Two Cases of Paraprostatic Cysts in Castrated Male Dogs

Introduction

Two castrated male dogs presented to the University of Minnesota Veterinary Medical Center for evaluation of tenesmus. Diagnosis of paraprostatic cysts was made in both dogs based on the results of abdominal ultrasonography, MRI (case 2 only), and histopathology of tissues obtained during the exploratory laparotomies. In both cases, the cysts were drained and debulked, and omentalization was performed. Tenesmus resolved in both cases. In case 1, a prostatic abscess developed 1 mo after the original surgery, which was drained under ultrasound guidance and injected with 180 mg (1.8 mL) of 10% gentamicin^a. This dog was euthanized 7 mo later for reasons unrelated to prostatic disease. In case 2, the dog developed a prostatic cystadenocarcinoma and an abdominal wall adenocarcinoma 1 yr after the initial surgery. A second surgery was performed to debulk both masses. Eight months later, recurrence of the abdominal wall mass was noted, and the dog was euthanized. A paraprostatic cyst should be considered in the differential diagnoses for an abnormal prostate in a castrated male dog.

Case Report

Case 2

An 11 yr old castrated male standard poodle presented for evaluation of tenesmus, diarrhea, hematochezia, and hyporexia. The dog had been castrated 2 yr previously. A 3 cm, soft, fluctuant, smooth, nonpainful mass associated with the prostate gland was palpated via digital rectal examination. Results of a serum biochemistry panel were unremarkable. Abdominal ultrasonography revealed a 3 cm, round, echogenic structure dorsal to the prostate. MRI revealed a 3 cm, right-sided, intrapelvic, paraprostatic cyst and a <1 cm, left-sided, intrapelvic paraprostatic cyst (Figures 1, 2).

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During an exploratory laparotomy, both cysts were drained and debulked, and omentalization was performed as previously described.¹ Histopathology of both cysts revealed a layer of well-differentiated, vacuolated, cuboidal to columnar epithelial cells consistent with paraprostatic cysts. In addition, vessels in the wall of the cysts contained small cells believed to be either hypertrophied endothelial cells or neoplastic epithelial cells. Subsequent immunohistochemical staining revealed a lack of cytokeratin staining, consistent with hypertrophied endothelial cells. Aerobic and anaerobic culture and sensitivity of fluid from both cysts showed no bacterial growth. The tenesmus and hematochezia resolved within 2 wk of surgery.

Approximately 1 yr later, digital rectal examination identified a prostatic mass. In addition, a 4 cm mass in the ventral abdominal wall just cranial to the pubic rim was found via abdominal palpation. No tenesmus or hematochezia was reported. Abdominal ultrasonography revealed a 3 cm, thick-walled, cavitated structure containing echogenic material dorsal to the prostate in approximately the same area as the paraprostatic cyst previously diagnosed. Exploratory laparotomy revealed a 4 cm, firm, round, nodular mass infiltrating the abdominal musculature cranial to, and attached to, the pubic brim, and an approximately 3 cm, round cyst attached to the prostate gland by a small stalk. The abdominal mass was debulked and the cystic structure was drained and debulked, and omentalization was performed as in the previous surgery.

Histopathology of the prostatic mass revealed a cyst intermittently lined by pleomorphic papillae lined with epithelium, which was consistent with a papillary cystadenocarcinoma. Histopathology of the abdominal wall mass was consistent with an adenocarcinoma.

Eight months after the second surgery, the dog was examined for recurrence of a firm abdominal wall mass just cranial to the pubic rim. No tenesmus or hematochezia was reported. Based on the mass’ proximity to the original abdominal wall mass, recurrence of adenocarcinoma was presumed. Abdominal ultrasonography and biopsy of the mass were recommended, but declined by the owners. The dog was humanely euthanized.

