Cancer

Probably the word we most dread to hear, yet one that all of us who have wolfhounds are likely to hear at some time, since it is estimated that one in ten dogs will have some form of tumour. It is also a subject on which there are many misunderstandings and misconceptions, not only among owners but in the veterinary profession. The last is probably due to the fact that widespread treatment of cancer in animals is still relatively new and has only been included in the course of study at veterinary schools in the UK over the past three decades.

What is cancer?

The proper name is neoplasia - literally "new growth" - and the features which define it are persistence/purposelessness/proliferation. There are two types of neoplasia: malignant which destroy, invade, and metastase (spread); and benign, which grow locally and by expansion rather than invasion. A benign tumour is like an egg, with well defined edges which can be felt around. It can usually be removed by surgery. Even a benign tumour, though, can cause problems through pressure, for example if it occurs in the brain. A malignant tumour has no clear edges and will ulcerate and spread. Since the boundary cannot be defined it is impossible to remove all the cancer cells by surgery. Malignant tumours metastase by lymphatic invasion, haematogenous route, or transcoelonic spread. Such metastasis in dogs is generally to the lungs but it can be to any part of the body. The only way to tell the difference between a benign and malignant tumour is to examine a sample under a microscope. Benign tumours can become malignant.

What kinds of cancer occur in dogs?

In dogs generally 50 per cent of all tumours are of the skin (of which about 40 per cent are malignant and 60 per cent benign), 20 per cent are of the mammary glands (which account for 58 per cent of tumours in bitches, of which about 50 per cent are benign), only 10 per cent are of the alimentary system (which is one of the prime sites in Man), 10 per cent of the lymphatic system, 5 per cent of the reproductive system, and 5 per cent others.

However, bone tumours are much more common in the large and giant breeds. They are most likely to occur at the ends of long bones in the growth plates but may occur on the skull, pelvis, etc. Bone tumours are mostly extremely malignant. Signs are swelling, pain, heat, and lameness (if the tumour is in a limb bone). However, although bone tumours on the forelimb (especially at the distal [lower] end of the radius/ulna - just above the knee joint and a favourite site) or those on the distal end of the tibia/fibula (just above the hock joint) on the hind leg are easy enough to detect, particularly when the typical hard swelling starts, those in other sites can be much more difficult to diagnose. This is especially so if they are on bones where there is heavy surrounding musculature, such as the shoulder-blade, upper arm, or thigh. These are all too likely to be diagnosed as pulled muscles, damaged tendons, arthritis, etc. Also, osteosarcoma (the most common form of bone cancer in these breeds) often starts up after some kind of trauma such as a collision or fall and is most likely to occur in the age range of 6 to 8 years, so the tendency is for a diagnosis of bruising, arthritis, etc. in any case.

Oral tumours are quite common and mostly malignant. One problem is that they are often well advanced before being noticed. Signs are bloody saliva, bad breath, difficulty in eating, or unwillingness to eat. Malignant melanoma are particularly dangerous and spread rapidly.

Nasal tumours are quite rare but mostly malignant. Signs are sneezing, difficulty in breathing, discharge from the nose, or nosebleeds. They do not spread quickly but cause much local damage.

Tumours of the gut are rare but usually malignant and generally well advanced by the time they are noticed. Spleen tumours are usually malignant with early secondaries. Signs of internal tumours are non-specific but include weight loss (sometimes rapid), diarrhoea or constipation, pain which may be shown by a stiffness of movement, passage of blood or vomiting of blood or "coffee-grounds", inappetance.

Tumours of the lungs (a prime site in Man) do not occur as primaries in dogs but frequently as secondaries. Signs are difficulty in breathing, particularly after exercise, coughing, and wheezing. This can often be misdiagnosed as lungworm.

Cancer of the lymph system is quite common and can involve a combination of lymph glands, bone marrow, circulating blood, and internal organs. Signs are enlargement of glands, depression, anorexia, and weight loss. These cancers include lymphosarcoma and leukaemia. There are two types of canine lymphoma: the multicentric which affects the whole body, and one which develops only in the alimentary, cutaneous, and thymic glands.

