarged and altered in shape immediately cranial to the calcified mass.

Percutaneous fine-needle aspirates of the mass were performed without sedation. Large clusters of spindle-shaped cells with round to oval nuclei and a coarse chromatin pattern were observed cytopathologically. A homogeneous magenta matrix surrounded the cells. The characteristics of these cells were considered most consistent with a benign mesenchymal tumor. The possibility of an obstructive urological emergency was discussed with the owner. Biopsy of the mass was recommended, but the owner declined further diagnostics at that time.

The dog was presented to the hospital 9 days later, following an episode of dysuria and inability to urinate. A 10-French polyvinyl feeding tube^b was passed, with some difficulty, through the urethra and into the bladder. Mild resistance was noted as the feeding tube was passed through the penile urethra in the region of the mass at the base of the os penis. Lateral and ventrodorsal abdominal radiographs were repeated, and no change in the size or shape of the calcified mass was observed. On retrograde contrast urethrocystogram, the urethral lumen deviated ventrally and to the right of the calcified mass [Figure 2]. The mass did not appear to directly invade the urethra; but the urethra was narrowed adjacent to the mass and dilated proximally. These findings confirmed the mass was compressing the urethral lumen and obstructing urine outflow.

On ultrasonographic evaluation, the mass measured 2.5 × 1.7 cm. The mass appeared discrete, contained multiple calcified areas, and appeared to be separate from the os penis. The bladder and prostate gland appeared normal, and no abdominal metastases were detected. There was no evidence of metastatic disease on three standard radiographic views of the thorax. Differential diagnoses for the periurethral mass included dystrophic calcification of the bulbus glandis, calcifying hematoma, or a mesenchymal neoplasm originating from the os penis such as a fibroma, fibrosarcoma, osteoma, or osteosarcoma.

Because the dog continued to have difficulty urinating and a definitive diagnosis for the mass had not been determined, excisional biopsy of the mass was recommended. The owners were prepared for the possibility that the dog might require penile amputation and scrotal urethrostomy if the mass was found at surgery to be highly invasive or if malignancy was considered probable based on intraoperative cytopathology.

A preoperative CBC and serum biochemical panel were within reference ranges. An 8-French polypropylene urinary catheter^c was advanced into the urinary bladder prior to surgery to facilitate intraoperative identification of the urethra. With the dog in dorsal recumbency, an 8-cm midline incision was made over the caudal penis and bulbus glandis. The retractor penis muscle, ventral artery and vein of the penis, and urethra were identified and retracted to the left side of the mass. The tunica albuginea was incised. Blunt and sharp dissection was used to resect the mass from the surrounding corpus spongiosum and tunica albuginea. The mass was attached to the caudal-most aspect of the os penis. Rongeurs were used to remove the mass and the caudal 1 cm of the os penis attached to the mass. All visible portions of the mass were removed. No visibly normal tissue surrounding the mass was resected because of the close proximity to the urethra. Intraoperative hemorrhage was minimal.

The mass was 3 × 2 cm in diameter, oval shaped, and firm. A population of individual spindle-shaped cells containing scant to moderate amounts of basophilic cytoplasm, small eosinophilic granules, and round to oval nuclei was observed on cytopathological evaluation of impression smears made from a cut surface of the mass that was obtained while the surgery was still in progress. Few mitotic figures, and mild anisocytosis and anisokaryosis were noted. The cytopathological findings were considered most consistent with a benign mesenchymal neoplasm but were not definitive. The decision was made to wait for a definitive diagnosis before considering more radical surgical resection. The tunica albuginea, subcutaneous, and subcuticular tissues were closed in layers.

The mass was fixed in 10% neutral buffered formalin, processed routinely, embedded in paraffin, sectioned at 4 to 5 μm, and stained with hematoxylin and eosin and with Masson trichrome. Microscopic evaluation of the mass revealed the periphery to be composed of generally isomorphic, densely packed, spindle-shaped cells arranged in interwoven bundles, often in a herringbone pattern. The cells were surrounded by a small amount of fibrillar extracellular matrix that stained blue with Masson trichrome stain. These features were consistent with collagen. The cells were interpreted to be neoplastic fibroblasts [Figures 3A, 3B]. There was no recognizable periosteal membrane bordering the lesion.