Discussion

Paraprostatic cysts are uncommon and are usually seen in older, large breed dogs.^2,3 Their previously reported incidence ranges from 2.6% to 5.3% of dogs with prostatic disease. All reported cases were intact male dogs.⁴ A more recent study showed the incidence to be 1.1% of 177 dogs with prostatic disease.⁵ The etiology of paraprostatic cysts is poorly understood. They are believed to arise from dilated portions of the uterus masculinus, a remnant of the Müllerian duct.^6,7 As the cyst grows, it remains associated with the prostate gland. Affected animals may be asymptomatic in the early stages, but as the cyst continues to grow, clinical signs may develop. The most commonly reported clinical signs include tenesmus, abdominal distension, dysuria, and/or hematuria.^3,4,7

To the authors’ knowledge, the two cases presented in this report are the first documented cases of paraprostatic cysts developing postcastration in male dogs. There are two other reports of paraprostatic cysts in castrated male dogs. In those cases, while the dogs were castrated at the time of diagnosis of the paraprostatic cysts, castration had been performed only one and three months prior to referral, respectively.^3,8 It seems unlikely that the cysts would have developed in such a short amount of time and grown large enough to cause the clinical signs that were described in those reports. It is possible that the paraprostatic cysts were testosterone-dependent and therefore developed while the dogs were intact, and not after castration. In addition, there is a single case report of a paraprostatic cyst occurring in a castrated male cat.⁹

The cysts reported in both of the cases in this report were not diagnosed until at least 2 yr after castration. It is possible that the cysts developed while the dogs were intact, but the size of the cysts was insufficient to lead to appreciable clinical signs. The dogs in this report were castrated at late ages (4.5 yr and 9 yr). Because the development of paraprostatic cysts can be stimulated by testosterone, it seems unlikely that a cyst would have grown so slowly prior to castration without causing some degree of clinical dysfunction.

Diagnosis of a paraprostatic cyst is multimodal. Digital rectal examination may reveal a soft, fluid-filled structure in the caudal abdomen. Multiple imaging modalities are available to aid in definitive diagnosis. Survey abdominal radiography with or without contrast cystography will often reveal a round structure of soft-tissue or mineralized opacity dorsal to, and possibly displacing, the urinary bladder.^{3,4,7,8,10,11} Abdominal ultrasonography can also be used to assess the prostate gland and any abnormal abdominal structures. Two studies have evaluated the ultrasonographic features of paraprostatic cysts.^12,13 In one study, the cyst contents were anechoic in 67% (6/9) of dogs, mixed echogenicity in 22% (2/9) of dogs, and echogenic in 1% (1/9) of dogs.¹² In the other study, the cystic contents of two dogs with paraprostatic cysts were hypoechoic to anechoic.¹³

There is a single reported case of a paraprostatic cyst contributing to the development of bilateral perineal hernias.² In that case, both prolonged rectal tenesmus and pressure on the pelvic diaphragm were believed to have contributed to hernia formation. Infection of paraprostatic cysts is uncommon because the cysts do not communicate with the urogenital tract.³ In one study, the cystic contents of 12 dogs with paraprostatic cysts were sterile, as in the two cases reported here.⁴

Of interest is the diagnosis of abdominal wall adenocarcinoma and prostatic papillary cystadenocarcinoma in case 2 diagnosed 1 yr after the original surgery. To the authors’ knowledge, there are no reported cases of sterile paraprostatic cysts giving rise to malignant neoplasia. The original histopathology report in case 2 noted the presence of small cells believed to be either hypertrophied endothelial cells or neoplastic epithelial cells. Whether the immunohistochemistry findings either represented a false negative result or there was transformation from an incompletely excised sterile paraprostatic cyst to a papillary adenocarcinoma is unknown. The median survival time for dogs diagnosed with prostatic carcinoma is 0.7 mo without cyclooxygenase (COX) inhibitor treatment and 6.9 mo with COX inhibitor treatment.¹⁴ The dog in case 2 did not receive a COX inhibitor during the 1yr period after the first surgery. Therefore, one would expect that progression of the disease would have occurred faster if the original sterile paraprostatic cyst was instead a neoplasm.

The preferred surgical therapy for a paraprostatic cyst includes exploratory laparotomy with cyst drainage, debulking, and either omentalization or marsupialization.¹ Castration is recommended because prostatic secretion is regulated by 5α-dihydrotestosterone, a metabolite of testosterone.¹⁵ If a paraprostatic cyst arises due to a dilation of the uterus masculinus secondary to prostatic secretions, removing the causative agent of the secretions (5α-dihydrotestosterone and testosterone) should prevent the cysts from developing. If surgery is not elected, ultrasound- guided percutaneous drainage of the cyst may be performed.^16,17

Conclusion

In this report, two cases were presented in which paraprostatic cysts apparently developed postcastration in male dogs. Paraprostatic cysts should be a differential when an abnormal prostate gland is found in a castrated male dog.