Also referred to as "malignant neoplasms" or "malignant tumours", cancer is the uncontrolled division of cells that normally should be restrictive in their growth. Tumours are split into two categories, malignant (cancerous) or benign. Benign tumours grow slowly, are surrounded by a capsule and do not invade neighbouring tissue or spread to other areas.
Cancers, on the other hand, tend to grow rapidly, invade surrounding tissue and spread to other parts of the body.
Cancer can occur in any cell type within the body and in any location and body system. For example the brain, lungs, liver, blood, bones, skin, intestines etc. There are different "types" of cancer and are classified by the type of cells involved. For example mast cell tumours, basal cell carcinoma, adenocarcinoma (originating from glandular cells such as breast or kidney), leukemias (cancers of the blood cells) etc. So, a cat may have skin cancer and it could be a melanoma, squamous cell carcinoma, basal cell carcinoma etc. All skin cancers, but originating from different cell types.
The most common cancers to affect cats are:
Lymphoma is cancer of the lymphatic system. It is the most common cancer found in cats and is responsible for 1/3rd of all cancers. It arises from lymphoid tissue, which is found throughout the body and may involve any organ or tissue. Cats with feline leukemia virus are 60 times more likely to acquire lymphosarcoma than those without. Cats living in smoking households are twice as likely to acquire lymphosarcoma. 
Fibrosarcoma is an aggressive type of malignant growth (cancer) that originates in the fibrous connective tissue. It is the third most common skin tumour of the cat.
Squamous cell carcinoma (SCC) is a common malignant tumour of the skin. Most often caused by excessive exposure to sunlight it is seen most often in white cats or cats with white patches. Tumours can appear anywhere on the body although areas most commonly affected are the ears, nose, mouth and eyelids. Older outdoor cats are more commonly affected than younger ones.
Cancer can occur in cats of any age, but it is most commonly seen in middle-aged to older cats. It is a leading cause of death in elderly cats.
What causes cancer in cats?
There are a number of causes of cancer in cats, some of which include:
- Carcinogens (agents which can cause cancer). Examples of carcinogens include UV radiation, X-Rays, certain chemicals, cigarettes etc.
- In many cases, the cause of cancer is not known.
What are the symptoms of cancer in cats?
Cancer symptoms will vary depending on the location and part of the body affected by cancer. Some common symptoms may include:
- Change in bowel or bladder habits (constipation, diarrhea etc)
- Blood in urine or feces
- Abdominal pain
- Difficulty breathing
- Loss of appetite
- Lumps, bumps, crusty lesions on any part of the body
- Swollen lymph nodes
- Vomiting (sometimes with blood)
- Wounds or ulcers that don't seem to heal
- Weight loss
What is the treatment for cancer in cats?
Treatment may vary depending on the location but may include:
- Surgery to remove the tumour and surrounding tissue.
Chemotherapy is usually administered at a specialist veterinary centre. It doesn't cause hair loss in cats but in my experience, did cause our cat to be lethargic and off her food for one to two days after administration.