In the vicinity of the oral cavity, the tumour can be located at the lower and upper jaw, behind the eye, at the transition between oral cavity and throat, the tonsils, the trachea.

In the skin, it can appear at multiple places as well: the nasal planum, eyelids, ear canals, penis and vaginal wall. They can also appear in the vicinity of the nailbed.

Behaviour

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.

Prevalence

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.

Breed predisposition

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.

Symptoms

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.

Metastases:

• 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.

Histology

Tissue samples of the tumour can be taken via a thin needle biopsy or a surgical biopsy. After a thin needle has been inserted into the tissue mass (aspiration), its contents can be examined under the microscope (by smearing and staining it on a microscope slide). This approach can be used to estimate whether the biopsied tissue mass is a malignant tumour of the skin. However, when it is necessary to know how far the tumour reaches in the skin (very superficially or deeply infiltrating), tissue samples are taken via a small surgical procedure (a tissue biopsy or punch biopsy).

A direct scraping/imprint of the tissue mass on a microscope slide can -after staining of the slide- sometimes contain cancer cells, but usually only inflammatory cells are seen. Therefore, this method is not likely to give a good representation of what is actually going on.

If the tissue mass occurs on the nasal planum or in the oral cavity, a short general sedation is necessary (the nasal planum is very sensitive and interventions in the oral cavity may initiate to a bite reflex). For digital SCC, a biopsy in every digit abnormality not responding to initial treatment should be carried out to avoid delays in diagnosis and subsequent treatment.

The lymph nodes are rarely involved, unless in an advanced stage.

Imaging

CT and/or MRI scans can provide clarity on the tumour depth, which is necessary to estimate whether surgical resection is possible. Besides the tumour, these scans can also be used to assess regional lymph nodes for indications of metastases. These scans allow a more detailed image than possible via radiographs (can trace potential small metastases that are not yet visible on a radiograph (roentgen photo)). For digital SCC, radiographs should be obtained, as bone involvement occurred in >90% of dogs and helps guide the extent of surgery.

Imaging of the chest will in most cases not show any metastases, as this tumour type has a low propensity to metastasize.

Although a preventive measure could consist of avoiding sun exposure, this is in reality usually not achievable for dogs. Regular inspection of the dog’s oral cavity can lead to early detection of the tumour and potentially less invasive therapeutic options. Depending on the depth of the tumour, the treatment will differ. More treatment options are available for a superficial than an invasive tumour.

Superficial tumour

This location can, when sufficient tumour-free margins can be obtained, treated adequately with surgical excision. Other options consist of local freezing, lasers, light therapy (photodynamic therapy), radiation, possibly combined with injecting chemo into the tumour and debulking (excising tumour tissue without complete margins). The disadvantage of these alternative techniques is that it’s not possible to check whether the entire tumour has been removed or treated. Of note, not all species are alike: radiation of this tumour type on the nasal planum has better results in cats than in dogs.

Invasive tumour

Surgery is the treatment option of choice. Wide surgical excision usually leads to long-term control of the tumour because of the low percentage of metastases.

Radiation therapy -in contrast to cats- does not appear to be very effective for the treatment of SCC of the skin and nose in dogs, though it may be used in combination with surgery if the tumour is not completely removed or if complete removal is not possible. However, good results have been obtained with radiation of the oral cavity (31% of dogs experienced local tumour recurrence after radiation). Chemotherapy gains traction as therapeutic option and is mostly indicated in case of elaborated tumour mass or when metastases have been detected.

Mental health

The mental health of the dog is a factor that should not be underestimated: sometimes the look/face of your pet can change a lot after surgical removal of a (large) tumour. This doesn’t bother the dog him/herself, however, it can alarm the owners, or passers-by and/or raise questions. In general, offering continued attention and love, can reduce feelings of pain in humans and animals. It’s important to avoid boredom, fatigue and isolation in your dog, as this lowers the pain threshold. A regular scheme and environment, sufficient sleep/rest, environmental enrichment, company, sympathy and avoiding unpleasant environments can considerably decrease the inconvenience associated with cancer.

The prognosis depends on the location and size of the tumour, as well as the chosen treatment.

Nasal planum

• When a non-invasive tumour on the nasal planum is detected early, the prognosis is good.
• In a late tumour stage, a cure can be obtained via aggressive surgery. When the entirety of the tissue mass has been removed at once, relapse after surgery is uncommon. However, when the tissue mass has not been completely removed, relapse can occur. When surgery is no longer an option, radiation and chemotherapy can be used with the purpose of delaying tumour growth as much as possible. Positive responses have been obtained with toceranib, a tyrosine kinase inhibitor.

Oral cavity

• A tumour in the front part of the oral cavity has lower chances of metastasis as opposed to a tumour in the back of the oral cavity or at the tonsils. Surgical interventions yield mostly good results and after one year, 57-91% of these dogs are still alive (the numbers vary between studies). Radiotherapy provides a good response for the oral location, with over 70% of dogs still alive after one year. The best results are obtained with surgery and/or radiotherapy. After radiotherapy, relapse was reported in 31% of cases and the most common survival time for only radiotherapy is 15 to 16 months and increases to up to 34 months when combined with surgery. The prognosis is in general good to very good.
• Positive prognostic factors for SCC of the oral cavity are: location in the front of the oral cavity, young age, early detection, location at the upper jaw.

Subungual

• According to the published studies, dogs undergoing digit amputation will have a 1- and 2-year survival rates of 81-95% and 60–74%, respectively.