Equis ISSN 2398-2977

Penis: neoplasia

Synonym(s): Penile neoplasia

Contributor(s): Prof G Barrie Edwards, Jean Pierre Held, Prof Derek Knottenbelt, Rob Lofstedt, Graham Munroe, Katrin Schmallenbach, Elaine Watson, Anna Hollis


  • Penile neoplasia includes: fibrous papilloma, squamous cell carcinoma, sarcoid, melanoma.
  • Tumors may be either benign (papilloma/sarcoid/melanoma) or malignant (sarcoid, squamous cell carcinoma).
  • Tumors are most commonly situated on the glans. Precarcinomatous changes can be seen on skin of penis away from glans.
  • Rarely occur on the body or other locations.
  • May occur in association with preputial tumors.



  • Origin of benign and malignant tumors is poorly understood.
  • Experiments have shown that benign tumors can be transmitted to other animals through cell-free filtrates.
  • Malignant tumors can be produced by certain chronic irritants.

Squamous cell carcinoma

  • Non-pigmented skin is more often involved.
  • It has been suggested that there is a direct transformation of virally induced squamous papillomas into squamous cell carcinoma in association with an environmental carcinogenic agent.
  • The carcinogenic agent associated with penile squamous cell carcinoma is thought to be smegma, a mixture of sebaceous material and shed keratin that forms in the preputial space and accumulates in the fossa glandis.

Predisposing factors


  • For spontaneous tumors, a predisposition to neoplastic disease may be inheritable.
  • Predisposition of gray horses with dark pigmented skin for melanomas.
  • An increased likelihood of squamous cell carcinoma has been suggested, if the penile integument is unpigmented.
  • Sarcoids Sarcoid tend only to be the nodular form and remain reasonably localized - a few ulcerate.


  • Penile tumors may be epithelial (squamous cell carcinoma and squamous papillomas) or mesenchymal (sarcoid/melanoma).

Squamous cell carcinoma

  • It does invade deeper structures.
  • It may metastasize to regional lymphatics and to general circulation, especially young geldings with aggressive forms.
  • Squamous papillomas were found in approximately one third of cases in association with a squamous cell carcinoma and is considered the benign counterpart of squamous cell carcinoma.
  • A pre-malignant stage is recognized histologically by cells in the basal layer of the hyperplastic epithelium which show an increased mitotic rate, loss of polarity but no invasion of the basement membrane.
  • Squamous cell carcinoma may arisede novoor from papilloma, which indicates that any papillomatous lesion present on the equine penis should be considered as a possible pre-malignant proliferation.


  • Other tumors are usually present at other predilection sites Sarcoid.


Sarcoma/squamous cell carcinoma/epithelioma

  • Occasionally behave in a malignant invasive manner.
  • Can rapidly destroy the body of the penis.


  • The metastasis rate for these tumors is low, and palpable swelling of the regional lymph node is usually hyperplastic in nature and not neoplastic. The response of the iliac lymph nodes is an important prognostic factor (rectal examination is probably obligatory).
  • Some animals affected have had benign papillomata removed from the same site several years previously.
  • Some highly malignant forms with generalized metastasis, including into vertebral bodies.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • van den Top J G B et al (2010) Penile and preputial tumours in the horse: Literature review and proposal of a standardised approach. Equine Vet J 42 (8), 746-757 PubMed.
  • van den Top J G B, de Heer N, Klein W R & Ensink J M (2008) Penile and preputial tumours in the horse: A retrospective study of 114 affected horses. Equine Vet J 40 (6), 528-532 PubMed.
  • Henson F M D & Dobson J M (2004) Use of radiation therapy in the treatment of equine neoplasia. Equine Vet Educ 16 (6), 315-318 VetMedResource.
  • Howarth S, Lucke V M & Pearso H (1991) Squamous cell carcinoma of the external genitalia - a review and assessment of penile amputation and urethrostomy as a surgical treatment. Equine Vet J 23 (1), 53-58 PubMed.
  • Goetz T E, Ogilvie G K, Keegan K G & Johnson P J (1990) Cimetidine for treatment of melanomas in three horses. JAVMA 196, 449-452 PubMed.
  • Caron J P, Barber S M & Bailey J V (1985) Equine testicular neoplasia. Comp Cont Ed Pract Vet 7, 53-59 VetMedResource.
  • Markel M, Wheat J D & Jones K (1988) Genital neoplasms treated by en-bloc resection and penile retroverion in horses - 10 cases (1977-1986). JAVMA 192 (3), 396-400 PubMed.
  • Schumacher J & Vaughan J T (1988) Surgery of the penis and prepuce. Vet Clin North Am Equine Pract 4, 473-493 PubMed.
  • Vaillancout D, Fretz P & Orr J P (1979) Seminoma in the horse - report of two cases. J Equine Med Surg 3, 213-218 VetMedResource.
  • Strafuss A C (1976) Squamous cell carcinoma in horses. JAVMA 168, 61-62 PubMed.
  • Smith H A (1954) Interstitial cell tumor of the equine testis. JAVMA 124, 356-357 PubMed.
  • Plaut A, Kohn-Speyer A C (1947) The carcinogenic action of smegma. Science 105, 391-392 PubMed.

Other sources of information

  • Vaughan J T (1980) Surgery of the Penis and Prepuce. In: Bovine and Equine Urogenital Surg. Lea & Febiger, USA.
  • McEntee K (1970) The Male Genital System. In: Pathology of Domestic Animals. 3rd edn. Academic Press, USA.