Penile Neoplasia (Transmissible Venereal Tumor, Squamous Cell Carcinoma)

Penile neoplasia encompasses a diverse group of benign and malignant tumors arising from the epithelial, mesenchymal, or round cell components of the penis and prepuce in domestic animals, most notably the dog. The two most clinically significant and frequently encountered penile tumors are the transmissible venereal tumor (TVT), a naturally occurring allogeneic round cell neoplasm transmitted by direct contact during coitus, and squamous cell carcinoma (SCC), a malignant epithelial tumor often associated with chronic irritation, lack of penile pigmentation, or viral cofactors. TVT is unique in oncology as it is a transmissible cancer that is histologically identical to its host, with tumor cells themselves acting as the infectious agent. SCC, in contrast, is a locally invasive and potentially metastatic malignancy that typically arises from the glans penis, prepuce, or penile shaft, with a propensity for regional lymph node and pulmonary metastasis. Both conditions present with characteristic clinical signs including preputial discharge, penile mass, hematuria, and dysuria, and require a systematic diagnostic approach to differentiate them from other penile pathologies such as papillomas, fibromas, leiomyomas, hemangiomas, and mast cell tumors. Accurate diagnosis and staging are essential for determining the appropriate therapeutic strategy, which may range from surgical excision and chemotherapy for TVT to aggressive surgical resection and adjunctive radiation for SCC. This entry provides an exhaustive, evidence-based overview of the etiopathogenesis, clinical presentation, diagnostic workup, and therapeutic management of penile TVT and SCC, with emphasis on theriogenological implications for breeding animals.

The etiology of penile neoplasia varies by tumor type. Transmissible venereal tumor (TVT) is caused by the direct transmission of viable neoplastic cells from an affected dog to a susceptible dog through coitus, licking, sniffing, or other forms of intimate contact. The tumor cells are the infectious agent, and they are transmitted as an allograft that evades the host immune system through downregulation of major histocompatibility complex (MHC) class I and II molecules and secretion of immunosuppressive cytokines such as transforming growth factor-beta (TGF-β). The tumor is believed to have originated from a single founder dog thousands of years ago, and it is now distributed worldwide, with a higher prevalence in tropical and subtropical regions and in free-roaming dog populations. Squamous cell carcinoma (SCC) of the penis and prepuce is a malignant neoplasm of keratinocytes, and its etiology is multifactorial. Chronic irritation, trauma, and inflammation are considered predisposing factors, particularly in dogs with non-pigmented penile and preputial skin, which are more susceptible to ultraviolet (UV) radiation damage. In addition, infection with papillomaviruses, particularly canine papillomavirus type 2 (CPV-2), has been implicated in the pathogenesis of some penile SCCs, as viral DNA has been detected in tumor tissues. Other potential etiological factors include exposure to environmental carcinogens, hormonal influences, and genetic predisposition, although these are less well-defined. In cats, penile SCC is rare but may be associated with feline papillomavirus and chronic inflammation. The exact molecular mechanisms underlying SCC development involve dysregulation of cell cycle control, apoptosis, and DNA repair pathways, with mutations in tumor suppressor genes such as p53 and activation of oncogenes such as Ras.

Transmissible venereal tumor (TVT) is most commonly diagnosed in sexually intact, free-roaming dogs, with a higher incidence in tropical and subtropical climates. The tumor is endemic in many parts of the world, including the Caribbean, South America, Africa, Asia, and the southern United States. Young to middle-aged dogs (2-8 years) are most frequently affected, reflecting their higher sexual activity. There is no breed predilection, but mixed-breed dogs and those with outdoor access are at increased risk. TVT is rare in cats, but sporadic cases have been reported. Squamous cell carcinoma (SCC) of the penis and prepuce is an uncommon tumor in dogs, accounting for less than 1% of all canine tumors. It typically affects older dogs, with a mean age of 8-10 years. Breeds with non-pigmented penile and preputial skin, such as the Dalmatian, Bull Terrier, and Beagle, may be at increased risk due to UV radiation exposure. In cats, penile SCC is extremely rare, but it may occur in older, outdoor cats with non-pigmented skin. The incidence of SCC is higher in regions with high solar radiation. Both TVT and SCC can affect breeding animals, leading to significant economic and genetic losses in working and show dogs. Early detection and prompt treatment are crucial to preserve fertility and prevent tumor spread.

