Testicular Neoplasia (Sertoli Cell Tumor, Seminoma, Leydig/Interstitial Cell Tumor)

Testicular neoplasia encompasses a group of primary tumors arising from the cellular components of the canine and feline testis. The three most common histotypes are Sertoli cell tumors (SCTs), seminomas (SEMs), and Leydig (interstitial) cell tumors (LCTs). These tumors are typically benign and slow-growing, but SCTs frequently produce estrogen, leading to feminization syndrome, bone marrow suppression, and other systemic effects. Seminomas arise from germinal epithelium, while Leydig cell tumors originate from the interstitial endocrine cells. The disease is most prevalent in older, intact male dogs, particularly in cryptorchid testes, where SCTs and seminomas are overrepresented. In cats, testicular tumors are rare, with SCTs being the most common. The clinical significance lies in the potential for malignant transformation, metastasis (especially in seminomas), and paraneoplastic syndromes. Accurate diagnosis requires a combination of physical examination, ultrasonography, fine-needle aspiration, and histopathology. Treatment of choice is surgical castration, which is curative in most cases, with a favorable prognosis for non-metastatic tumors.

The exact etiology of testicular neoplasia is multifactorial. Hormonal imbalances, particularly elevated levels of gonadotropins (LH and FSH) due to aging or testicular dysfunction, are implicated in the pathogenesis of Sertoli cell tumors and Leydig cell tumors. Cryptorchidism is a major risk factor, as the intra-abdominal or inguinal location exposes the testis to higher body temperature, which disrupts spermatogenesis and predisposes to neoplastic transformation. Chronic inflammation, trauma, and exposure to environmental toxins (e.g., pesticides, heavy metals) may also contribute. Genetic factors are suspected, with certain breeds (e.g., Boxer, German Shepherd, Weimaraner) showing increased incidence. In Sertoli cells, estrogen production is often elevated, leading to a hyperestrogenic state that can cause bone marrow suppression and feminization. In Leydig cell tumors, excessive testosterone or estrogen secretion may occur. Seminomas arise from germ cells and are often associated with cryptorchidism, but the precise molecular triggers are unknown. Viral etiologies have not been confirmed in domestic animals.

Testicular neoplasia is the most common tumor of the canine male reproductive tract, accounting for approximately 4-7% of all canine tumors. The mean age of affected dogs is 10 years, with a range of 5-15 years. Cryptorchid testes are 13.6 times more likely to develop Sertoli cell tumors and 9.2 times more likely to develop seminomas compared to scrotal testes. Leydig cell tumors are less associated with cryptorchidism. Breed predispositions include Boxer, German Shepherd, Weimaraner, Shetland Sheepdog, and Collie. In cats, testicular tumors are rare, with Sertoli cell tumors being the most frequently reported, often in cryptorchid cats. The incidence in cats is less than 1% of all feline tumors. No significant sex predilection exists as the disease occurs in intact males. The prevalence of testicular tumors increases with age, and most tumors are benign; however, seminomas have a higher metastatic potential (up to 10-15%).

The pathophysiology of testicular neoplasia involves dysregulation of normal testicular cell growth and differentiation. Sertoli cell tumors arise from the sustentacular cells of the seminiferous tubules. These tumors often secrete estrogen, leading to hyperestrogenism, which causes feminization (gynecomastia, pendulous prepuce, attraction of other males), squamous metaplasia of the prostate, and bone marrow suppression (pancytopenia). The estrogen-induced bone marrow suppression is due to inhibition of hematopoietic stem cells, leading to anemia, leukopenia, and thrombocytopenia. Seminomas originate from germ cells and are composed of large, round cells with clear cytoplasm. They may be intratubular or diffuse, and while they are generally benign, they can metastasize to regional lymph nodes and lungs. Leydig cell tumors arise from the interstitial cells and may secrete testosterone or estrogen, causing hormonal imbalances. The growth of these tumors is often slow, and they may be incidental findings. The local effects include testicular enlargement, atrophy of the contralateral testis, and potential obstruction of the spermatic cord. Malignant transformation is rare but can occur, especially in seminomas.

Intrinsic factors include age (older dogs), breed (Boxer, German Shepherd, Weimaraner), cryptorchidism (retained testis), and genetic predisposition. Hormonal imbalances, such as elevated LH and FSH due to testicular degeneration, may promote tumor development. Extrinsic factors include environmental toxins, exposure to exogenous hormones (e.g., anabolic steroids), and possibly trauma. Poor breeding management, such as not castrating cryptorchid dogs, increases the risk. Additionally, dogs with bilateral cryptorchidism are at higher risk. In cats, cryptorchidism is also a risk factor. The presence of one testicular tumor may predispose to the development of tumors in the contralateral testis, as multiple tumors are common.

