Prostatic Adenocarcinoma

Prostatic adenocarcinoma is a malignant neoplasm arising from the glandular epithelium of the prostate gland, predominantly affecting intact and castrated male dogs, and rarely cats. It is the most common primary prostatic malignancy in dogs, characterized by local invasion, regional lymph node metastasis, and distant spread to bones, lungs, and other organs. The disease is hormonally influenced, with androgens and estrogens playing roles in its pathogenesis, although castration does not prevent its development and may even increase risk. Clinically, it presents with lower urinary tract signs, tenesmus, and systemic signs of malignancy. Diagnosis requires histopathology, as imaging and cytology are often inconclusive. Prognosis is poor, with median survival times of a few months despite treatment.

The exact etiology of prostatic adenocarcinoma is unknown, but several factors are implicated. Hormonal influences are significant: androgens (testosterone and dihydrotestosterone) promote prostatic growth, and estrogens may contribute to squamous metaplasia and neoplastic transformation. Chronic inflammation (prostatitis) has been proposed as a risk factor, though a direct causal link is not established. Genetic mutations, including alterations in tumor suppressor genes (e.g., p53) and oncogenes (e.g., c-erbB-2), have been identified. Environmental factors, such as exposure to certain chemicals, may also play a role. In dogs, castration has been associated with an increased risk of prostatic adenocarcinoma, possibly due to altered hormonal milieu. No infectious agent has been consistently implicated.

Prostatic adenocarcinoma is primarily a disease of older male dogs, with a mean age of onset around 9-10 years. It occurs in both intact and castrated dogs, but castrated dogs may have a higher relative risk, contrary to benign prostatic hyperplasia which is androgen-dependent. Certain breeds may be predisposed, including medium to large breeds such as Boxers, Doberman Pinschers, Scottish Terriers, and German Shepherds, though any breed can be affected. It is rare in cats, with only sporadic case reports. No sex predilection exists beyond males, as females lack a prostate. The incidence is relatively low compared to benign prostatic conditions, but it is the most common malignant prostatic tumor in dogs. There is no known parity or breeding status influence.

Prostatic adenocarcinoma arises from the acinar or ductal epithelial cells of the prostate. The neoplastic cells exhibit uncontrolled proliferation, invasion into the prostatic stroma and capsule, and metastasis via lymphatic and hematogenous routes. Histologically, tumors may be well-differentiated (forming acini) or poorly differentiated (solid sheets of cells). Hormonal influences: androgens stimulate cell proliferation, while estrogens may induce squamous metaplasia and potentially malignant transformation. In castrated dogs, the lack of androgen feedback may lead to altered estrogen/testosterone ratios, promoting tumorigenesis. The tumor often causes urethral obstruction due to local invasion and enlargement, leading to dysuria and urinary retention. Metastasis commonly occurs to the iliac lymph nodes, lungs, bones (especially lumbar vertebrae and pelvis), and occasionally the liver and spleen. Paraneoplastic syndromes, such as hypercalcemia, may occur due to secretion of parathyroid hormone-related protein.

Intrinsic factors include advanced age (most common in dogs over 8 years), breed predisposition (e.g., Boxers, Dobermans, Scottish Terriers), and possibly genetic susceptibility. Castration is a significant risk factor; studies show that castrated dogs have a higher incidence of prostatic adenocarcinoma compared to intact dogs, possibly due to hormonal imbalances. Chronic prostatitis may predispose to malignant transformation, though evidence is limited. Extrinsic factors include environmental carcinogens, though specific agents are not well-defined. Obesity and diet may influence hormonal status, but no direct link is established. There is no evidence that breeding status or sexual activity affects risk.

Clinical signs are often insidious and may be absent in early stages. Common signs include stranguria, dysuria, hematuria, and urinary incontinence due to urethral compression or invasion. Tenesmus and constipation may occur from rectal compression. Systemic signs include weight loss, lethargy, anorexia, and fever. On physical examination, a firm, irregular, painful prostate may be palpated per rectum or via abdominal palpation. The prostate may be asymmetrically enlarged, and there may be pain on palpation. In advanced cases, signs of metastasis may be present, such as lameness (bone metastasis), respiratory distress (lung metastasis), or neurological deficits (spinal metastasis). Paraneoplastic hypercalcemia may cause polyuria, polydipsia, and weakness.