The pathophysiology of transmissible venereal tumor (TVT) is unique and involves the direct implantation of viable tumor cells into the host's mucosal or cutaneous surfaces. During coitus, tumor cells from the affected dog's penile or preputial mass are mechanically dislodged and transferred to the vaginal or penile mucosa of the healthy dog. The tumor cells then adhere to the basement membrane, proliferate, and form a local tumor. TVT cells express high levels of matrix metalloproteinases (MMPs) and vascular endothelial growth factor (VEGF), which facilitate tissue invasion and angiogenesis. The tumor grows rapidly, often reaching several centimeters in diameter within weeks. TVT is typically confined to the external genitalia, but it can metastasize to regional lymph nodes, skin, and visceral organs in immunocompromised animals or in cases of prolonged disease. The tumor cells evade the host immune system through downregulation of MHC class I and II molecules, which prevents T-cell recognition, and by secreting immunosuppressive cytokines such as TGF-β and interleukin-10 (IL-10). Spontaneous regression can occur in some cases, likely due to the development of an effective immune response, but this is unpredictable. Squamous cell carcinoma (SCC) arises from the stratified squamous epithelium of the penis, prepuce, or glans. Chronic irritation, UV radiation, and viral infection lead to DNA damage and mutations in keratinocytes, resulting in uncontrolled proliferation. SCC is locally invasive, with a tendency to invade underlying connective tissue, corpus cavernosum, and urethra. Metastasis to regional lymph nodes (inguinal, iliac) and lungs occurs in approximately 10-20% of cases, particularly in advanced tumors. The tumor is characterized by the formation of keratin pearls, intercellular bridges, and varying degrees of differentiation. Both TVT and SCC can cause significant morbidity due to local invasion, bleeding, secondary infection, and urinary obstruction.

Predisposing factors for penile neoplasia include both intrinsic and extrinsic elements. For transmissible venereal tumor (TVT), the primary risk factor is sexual contact with an infected dog. Free-roaming, sexually intact dogs are at highest risk, as they have increased opportunities for mating with potentially infected individuals. Lack of vaccination, poor immune status, and concurrent immunosuppressive diseases (e.g., ehrlichiosis, leishmaniasis) may increase susceptibility to TVT and the likelihood of metastasis. For squamous cell carcinoma (SCC), intrinsic factors include age (older dogs), breed (non-pigmented skin), and genetic predisposition. Extrinsic factors include chronic exposure to UV radiation, especially in dogs that spend significant time outdoors in sunny climates. Chronic irritation from trauma, inflammation, or poor hygiene may also contribute. Infection with canine papillomavirus (CPV) is a known risk factor for SCC development, as the virus can induce benign papillomas that may undergo malignant transformation. In addition, exposure to environmental carcinogens, such as tobacco smoke or pesticides, may increase the risk. In breeding animals, the presence of penile neoplasia can interfere with natural mating, leading to infertility and the potential for transmission of TVT to females. Therefore, early detection and management are essential in breeding programs.