Toward the center of the mass, the fibroblastic cells merged with a less cellularly dense tissue composed of round to oval cells within lacunae and surrounded by a deeply eosinophilic, extracellular matrix interpreted to be osteoid. The extracellular matrix was birefringent under polarized light and appeared fibrillar and haphazardly arranged. This tissue was interpreted to be woven or immature bone. Plump cells at the interface between the fibrous tissue and the woven bone were interpreted to be osteoblasts [Figure 3C].

In several areas, large, multinucleated osteoclasts were located along segments of woven bone. Rows of smaller osteoblasts were present between thin seams of lamellar bone and the surfaces of the woven bone. This finding was interpreted to be consistent with remodeling of the bone. Within foci of remodeled or mature bone, there was a loose connective tissue infiltrated with hematopoietic cells, including megakaryocytes, typical of normal bone marrow [Figure 3D]. Based on these findings, the mass was diagnosed as an ossifying fibroma of the os penis. Neoplastic cells extended to the periphery of all edges of the tumor. Complete resection of all neoplastic cells was considered highly unlikely in this case, given that the mass had been removed from the base of the os penis with rongeurs.

The dog recovered well postoperatively. Postoperative analgesia was provided by a single epidural injection of morphine^d (0.1 mg/kg) administered prior to recovery from anesthesia. Minimal swelling and bruising developed at the surgical site, and a larger stream of urine was observed after recovery from anesthesia. Penile amputation and scrotal urethrostomy were recommended if the tumor recurred.

The dog continued to urinate normally for 17 months postoperatively. Nineteen months after surgery, the dog presented for reevaluation because the owner noticed that the dog’s urine stream had diminished over a 2-month period. A CBC, serum biochemical panel, and urinalysis were within reference ranges. A 1.5-cm, hard mass was palpable and firmly adhered to the base of the os penis under the old surgical incision. For financial reasons, the owner declined recommended preoperative radiographic and cytopathological evaluations of the mass.

The site was surgically explored. The mass was found to be intimately associated with the urethra at the caudal aspect of the os penis. Excision of the mass with maintenance of an intact urethra was not possible. The owner gave consent for a scrotal urethrostomy to be performed, but declined complete resection of the mass since the latter procedure would have necessitated penile amputation. The owner did not allow another biopsy of the mass. The dog recovered well after scrotal urethrostomy and urinated a steady stream of urine without difficulty. Seventeen months after the second surgery, the dog continues to produce a solid stream of urine without difficulty. According to the owner, the mass at the base of the os penis is not painful and continues to slowly enlarge.

Discussion

Pathological conditions of the os penis in dogs are rare, with fractures of the os penis and penile hematomas being the most common. Urethral obstruction can occur secondary to callus formation after fracture of the os penis.¹ Urinary obstruction resulting from an organized hematoma of the caudal os penis has been reported in a 6-year-old, intact male Boston terrier with a clinical history very similar to the dog in this case.²

Transmissible venereal tumor is the most common penile tumor in the dog.³ Other reported genital neoplasms include squamous cell carcinoma of the penile, preputial, and urethral mucosa;¹³ papilloma¹³ and hemangiosarcoma of the prepuce; and mast cell tumor³ and lymphosarcoma of the penis.⁴ A chondrosarcoma of the os penis was reported in an 8-year-old, castrated male Great Dane with a 2-week history of incontinence and dysuria.⁵ To the authors’ knowledge, the case reported here is only the second published report of a tumor of the os penis in a dog. This report also documents the first reported case of an ossifying fibroma in a dog in a location other than the mandible or maxilla.

An ossifying fibroma is a relatively rare, benign, fibro-osseous tumor that is classically a tumor of the skull, and it most frequently involves the mandible or maxilla of horses, cattle, and humans.^6–10 Ossifying fibromas of the mandible and maxilla have been reported in other species including a greater kudu,¹¹ a llama,¹² an Anglo-Nubian goat,⁹ a cat,⁶ and four dogs.¹³ Ossifying fibromas in the jaw are usually solitary expansile lesions, which destroy preexisting bone structure and replace alveolar and cortical bone. They can cause loss of teeth and malocclusion, interfere with mastication, and predispose to pathological fracture.⁶⁷