Differential diagnoses include: 1) Benign Prostatic Hyperplasia (BPH): common in intact older dogs, symmetrical enlargement, no metastasis, responsive to castration. 2) Prostatitis: acute or chronic bacterial infection, often with fever, leukocytosis, and painful prostate; responds to antibiotics. 3) Prostatic Cyst: fluid-filled cavity, may be paraprostatic, often associated with BPH; imaging shows anechoic structure. 4) Prostatic Abscess: severe infection with pus, systemic signs, ultrasonography shows cavitated lesions. 5) Transitional Cell Carcinoma (TCC) of the urethra/prostate: arises from urothelium, may involve prostate secondarily; cytology and histopathology differentiate. 6) Squamous Cell Carcinoma of the prostate: rare, associated with squamous metaplasia. 7) Lymphoma: systemic disease, may involve prostate; cytology shows lymphoblasts. 8) Leiomyosarcoma or other mesenchymal tumors: rare, histopathology differentiates. 9) Prostatic urethral polyp: benign, but can cause similar signs. 10) Granulomatous prostatitis: chronic inflammation, may mimic neoplasia on imaging.

1) Clinical triage: history, physical exam, rectal palpation. 2) Minimum database: CBC, serum biochemistry, urinalysis, urine culture. 3) Imaging: abdominal radiography (may show prostatomegaly, mineralization, or metastasis), thoracic radiography (for lung metastasis), abdominal ultrasonography (prostate size, echotexture, cysts, lymphadenopathy). 4) Prostatic sampling: fine-needle aspiration (FNA) for cytology, or biopsy (Tru-cut or surgical) for histopathology. FNA may be guided by ultrasound. 5) Staging: if neoplasia confirmed, consider CT or MRI for local invasion and metastasis, and bone scintigraphy if bone metastasis suspected. 6) Histopathology is definitive. 7) Additional tests: serum prostate-specific antigen (PSA) is not reliable in dogs; canine prostate-specific esterase (CPSE) may be elevated in prostatic disease but not specific for cancer.

Hematology: may show leukocytosis due to inflammation or stress, or normal. Biochemistry: hypercalcemia may be present (paraneoplastic), elevated alkaline phosphatase (ALP) due to bone metastasis or cholestasis, azotemia if urinary obstruction. Urinalysis: hematuria, pyuria, proteinuria; urine culture may be negative unless secondary infection. Prostatic fluid evaluation: if obtained, may show neoplastic cells on cytology. Serum hormone levels: testosterone may be low in castrated dogs, but not diagnostic. Tumor markers: CPSE may be elevated, but not specific. Histopathology: definitive diagnosis; grading may have prognostic value.

Abdominal radiography: may show an enlarged prostate, mineralization (dystrophic calcification), and loss of detail. Caudal abdominal mass effect. Thoracic radiography: detect pulmonary metastases (nodules). Ultrasonography: prostate may be asymmetrically enlarged, with mixed echogenicity, irregular margins, and areas of mineralization or cavitation. Iliac lymph nodes may be enlarged. Color Doppler may show increased vascularity. CT and MRI: provide detailed assessment of local invasion (urethra, bladder, rectum) and metastasis. CT is preferred for staging. Vaginoscopy is not applicable in males; urethrocystoscopy may be used to assess urethral involvement and obtain biopsies.

Fine-needle aspiration cytology: may show clusters of epithelial cells with anisocytosis, anisokaryosis, prominent nucleoli, and high nuclear-to-cytoplasmic ratio. However, cytology has low sensitivity and specificity; well-differentiated tumors may be difficult to distinguish from hyperplasia. Histopathology: biopsy reveals acinar or solid patterns of malignant epithelial cells, with invasion into stroma and capsule. Immunohistochemistry may be used to differentiate from TCC (e.g., positive for cytokeratin 7 and negative for uroplakin III). Special stains: mucicarmine may be positive in some adenocarcinomas. Grading systems (e.g., Gleason-like) may be applied, but prognostic value is limited.

Treatment is often palliative, as complete surgical excision is difficult due to local invasion. Options include: 1) Medical therapy: anti-androgens (e.g., finasteride 0.1-0.5 mg/kg PO q24h) may slow growth but not curative. Estrogens (e.g., diethylstilbestrol) are contraindicated due to side effects. Chemotherapy: mitoxantrone (5-6 mg/m² IV q21d) or carboplatin (300 mg/m² IV q21d) may provide partial responses. NSAIDs (e.g., piroxicam 0.3 mg/kg PO q24h) have shown some activity in TCC but less in prostatic adenocarcinoma. 2) Surgical: total prostatectomy is rarely feasible due to high morbidity (urinary incontinence). Debulking or cystostomy tube placement may relieve obstruction. 3) Radiation therapy: may provide palliation of pain and obstruction. 4) Supportive care: pain management (NSAIDs, opioids), antibiotics for secondary infections, and management of hypercalcemia (fluid therapy, furosemide, prednisone). 5) Stenting of the urethra may relieve obstruction. Prognosis is poor; median survival is 1-3 months without treatment, and up to 6-12 months with aggressive therapy.