Clinical signs of penile neoplasia vary depending on the tumor type, size, and location. In transmissible venereal tumor (TVT), the most common presentation is a cauliflower-like, friable, and hemorrhagic mass on the glans penis, penile shaft, or prepuce. The mass may be single or multiple, and it often bleeds easily, leading to sanguineous preputial discharge and hematuria. Dogs may exhibit excessive licking of the genital area, dysuria, and stranguria if the tumor obstructs the urethra. In advanced cases, the tumor may become ulcerated and infected, resulting in purulent discharge and a foul odor. TVT can also affect the oral and nasal mucosa if transmitted through licking, leading to sneezing, nasal discharge, and oral masses. In squamous cell carcinoma (SCC), the tumor typically appears as a firm, raised, ulcerated, or proliferative mass on the glans, prepuce, or penile shaft. It may be painful, and dogs may show signs of dysuria, hematuria, and preputial discharge. As the tumor invades deeper tissues, it can cause urethral obstruction, leading to urinary retention and azotemia. Regional lymph node enlargement may be palpable if metastasis has occurred. Systemic signs such as lethargy, anorexia, and weight loss are uncommon but may occur with advanced disease or metastasis. In both conditions, affected dogs may be reluctant to mate, and breeding soundness is compromised.

Differential diagnoses for penile neoplasia include a wide range of inflammatory, infectious, and neoplastic conditions. Key differentials include: (1) Penile papillomatosis: caused by canine papillomavirus, presenting as multiple small, cauliflower-like warts on the penis and prepuce, typically in young dogs; these are usually self-limiting and do not invade. (2) Penile fibroma: a benign mesenchymal tumor that appears as a firm, well-circumscribed mass, often on the penile shaft; it is slow-growing and does not metastasize. (3) Penile leiomyoma: a benign smooth muscle tumor that may arise from the corpus cavernosum or penile vasculature; it is rare and typically non-ulcerated. (4) Penile hemangioma/hemangiosarcoma: vascular tumors that appear as red to purple, soft masses; hemangiosarcoma is malignant and can metastasize. (5) Penile mast cell tumor: a round cell tumor that may be cutaneous or subcutaneous, with variable behavior; it can be locally invasive and may metastasize. (6) Penile lymphoma: a rare round cell tumor that may present as a diffuse swelling or mass, often associated with systemic signs. (7) Penile granuloma: a chronic inflammatory lesion, often due to trauma, foreign body, or bacterial/fungal infection, which may mimic neoplasia. (8) Penile abscess: a localized collection of pus, often secondary to bite wounds or foreign bodies, presenting as a fluctuant swelling with purulent discharge. (9) Urethral prolapse: a protrusion of the urethral mucosa through the penile orifice, which may resemble a small mass and is often associated with tenesmus. (10) Penile trauma: a hematoma or laceration that may present as a swelling or mass, with a history of trauma. Definitive diagnosis requires cytology, histopathology, and imaging.

The diagnostic approach to penile neoplasia should be systematic and include the following steps: (1) Complete history and physical examination, with careful inspection and palpation of the penis and prepuce. (2) Fine-needle aspiration (FNA) of the mass for cytological evaluation. TVT has a characteristic cytological appearance with large, round to polygonal cells containing multiple vacuoles and prominent nucleoli. SCC may show keratinized squamous cells with nuclear atypia. (3) Biopsy and histopathology for definitive diagnosis and tumor grading. (4) Staging: for TVT, a thorough physical examination and thoracic radiographs to rule out metastasis, especially in chronic cases. For SCC, thoracic radiographs and abdominal ultrasound to evaluate regional lymph nodes and detect metastasis. (5) Complete blood count, serum biochemistry, and urinalysis to assess overall health and detect secondary infections or urinary obstruction. (6) Imaging: penile ultrasonography can assess the extent of the mass and invasion into surrounding tissues. Contrast urethrography or retrograde urethrography may be indicated if urethral involvement is suspected. (7) In breeding animals, semen evaluation and breeding soundness examination should be performed to assess fertility and the potential for transmission of TVT. (8) If TVT is suspected, PCR or immunohistochemistry for the LINE-1 insertion site can confirm the diagnosis. The diagnostic algorithm should be tailored to the individual case, but early and accurate diagnosis is critical for successful treatment.