Ossifying fibromas in sites other than the skull of domestic animals have been reported, but they are very rare.¹⁴ Little is known about the behavior of ossifying fibromas because of the paucity of reported cases. They are typically solitary, slow-growing lesions.¹⁵ Malignant transformation or metastasis has not been reported.⁸⁹ The pathological consequences of ossifying fibromas appear to be limited to the expansile distortion of normal bone and the resultant effects on the structure and function of surrounding tissues. Depending on their location, ossifying fibromas can cause compression of physiologically important structures such as the urethra, as seen in this case, or they may remain clinically silent.⁶⁷ Local recurrence is common if surgical excision is incomplete.⁹¹⁵

Radiographically, ossifying fibromas are usually well-demarcated, solitary lesions of moderate radiodensity.⁷ Expansion of the normal contour of the affected bone is typical.⁷ Histopathologically, an ossifying fibroma is formed by osseous metaplasia of a fibroma in which fibroblasts undergo transformation into osteoblasts.¹⁶ These osteoblasts line irregularly arranged spicules of primarily woven bone.⁷⁸ This rimming of bone spicules with osteoblasts is characteristic of ossifying fibromas and distinguishes this tumor from other fibro-osseous lesions.¹⁵ Ossification occurs randomly throughout the expansile lesion. Evidence of a few large, irregular, bony trabeculae undergoing osteoclastic resorption may be found.⁷ The presence of lamellar bone is reportedly rare but may be seen in an ossifying fibroma when trabeculae of woven bone are resorbed and replaced by lamellar bone.⁷⁸¹⁵ A periosteal membrane does not typically border the developing lesion.⁷

Ossifying fibromas are histopathologically distinguished from other fibro-osseous lesions such as fibrous dysplasia, osteoma, and osteosarcoma. Fibrous dysplasia is distinguished by whorling, loose fibrous connective tissue and proliferation of bony spicules of woven bone that are not rimmed by osteoblasts.⁹¹⁴¹⁶ Osteomas are distinguished by a predominance of lamellar bone, the presence of marrow between the trabeculae, and less connective tissue than ossifying fibromas.⁸⁹ In osteosarcoma, the neoplastic cells are more pleomorphic and hyperchromic and are characterized by a higher mitotic index.⁸⁹ Well-structured bone is also not typical of an osteosarcoma.⁸

The presence of osteoblasts lining spicules of woven bone and the presence of dense, fibrous tissue undergoing osseous metaplasia distinguished this tumor from fibrous dysplasia and osteoma. Additionally, the absence of a periosteal membrane bordering the lesion distinguished this tumor from an osteoma.

A significant component of lamellar bone and bone marrow were also present in this lesion. As bone is not an inert tissue but normally undergoes continual remodeling, it is reasonable to assume that remodeling of woven bone and replacement by lamellar bone occurs in ossifying fibromas.⁸ An ossifying fibroma undergoing remodeling could resemble an osteoma, in that it would contain a significant component of lamellar bone and hematopoietic tissue between bony trabeculae.⁸

The high frequency of local recurrence after incomplete surgical excision of ossifying fibromas in young horses and in humans suggests the tumor could behave similarly in other species. Too few cases of ossifying fibroma have been reported in the dog to predict how much unaffected tissue surrounding the tumor should be resected to prevent local recurrence. Initial surgical excision for histopathological confirmation, the location of the ossifying fibroma, and the potential impact on surrounding tissues all affect the decision on how much surrounding tissue can be resected in each individual case. In the case of the ossifying fibroma of the os penis presented here, an excisional biopsy was performed. The proximity of the mass to the urethra made wide surgical resection impossible. A penile amputation and scrotal urethrostomy were the only options that would have ensured complete excision of the ossifying fibroma.

Regrowth of a firm mass at the base of the os penis and recurrence of clinical signs in this dog were consistent with presumed recurrence of the ossifying fibroma. The behavior of this tumor was typical of an ossifying fibroma, in that it recurred in the absence of wide surgical resection, was slow growing, and produced only localized effects (distorted the structure of the os penis and the function of the adjacent urethra).

Conclusion

This case report is unique in several respects. It represents a very rare cause of urinary obstruction in the male dog. It is only the second case of a tumor of the os penis reported in the dog (the other case was a chondrosarcoma), and it is the first reported case of an ossifying fibroma outside of the mandible and maxilla in the dog.

Acknowledgments

The authors thank Drs. Saundra Hewitt, Carl Porter, and India F. Lane for their assistance.