Michael H. Goldschmidt, MSc, BVMS, MRCVS, Diplomate ACVP Professor and Head, Laboratory of Pathology and Toxicology Chief, Surgical Pathology Department of Pathobiology
Frances S. Shofer, PhD, Adjunct Associate Professor of Epidemiology and Biostatistics
Introduction
Definition: A malignant tumor of epidermal cells with varying degrees of keratinocyte (squamous cell) differentiation
Synonyms: epidermoid carcinoma or prickle-cell carcinoma
Etiology
Some cases of squamous cell carcinoma are thought to arise from excessive short-wave ultraviolet light (UVB) exposure, which can cause epithelial changes including clonal mutation and DNA damage to keratocytes. As a result, these tumors commonly arise in non-pigmented, either sparsely- haired or un-haired skin. Since increased sun exposure is a known risk factor for the tumor, there is a higher incidence of squamous cell carcinoma in the south and southwestern USA and in other parts of the world with a similar climate.
Epidemiology
| Sex | N | Percent |
| Female | 214 | 21% (21%) |
| Female Spayed | 341 | 33% (33%) |
| Male | 292 | 29% (25%) |
| Male Castrated | 172 | 17% (21%) |
| (Normal Population %) |
| Breeds at Increased Risk |
N | Probability | OR | 95% Confidence Interval |
|
| Bloodhound | 5 | 0.0215 | 3.9 | 1.4 | 10.5 |
| Keeshond | 14 | <0.0001 | 3.5 | 2.0 | 5.9 |
| Basset Hound | 32 | <0.0001 | 3.0 | 2.1 | 4.3 |
| Mastiff | 6 | 0.0157 | 2.8 | 1.3 | 5.8 |
| Standard Poodle | 27 | 0.0002 | 2.2 | 1.5 | 3.2 |
| Shih Tzu | 29 | 0.0005 | 2.0 | 1.4 | 2.9 |
| Breeds at Decreased Risk |
N | Probability | OR | 95% Confidence Interval |
|
| Doberman Pinscher | 15 | 0.0108 | 0.54 | 0.33 | 0.88 |
| Yorkshire Terrier | 6 | 0.0489 | 0.46 | 0.21 | 1.02 |
| Boxer | 10 | <0.0001 | 0.32 | 0.17 | 0.60 |
| Scottish Terrier | 2 | 0.0414 | 0.26 | 0.07 | 1.04 |
| Pug | 1 | 0.0271 | 0.11 | 0.01 | 1.78 |
| Site | Percent |
| Head | 39.0% |
| Forelimb | 13.3% |
| Abdomen | 12.3% |
| Hindlimb | 11.9% |
| Neck | 6.3% |
| Perineum | 5.5% |
| Thorax | 4.1% |
| Tail | 3.5% |
| Back | 2.6% |
| Multiple Sites | 0.8% |
| Scrotal | 0.7% |
Clinical Presentation/Physical Exam Findings
- Focal solar dermatitis or solar keratitis are the earliest lesions caused by sun damage
- seen in areas with sparse or no hair, and/or non-pigmented skin, primarily the head, and ventral abdomen, a.k.a., the sites seen in the sun bathing dog lounging on its back
- The area may appear sunburned, with redness, swelling and scaling
- The tumor is followed over a period of time by thickening, hardening and wrinkling
- hardened skin may appear brown, raised or crusty
- Early or pre-neoplastic disease signs may not be noticed
- lesion may also appear as a non-healing ulcer that grows and infiltrates the underlying tissue to become a firm intradermal and subcutaneous mass
Tumor Pathology
Histopathology
The most significant histopathologic findings are:
- Islands, cords and trabeculae of invasive epithelial cells
- Formation of keratin pearls (lamellae of keratin within the tumor) by the invasive neoplastic cells
- Desmosomes (intercellular bridges) can be identified between tumor cells and intracytoplasmic keratin proteins
- Cells and nuclei are large, the nuclei are hyperchromatic, the chromatin often appears clumped, nucleoli may be prominent
- Desmoplastic reaction in the surrounding dermis and subcutaneous tissue in response to the invasive neoplastic squamous epithelial cells
Cytology
- Moderately large to giant-sized cells
- Round, oval, clefted or lobulated nuclei; coarse, ropey chromatin; multiple, large, often irregularly-shaped nucleoli; binucleate and multinucleate forms common
- Marked anisocytosis and anisokaryosis
- Frequent perinuclear vacuolization
- Blue to turquoise-blue cytoplasm
- Often contains evidence of acute or chronic active inflammation (therefore, clean and partially debride prior to forming a scraping for cytology smears)
Clinical Behavior
- Metastasis is uncommon
- Tumors are invasive and may extend into the nerves within the dermis and subcutaneous tissue
- SCC nay invade the dermal and subcutaneous lymphatics with subsequent spread (metastasis) to the regional lymph nodes
- Metastasis is more likely to occur with poorly differentiated tumors
- SCC may also recur in certain locations following local excision. This is usually in areas where wide excision might be difficult to accomplish e.g. the eyelid.
References
- Goldschmidt, M.H., & Hendrick, M.J. (2002). Tumors of the skin and soft tissue. In D.J. Meuten (Ed.), Tumors in domestic animals 4 th ed. (pp. 45-119). Iowa: Iowa State Press
- Goldschmidt, M.H., & Shofer, F.S. (1998). Skin tumors of the dog and cat. Woburn, MA: Butterworth-Heinemann
- Gross, T.L., Ihrke, P.J., & Walder, E.J. (1992). Veterinary dermatopathology: A macroscopic and microscopic evaluation of canine and feline skin disease. (pp. 327-485). St. Louis, Missouri: Mosby Year Book
- World Health Organization (1998). Histological classification of epithelial and melanocytic tumors of the skin of domestic animals (2 nd series, vol 3). Washington, DC: Armed Forces Institute of Pathology
- Yager, J.A. & Wilcock, B.P. (1994). Color atlas and text of surgical pathology of the dog and cat. Ontario, Canada: Mosby Year Book.
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