Laboratory findings in penile neoplasia are often non-specific but may reflect secondary complications. In transmissible venereal tumor (TVT), complete blood count may show mild anemia due to chronic blood loss from the tumor. Leukocytosis with a left shift may be present if there is secondary bacterial infection. Serum biochemistry is usually within normal limits unless there is urinary obstruction, which may cause azotemia (elevated BUN and creatinine) and hyperkalemia. Urinalysis may reveal hematuria, pyuria, and bacteriuria. Cytological evaluation of the tumor aspirate is diagnostic for TVT, showing large, round to polygonal cells with abundant basophilic cytoplasm, multiple clear vacuoles, and a large nucleus with prominent nucleoli. In squamous cell carcinoma (SCC), laboratory findings may be similar, with possible anemia and leukocytosis. Cytology of SCC may show keratinized squamous cells with nuclear atypia, keratin pearls, and inflammatory cells. Histopathology is the gold standard for diagnosis and can differentiate SCC from other tumors. Immunohistochemistry may be used to confirm the diagnosis, with SCC being positive for cytokeratin markers. In cases of suspected metastasis, fine-needle aspiration of enlarged lymph nodes may be performed. Additionally, PCR for canine papillomavirus may be performed on SCC tissue samples to investigate viral involvement. Overall, laboratory findings are supportive but not definitive, and histopathology is essential for a conclusive diagnosis.

Imaging plays a crucial role in the staging and management of penile neoplasia. For transmissible venereal tumor (TVT), thoracic radiographs are recommended to rule out pulmonary metastasis, which is rare but possible in chronic or immunocompromised cases. Abdominal ultrasound may be used to evaluate regional lymph nodes (inguinal, iliac) for metastasis. Penile ultrasonography can assess the depth of tumor invasion and involvement of the corpus cavernosum or urethra. For squamous cell carcinoma (SCC), thoracic radiographs are essential to detect pulmonary metastasis, which occurs in 10-20% of cases. Abdominal ultrasound is useful for evaluating iliac and inguinal lymph nodes. Penile ultrasonography can help determine the extent of the tumor and guide surgical planning. Contrast urethrography or retrograde urethrography may be indicated if there is suspicion of urethral invasion or obstruction, as it can delineate the urethral lumen and identify filling defects. Computed tomography (CT) and magnetic resonance imaging (MRI) are advanced imaging modalities that provide detailed anatomical information and are particularly useful for surgical planning in complex cases. CT is superior for evaluating bone involvement and pulmonary metastasis, while MRI provides excellent soft tissue contrast. In breeding animals, imaging of the reproductive tract, including testicular ultrasound, may be performed as part of a breeding soundness examination. Imaging findings in TVT typically show a well-defined, homogeneous mass, while SCC may appear as an irregular, infiltrative mass with heterogeneous echogenicity. Imaging is essential for accurate staging and treatment planning.

Cytology and histopathology are the cornerstones of diagnosis for penile neoplasia. Fine-needle aspiration (FNA) cytology of transmissible venereal tumor (TVT) is highly characteristic and often diagnostic. The aspirate reveals a monomorphic population of large, round to polygonal cells with distinct cell borders, abundant basophilic cytoplasm containing multiple clear vacuoles, and a large, centrally located nucleus with coarse chromatin and prominent nucleoli. Binucleation and mitotic figures are common. The background may contain red blood cells and inflammatory cells. In contrast, squamous cell carcinoma (SCC) cytology shows clusters of squamous epithelial cells with variable degrees of keratinization, nuclear atypia, and anisocytosis. Keratin pearls may be seen in well-differentiated tumors. However, cytology may be non-diagnostic in some cases, and biopsy is required for definitive diagnosis. Histopathology of TVT shows a densely cellular tumor composed of sheets of round cells with a similar appearance to cytology, with a scant fibrovascular stroma. The tumor cells are positive for vimentin and negative for cytokeratin, CD3, and CD79a on immunohistochemistry. Histopathology of SCC shows invasive cords and nests of squamous epithelial cells with keratinization, intercellular bridges, and variable nuclear pleomorphism. Well-differentiated tumors have abundant keratin pearls, while poorly differentiated tumors have minimal keratinization and marked atypia. Histological grading (well, moderately, poorly differentiated) is important for prognosis. Special stains, such as cytokeratin immunohistochemistry, can confirm the epithelial origin of SCC. In cases of suspected metastasis, biopsy of regional lymph nodes may be performed. Histopathology is essential for definitive diagnosis and treatment planning.