Prognosis is grave. Median survival times are short: without treatment, 1-3 months; with chemotherapy or radiation, 6-12 months. Factors indicating worse prognosis include presence of metastasis at diagnosis, high histologic grade, and clinical signs of urinary obstruction. Paraneoplastic hypercalcemia is associated with poor outcome. Local recurrence is common after debulking. Quality of life is a major consideration; euthanasia is often elected due to progressive signs.

Follow-up should include regular physical examinations, including rectal palpation, every 1-3 months. Serial imaging (ultrasonography or CT) to assess tumor size and metastasis. Thoracic radiographs every 2-3 months to monitor lung metastasis. Serum biochemistry to monitor for hypercalcemia and renal function. If treated with chemotherapy, monitor CBC and biochemistry before each dose. Adjust pain management as needed. Discuss prognosis and quality of life with the owner.

Pearls: 1) Prostatic adenocarcinoma can occur in castrated dogs; do not rule out based on castration status. 2) Rectal palpation is essential; a firm, irregular, painful prostate warrants further investigation. 3) FNA may be non-diagnostic; biopsy is often needed. 4) Always stage for metastasis, especially to iliac lymph nodes and lungs. 5) Consider paraneoplastic hypercalcemia in any dog with prostatic mass and polyuria/polydipsia. Pitfalls: 1) Misdiagnosing as BPH or prostatitis, leading to delayed treatment. 2) Performing castration as treatment for suspected BPH without biopsy, which may worsen adenocarcinoma. 3) Overlooking metastasis on initial workup. 4) Using estrogen therapy, which can cause fatal bone marrow suppression. 5) Assuming that a negative urine culture rules out prostatic infection, as prostatic fluid culture is more sensitive.

1) Finasteride: 0.1-0.5 mg/kg PO q24h, used to reduce androgen-dependent growth, but limited efficacy in adenocarcinoma. 2) Piroxicam: 0.3 mg/kg PO q24h, may have anti-tumor effects via COX-2 inhibition; monitor for GI toxicity. 3) Mitoxantrone: 5-6 mg/m² IV q21d, for chemotherapy; monitor for myelosuppression. 4) Carboplatin: 300 mg/m² IV q21d, alternative; nephrotoxic, ensure adequate hydration. 5) For hypercalcemia: 0.9% NaCl IV at 60-100 ml/kg/day, furosemide 1-2 mg/kg IV q8-12h, prednisone 0.5-1 mg/kg PO q12h. 6) Pain management: NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h) or opioids (e.g., tramadol 2-5 mg/kg PO q8-12h). 7) Antibiotics for secondary urinary tract infection: choose based on culture and sensitivity, e.g., amoxicillin-clavulanate 13.75 mg/kg PO q12h. 8) Urethral stenting: interventional radiology procedure, not a drug. 9) Palliative radiation: not a drug protocol, but may be used. 10) Supportive care: antiemetics if chemotherapy-induced nausea (e.g., maropitant 1 mg/kg SC q24h).

Landmark studies: 1) Bell et al. (1991) reported that castration does not prevent prostatic adenocarcinoma and may increase risk. 2) LeRoy and Northrup (2009) reviewed prostatic carcinoma in dogs, noting poor prognosis and limited treatment options. 3) Sorenmo et al. (2004) evaluated chemotherapy with mitoxantrone and piroxicam, showing partial responses in some dogs. 4) A study by Bryan et al. (2007) found that castrated dogs had a higher risk of prostatic adenocarcinoma compared to intact dogs. 5) Consensus guidelines from the American College of Veterinary Internal Medicine (ACVIM) and the European Society of Veterinary Oncology (ESVONC) recommend histopathologic diagnosis and staging before treatment. 6) A retrospective study by Cornelis et al. (2018) reported median survival of 6 months with radiation therapy. 7) The use of COX-2 inhibitors is supported by studies showing overexpression of COX-2 in prostatic adenocarcinoma. 8) Overall, evidence is limited to retrospective studies and small case series; no prospective randomized trials exist due to the rarity and poor prognosis of the disease.