Clinical signs vary depending on the tumor type and hormone secretion. Many dogs are asymptomatic, and the tumor is detected as an incidental finding on palpation. In scrotal testes, a firm, non-painful mass may be palpable. In cryptorchid testes, an abdominal or inguinal mass may be detected. Sertoli cell tumors often cause feminization syndrome: bilateral symmetric alopecia, gynecomastia, pendulous prepuce, attraction of other male dogs, and prostatic squamous metaplasia. Bone marrow suppression may lead to lethargy, pale mucous membranes, petechiae, and increased susceptibility to infections. Seminomas may cause testicular enlargement and, if metastatic, weight loss, dyspnea, or lameness. Leydig cell tumors are usually non-functional but may cause perianal gland hyperplasia or prostatic disease due to testosterone secretion. In cats, signs may include testicular mass, alopecia, and rarely feminization.

Differential diagnoses include: 1) Testicular torsion: acute pain, swelling, and systemic signs; ultrasonography shows absent blood flow. 2) Orchitis/epididymitis: fever, pain, swelling, and systemic signs; ultrasonography shows diffuse changes, and culture may be positive. 3) Testicular hematoma: history of trauma, ultrasonography shows echogenic fluid. 4) Inguinal hernia: reducible mass, bowel sounds, and ultrasonography shows intestinal loops. 5) Spermatocele: cystic dilation of the epididymis, usually asymptomatic. 6) Testicular cyst: fluid-filled cavity, benign. 7) Granulomatous disease (e.g., brucellosis): firm, nodular testis, positive serology. 8) Testicular abscess: painful, fluctuant mass, fever, and leukocytosis. 9) Metastatic neoplasia (e.g., lymphoma): bilateral enlargement, systemic signs. 10) Normal aging changes: testicular atrophy, which may be asymmetric. Definitive diagnosis requires histopathology.

The diagnostic workup begins with a thorough history and physical examination, including palpation of both testes and the scrotum. If a testicular mass is suspected, the next step is abdominal ultrasonography to evaluate the contralateral testis and check for metastasis. Fine-needle aspiration (FNA) of the mass can be performed for cytology, but histopathology after surgical excision is the gold standard. Preoperative blood work should include a complete blood count (CBC) to assess for bone marrow suppression, serum biochemistry, and hormonal assays (estradiol, testosterone, LH). Thoracic radiographs are recommended to rule out pulmonary metastasis, especially for seminomas. If cryptorchidism is present, abdominal ultrasonography or CT may be needed to locate the retained testis. The algorithm proceeds from clinical suspicion to imaging, cytology, and ultimately histopathology.

Complete blood count may reveal non-regenerative anemia, leukopenia, and thrombocytopenia in cases of Sertoli cell tumor with hyperestrogenism. Serum biochemistry may show elevated liver enzymes (ALP, ALT) due to estrogen hepatotoxicity. Hormonal assays: elevated serum estradiol (often >20 pg/mL) in Sertoli cell tumors; elevated testosterone in Leydig cell tumors; LH may be elevated due to negative feedback disruption. Urinalysis may show hematuria or proteinuria if prostatic disease is present. Vaginal cytology is not applicable in males. Fine-needle aspirate cytology of the testicular mass may show clusters of large polygonal cells with abundant cytoplasm (Leydig cells), large round cells with clear cytoplasm (seminoma), or columnar cells with palisading (Sertoli cells). Histopathology is definitive.

Ultrasonography of the scrotum and abdomen is the primary imaging modality. Testicular tumors appear as hypoechoic or mixed echogenic masses within the testicular parenchyma. Sertoli cell tumors may have a multilobulated appearance. Seminomas are often homogeneous and hypoechoic. Leydig cell tumors may be small and hyperechoic. Color Doppler can assess vascularity. Abdominal ultrasonography is essential to locate cryptorchid testes and to evaluate for metastasis in the sublumbar lymph nodes, liver, and spleen. Thoracic radiographs are indicated to detect pulmonary metastases, which appear as nodular interstitial patterns. CT and MRI are advanced imaging options for surgical planning and staging, especially in complex cases.

Fine-needle aspiration cytology can provide a presumptive diagnosis. Sertoli cell tumors: clusters of elongated cells with oval nuclei and moderate cytoplasm, sometimes with vacuolation. Seminomas: large, round cells with distinct borders, high nuclear-to-cytoplasmic ratio, and prominent nucleoli. Leydig cell tumors: sheets of polygonal cells with abundant eosinophilic cytoplasm and small, round nuclei. Histopathology after castration is definitive. Sertoli cell tumors show tubular or solid patterns, with cells having pale, vacuolated cytoplasm and elongated nuclei. Seminomas show sheets of large round cells with clear cytoplasm and central nuclei. Leydig cell tumors show cords or nests of cells with eosinophilic cytoplasm and lipofuscin pigment. Immunohistochemistry can be used to differentiate: inhibin-α and vimentin for Sertoli cells, c-kit and OCT4 for seminomas, and calretinin and melan-A for Leydig cells.