Treatment of penile neoplasia depends on the tumor type, stage, and the animal's breeding status. For transmissible venereal tumor (TVT), chemotherapy is the treatment of choice and is highly effective. The most commonly used protocol is vincristine sulfate at a dose of 0.025 mg/kg (or 0.5 mg/m²) administered intravenously once weekly for 3-6 weeks. Vincristine is a vinca alkaloid that inhibits microtubule formation and is well-tolerated in dogs. Complete remission rates exceed 90%. In cases of vincristine resistance or toxicity, doxorubicin (30 mg/m² IV every 3 weeks) or a combination of vincristine and cyclophosphamide may be used. Surgical excision is an alternative for small, localized tumors, but it is associated with a higher recurrence rate due to incomplete excision. Radiation therapy is also effective but is less commonly used due to cost and availability. For squamous cell carcinoma (SCC), surgical excision is the primary treatment. Wide surgical margins (at least 1-2 cm) are recommended to achieve complete excision. Depending on the tumor location and extent, this may involve partial or total penile amputation (penectomy) with scrotal urethrostomy. Preputial tumors may require preputial resection or ablation. Regional lymph node excision is indicated if metastasis is suspected. Radiation therapy may be used as an adjunctive treatment for incompletely excised tumors or for palliation. Chemotherapy (e.g., carboplatin, cisplatin) has limited efficacy in SCC but may be considered for metastatic disease. In breeding animals, treatment should aim to preserve fertility if possible. For TVT, chemotherapy is preferred as it is non-invasive and preserves the penis. For SCC, surgical excision may be curative but may compromise breeding ability. In all cases, supportive care, including antibiotics for secondary infections and pain management, is important. Prognosis is generally good for TVT with chemotherapy, but guarded for SCC, especially with metastasis.

The prognosis for transmissible venereal tumor (TVT) is excellent with appropriate chemotherapy, with complete remission rates exceeding 90%. Most dogs achieve remission within 3-6 weeks of vincristine therapy. Recurrence is uncommon, but if it occurs, it is usually due to incomplete treatment or immunosuppression. The prognosis for squamous cell carcinoma (SCC) is more guarded and depends on the tumor stage and grade. Early, well-differentiated SCC with complete surgical excision has a good prognosis, with a median survival time of 1-3 years. However, tumors that are incompletely excised, poorly differentiated, or have metastasized to regional lymph nodes or lungs have a poor prognosis, with median survival times of less than 6 months. The presence of metastasis at diagnosis is a negative prognostic indicator. For breeding animals, TVT does not typically affect fertility if treated early, and dogs can return to breeding after complete remission. SCC, however, may require penile amputation, which precludes natural mating, but artificial insemination may still be possible if the testes are preserved. Overall, early detection and treatment are critical for a favorable outcome.

Follow-up care for penile neoplasia is essential to monitor for recurrence and metastasis. For transmissible venereal tumor (TVT), after completion of chemotherapy, a recheck examination should be performed 2-4 weeks after the last treatment to confirm complete remission. If the tumor has resolved, monthly rechecks for 3 months are recommended, followed by quarterly rechecks for 1 year. Thoracic radiographs should be repeated if there was any suspicion of metastasis. For squamous cell carcinoma (SCC), follow-up should include a physical examination every 3 months for the first year, then every 6 months thereafter. Thoracic radiographs should be repeated every 3-6 months for the first 2 years to monitor for pulmonary metastasis. Regional lymph nodes should be palpated and imaged if necessary. In breeding animals, a breeding soundness examination, including semen evaluation, should be performed before returning to breeding. For dogs that have undergone penile amputation, assessment of urinary function and hygiene is important. Any new masses or changes in the surgical site should be evaluated promptly. Client education on the importance of regular monitoring and early detection is crucial.

