Adenocarcinoma in dogs: an introduction
Canine adenocarcinoma is a type of cancer that arises from glands in the body and can affect multiple organs and organ systems, including lungs, kidneys, mammary glands, intestines, prostate or the pancreas.
These cancers are usually very aggressive, characterised by high rate of metastasis, early metastasis, very quick growth and a high degree of local invasion [1, 2, 3, 4, 5, 6].
Signs and Symptoms of Canine Adenocarcinoma
As adenocarcinomas can occur in numerous tissues and organs, they can present with a wide range of symptoms depending on the organs affected:
- Lung adenocarcinoma often presents with cough (seen in 52% of cases), difficulty breathing (dyspnea, seen in 23.8% of cases), lethargy (in 18.1% if cases), weight loss (in 12.4% of cases), and rapid breathing / panting (tachypnea, in 4.8% of cases) [4].
- Gastrointestinal adenocarcinomas usually present with symptoms like anorexia, diarrhoea, appetite loss, vomitting, and weight loss [6].
- In the case of prostatic adenocarcinoma, dogs may present with difficulty urinating, back pain and kyphosis (arching of the spine), and nodules on the skin [1, 5].
- Uterine adenocarcinoma often causes reduced appetite, occasional vomiting, and a change of behaviour to general dullness [7].
- Renal adenocarcinoma may present with weight loss and lumbar pain [5].
- Pancreatic adenocarcinoma can cause symptoms like jaundice (yellowing of the eye whites), anorexia, and vomiting [8].
- Mammary adenocarcinomas can present with a variety of symptoms depending on the stage and location of the tumor, however the most common presentation is a palpable lump or mass which is growing rapidly [4].
Diagnosis and Treatment
The diagnosis of adenocarcinoma in dogs is usually achieved through a combination of clinical examination, imaging and histopathology.
On clinical examination, the veterinarian will observe the signs such as cough, dyspnea or will palpate (feel) for masses which could indicate a tumour [9, 10].
Imaging such as X-ray or computed tomography (CT) is used to confirm the location and size of the tumour. More advanced modalities, including positron emission tomography (PET) are available, however, their usage is at the discression of the owner, and is often associated with greater expense. However, PET is a very useful diagnostic tool as it allows to ascertain the metabolic activity of the tumour, meaning it can be used to predict how quickly the tumour will grow and/or metastasise [11].
In addition, the tumour tissue and blood can be analysed for cancer markers. The classical adenocarcinoma biomarkers include the presence of cancer stem cells or signet ring cells and pleomorphic cells (cells with various shapes and sizes) in the tumour tissue, or molecules such as TTF-1, Napsin A, SP-A, CD44, and CD133 [12, 7].
Next-generation sequencing is also a potential diagnostic modality for detecting cancers in dogs and humans alike. DNA from blood is analysed to identify mutations or presence of cancer-associated strands of nucleic acids. In dogs, sequencing demonstrated a 54.7% sensitivity and a 98.5% specificity in detecting multiple canine cancers, including adenocarcinomas [13]. This is, however, also at the consideration of the owner, and is very expensive if paid out of pocket.
Treatment for Canine Adenocarcinoma
The treatment for a cancer in dogs is similar to that in humans, including a combination of surgery, chemotherapy and/or radiotherapy.
For canine adenocarcinomas, surgery is the first-line treatment option [8, 14]. Surgery focuses on removing the whole tumour, often with safety margins (portions of normal tissue surrounding the tumour in order to ensure that all cancer cells have been taken out). Additionally, radiotherapy to the tumour site can be used to shrink the tumour or instead of surgery; dogs with nasal adenocarcinoma with radiation therapy had a longer survival time than those with no treatment in a clinical study [8]. However, similarly to human cancers, some canine adenocarcinomas may not respond to radiotherapy [15].
Chemotherapy can also be used, either pre-operatively or following the surgery, especially in the case of metastatic cancers. Drugs of choice include a combination of fluorouracil and cyclophosphamide, or mitoxantrone [8]. Carboplatin, arguably the most commonly used chemotherapy in humans, is also used to treat canine adenocarcinomas [16].
In some cases, targeted therapies such as monoclonal antibodies may be used. For example, an anti-canine PD-1 monoclonal antibody was used to treat a dog with stage III oral adenocarcinoma, resulting in partial remission of the tumor [17]. These are, however, not used routinely in clinical practice due to their high cost.
Prognosis and survival without treatment
The survival rate for canine adenocarcinomas depends on multiple factors, primarily the size, stage and location of the tumour.
In colorectal adenocarcinoma, dogs that did not undergo surgery survived for 15 months on average [18].
For nasal adenocarcinoma (typically treated with radiation), dogs without treatment survive for 126 days on average [8]. Additionally, nosebleeds (epistaxis) are an important prognostic factor: dogs with nosebleeds survive for 88 days on average from diagnosis, whereas dogs without nosebleeds survive for longer, 226 days on average [19].
For dogs with gastrointestinal adenocarcinomas, 60% are expected to live for one year post diagnosis, and 36% are expected for 2 years [20]. Another study, however, indicates a median survival of only 10 months for some intestinal tumours, as well as the one- and two-year survival rates were 40.5% and 33.1%, respectively [21].
