The skin consists of multiple cell layers, of which the cells of the top layer (the epithelium) are called squamous cells. These cells can degenerate into cancer cells which multiply, thus forming a tumour called a squamous cell carcinoma.
These skin tumours can appear on multiple sites in the body and tend to behave more aggressively in the oral cavity than in the skin. In general, they mostly grow locally and have a low tendency to metastasize. In dogs with subungual tumours similar tumours may develop on other digits in 2 or 3 years following initial diagnosis, but the risk of local recurrence or metastatic spread is extremely low.
Squamous cell carcinomas (SCC) affect primarily older dogs of larger breeds. It’s the most prevalent malignant tumour of the epidermis and the second most prevalent of the oral cavity. Especially light-haired and thin-haired locations of the skin are sensitive to the development of SCC, presumably due to UV radiation.
The most common age at which this tumour type appears is 10-11 years in dogs with tumours on the nasal planum, whereas for the oral cavity the average age is 8-10 years.
Squamous cell carcinomas arising from the nail bed are the most prevalent digital neoplasms in dogs, representing between 36-50% of all digital neoplasms in dogs. They occur primarily in medium- and large-breed dogs with a black or predominantly black haircoat, with a mean and median age of 10 years at the time of first diagnosis.
This tumour type appears more often on the nasal planum of the following breeds: Labrador and Golden retriever, Blood hound, Basset and Standard poodle (in general especially larger dog breeds).
Moderately large black, or black and tan, dogs have a marked increase in the prevalence of subungual squamous cell carcinomas. This tumour type appears more often at or near the nailbed of the following breeds: giant schnauzers, standard schnauzers, black standard poodles, Rottweilers, Gordon setters, Briards, and Kerry blue terriers. The prevalence in giant schnauzers has consistently been substantially higher than in any other breed.
The development of invasive squamous cell carcinoma (SCC) of the skin stretches over an extended time line (months to years) and mostly follows the following stages during its development (which are not necessarily all noticed by the owner):
- Stage 1: redness and crusts
- Stage 2: superficial erosion and ulceration
- Stage 3: deeply invasive and eroded lesions
If the tumour is located in the oral cavity, following symptoms can occur: increased drooling, a facial swelling, bleedings from the mouth, weight loss, bad breath, difficulties swallowing, pain when opening the mouth. Loose teeth can be an indication that the tumour has affected the jaw.
Appearance: very variable (can be papillary, resemble a crater or have the shape of a mushroom). In the oral cavity, its appearance can vary between a red elevation, a cauliflower-like or ulcerated swelling. For oral SCC, in more than 70% of dogs the underlying bone is affected.
Clinical signs of digital squamous cell carcinoma mainly include digital swelling and lameness, with potential loss of the nail of the affected toe. For digital SCC, in more than 90% of dogs the underlying bone was affected. Digital SCC can be difficult to diagnose because its presentation can mimic other conditions such as infections, cysts and other tumour types. In one study, a duration of symptoms of >90 days was significantly associated with higher risk for tumour progression. The diagnostic delay is common because clinical signs are initially mild and may be disregarded at first.
- The skin variant: metastases seem to appear very little
- The nasal planum variant: metastases mostly occur towards the local lymph nodes
- The oral form located in front of the oral cavity: metastases occur in less than 10% towards the local lymph nodes and towards the lungs in 3-36% of dogs
- The oral form located at the back of the oral cavity (the tonsils and tongue base): metastases occur more easily (in up to 73% of dogs). At this time, few documentation is available.
- For large black (or predominantly black) dogs with subungual squamous cell carcinoma, there is a substantial risk that the dog will develop similar tumours on other digits in 2 or 3 y following initial diagnosis, but the risk of local recurrence or metastatic spread is extremely low.