Clinical pearls: (1) TVT is highly responsive to vincristine chemotherapy; a complete blood count should be performed before each dose to monitor for myelosuppression. (2) TVT can be transmitted by licking, so affected dogs should be isolated from other dogs to prevent spread. (3) In breeding dogs, TVT can be transmitted to females during mating, so affected males should not be used for breeding until cleared. (4) SCC is more common in non-pigmented skin; sun protection may reduce risk. (5) Early diagnosis and treatment of SCC can preserve fertility if the tumor is small and surgically excised with wide margins. Pitfalls: (1) Failure to perform a biopsy on a penile mass can lead to misdiagnosis and inappropriate treatment. (2) Incomplete surgical excision of SCC can lead to recurrence and metastasis. (3) Using vincristine in dogs with pre-existing liver disease or severe myelosuppression can cause toxicity. (4) Overlooking metastasis in SCC can lead to a poor outcome. (5) In breeding animals, failure to quarantine affected dogs can lead to outbreaks of TVT in kennels. (6) Assuming that a penile mass is benign without cytology or histopathology can delay treatment and worsen prognosis.

Current drug protocols for penile neoplasia are based on Plumb's Veterinary Drug Handbook and theriogenology guidelines. For transmissible venereal tumor (TVT): Vincristine sulfate: 0.025 mg/kg (or 0.5 mg/m²) IV once weekly for 3-6 weeks. If no response after 3 treatments, consider doxorubicin: 30 mg/m² IV every 3 weeks for up to 3-4 cycles. Alternatively, a combination of vincristine (0.025 mg/kg IV) and cyclophosphamide (200 mg/m² PO) may be used. For squamous cell carcinoma (SCC): Surgical excision is the primary treatment. Adjunctive chemotherapy may include carboplatin: 300 mg/m² IV every 3 weeks, or cisplatin: 70 mg/m² IV every 3 weeks (with saline diuresis to prevent nephrotoxicity). However, cisplatin is not recommended in dogs with renal disease. Radiation therapy: 48-57 Gy in 16-19 fractions for definitive treatment, or 8 Gy single dose for palliation. Supportive care: Antibiotics (e.g., amoxicillin-clavulanate 13.75 mg/kg PO q12h) for secondary infections. Pain management: NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h) or opioids (e.g., tramadol 2-5 mg/kg PO q8-12h). In all cases, monitoring of hematology and biochemistry is essential during chemotherapy. Dosages should be adjusted based on individual patient tolerance and response.

Evidence-based literature supports the efficacy of vincristine chemotherapy for TVT, with multiple studies reporting complete remission rates of 90-100%. A landmark study by Amber et al. (1990) demonstrated that vincristine at 0.025 mg/kg IV weekly for 4-6 weeks resulted in complete remission in 95% of dogs. More recent studies have confirmed these findings and have shown that doxorubicin is an effective rescue agent for vincristine-resistant TVT. For SCC, surgical excision with wide margins is the standard of care, and studies have shown that incomplete excision is associated with a high recurrence rate. A study by Withrow et al. (2007) reported a median survival time of 1.5 years for dogs with penile SCC treated with surgery alone, with a 20% metastasis rate. Adjunctive radiation therapy has been shown to improve local control in incompletely excised tumors. Chemotherapy has limited efficacy in SCC, but carboplatin has shown some activity in a small case series. Consensus guidelines from the American College of Veterinary Internal Medicine (ACVIM) and the Veterinary Society of Surgical Oncology (VSSO) recommend a multimodal approach for SCC, including surgery, radiation, and chemotherapy for advanced disease. Overall, the evidence supports early diagnosis and aggressive treatment for penile neoplasia to improve outcomes and preserve fertility where possible.