The treatment of choice is surgical castration (bilateral orchiectomy). For scrotal tumors, a standard prescrotal approach is used. For cryptorchid tumors, abdominal or inguinal exploration is required. Preoperative stabilization is necessary if bone marrow suppression is severe; blood transfusions or supportive care may be needed. Castration is curative for benign tumors and resolves hormonal signs within weeks. For metastatic seminomas, adjunctive chemotherapy (e.g., cisplatin, carboplatin) or radiation therapy may be considered, but prognosis is guarded. Hormonal therapy is not effective. Supportive care includes antibiotics for secondary infections, and in cases of severe pancytopenia, granulocyte colony-stimulating factor may be used. Postoperative monitoring includes serial CBCs to assess bone marrow recovery.

The prognosis for benign testicular tumors is excellent after castration, with resolution of clinical signs and no recurrence. For Sertoli cell tumors, the prognosis is good if no metastasis is present; however, bone marrow suppression can be fatal if severe. Seminomas have a higher metastatic rate (10-15%), and if metastasis is present, the prognosis is guarded. Leydig cell tumors are almost always benign. The overall survival rate is high, and fertility is not a concern as the dog is castrated. Negative prognostic indicators include metastasis, severe pancytopenia, and large tumor size.

Post-castration, patients should be re-examined at 2 weeks for surgical site healing. A CBC should be repeated at 2-4 weeks to monitor bone marrow recovery. If metastasis was present, thoracic radiographs and abdominal ultrasound should be repeated every 3 months for the first year, then every 6 months thereafter. Hormonal assays (estradiol, testosterone) can be checked to confirm normalization. For dogs with cryptorchidism, the contralateral testis should be removed to prevent future tumors. Owners should be advised to monitor for signs of recurrence or metastasis.

Pearls: 1) Always palpate both testes in older intact male dogs; a small, firm nodule may be a tumor. 2) Cryptorchid testes are at high risk for Sertoli cell tumors and seminomas; recommend castration early. 3) Feminization syndrome (alopecia, gynecomastia) is a classic sign of Sertoli cell tumor; check estradiol levels. 4) Bone marrow suppression can be life-threatening; perform a CBC before surgery. 5) Histopathology is essential for definitive diagnosis and prognosis. Pitfalls: 1) Do not assume a testicular mass is benign; always submit for histopathology. 2) Avoid fine-needle aspiration of a suspected seminoma if metastasis is possible; it may seed tumor cells. 3) Do not delay surgery in a dog with severe pancytopenia; stabilize with transfusions first. 4) Do not forget to check for a contralateral testicular tumor, as multiple tumors are common.

There are no specific medical therapies for testicular neoplasia. Chemotherapy protocols for metastatic seminoma include: 1) Cisplatin: 60-70 mg/m² IV every 3 weeks, with saline diuresis and antiemetics. 2) Carboplatin: 300 mg/m² IV every 3 weeks. 3) Doxorubicin: 30 mg/m² IV every 3 weeks (for Sertoli cell tumors with metastasis). Supportive care for bone marrow suppression: 1) Granulocyte colony-stimulating factor (G-CSF): 5 mcg/kg SC once daily for 3-5 days. 2) Broad-spectrum antibiotics (e.g., amoxicillin-clavulanate 13.75 mg/kg PO q12h) if neutropenic. 3) Blood transfusion if anemia is severe. Hormonal therapy is not recommended. All dosages are based on Plumb's Veterinary Drug Handbook.

Landmark studies include: 1) Hayes and Pendergrass (1976) established the association between cryptorchidism and Sertoli cell tumors and seminomas in dogs. 2) Lipowitz et al. (1973) described the clinical and pathological features of canine testicular tumors. 3) A retrospective study by Grieco et al. (2008) evaluated the immunohistochemical expression of inhibin-α in canine testicular tumors. 4) A study by Liao et al. (2009) reported the metastatic rate of seminomas in dogs. 5) The World Health Organization (WHO) classification of testicular tumors in domestic animals provides standardized criteria. Consensus guidelines from the American College of Theriogenologists (ACT) and the European Society for Veterinary Reproduction (EVSSAR) recommend castration as the treatment of choice and emphasize the importance of histopathology. Recent studies have explored the role of chemotherapy in metastatic seminomas, but evidence is limited to case reports.