Skin tumours can vary from granuloma, which are pea-sized, to vast lumps. Not all lumps which appear on the skin are tumours. Sebaceous cysts are very common, particularly in the elderly dog, but are only blocked sebaceous glands and quite harmless, although they do sometimes burst. It is a good rule, though, to have any lump removed and tested.

What causes cancer?

The cause is generally unknown but some factors have been identified. Hormones affect the growth of some tumours such as mammary tumours, and perianal tumours which depend on male hormones. A virus which causes oral papillomas has been identified, and air pollution is important in the development of tonsil and lung tumours. Sunlight is believed to cause some skin tumours. At the BSAVA Hereditary Diseases Seminar in May, 1988, Professor Patterson mentioned that osteosarcoma in the giant breeds is due to a cancer gene. A possible cause of cancer is geopathic stress. Click here for more.

Can cancer be prevented?

There is a significant decrease in the incidence of mammary tumours in bitches which are spayed before their second season. Tumours of the uterus and ovaries are prevented by spaying, and testicular and perianal tumours by castration. Retained testicles are prone to tumour formation and should be removed before tumours occur. Oral warts (papillomas) can be prevented by avoiding contact with affected dogs. Since tumours of the digestive tract are not common in dogs, it is unlikely that diet can be used as a preventative measure (as it can in Man), although anyone who had several dogs suffer such tumours should perhaps review their feeding methods. (This is a conventional veterinary view. In my opinion diet does more than affect the digestive system; it affects the whole body) Lines prone to suffer osteosarcoma should be avoided in breeding programmes.

Is there any treatment for cancer?

Not all types of cancer are treatable (nasal tumours, for example, are not) but many are and there are several different types of treatment which can be used. Some will actually effect a cure, in other cases the tumour will not be cured but may be controlled and the effects of the tumour (e.g. pain) alleviated.

Benign tumours can generally be cured by surgery and surgery remains the major form of treatment in most cases of cancer. Although surgery does not address the problem of very invasive tumours, because of the difficulty of removing all the cancer cells, it can still be used in conjunction with other forms of treatment.

Other methods of treatment are radiotherapy, chemotherapy, cryosurgery, and hyperthermia, although there are new methods of cancer therapy starting to come into use, such as gene therapy for osteosarcoma.

At one time in the UK the favoured treatment for osteosarcoma was radiotherapy but now it seems to be amputation of the affected limb together with chemotherapy. Radiotherapy did not cure but does remove the pain and slow down the rate of growth of the tumour. A high proportion of bone tumours metastase to the lungs; radiotherapy does not prevent metastasis but nor does amputation. Amputation only removes the primary tumour and radiotherapy does much the same while leaving the dog with all four limbs. The main danger with radiotherapy is pathological fracture but the danger is reduced the earlier treatment is begun. If treatment is not carried out early, or if the tumour is very aggressive and fast growing, a large part of the bone may have been invaded by cancer cells and, when these are killed by radiation, there is obviously going to be a narrower than normal portion of bone. Fractures of bones treated with radiation will not heal, so that if a fracture occurs the only alternatives are amputation or euthanasia.

Radiotherapy in dogs does not cause the awful side effects that it does in humans. This is partly because it is not used for widespread neoplasia or for tumours in major organs, and partly because of the dosage and frequency of treatments. It is not true that radiotherapy treatment will cause a dog to suffer the most appalling side effects which will make the few weeks of added life a misery. The only side effects likely to occur in a dog are loss of hair over the treatment site (which generally grows back white) and sometimes an effect on the skin over the treatment site similar to sunburn. Neither effect bothers the dog in the slightest. It is not true, either, that radiotherapy will give only a few weeks extra life. If a dog is accepted for radiotherapy it is impossible to say how long it will have as this depends so much on how long the tumour has been there, how aggressive it is, and how quickly it metastases, but the likelihood is that the extra lifespan will be measurable in months rather than weeks. The factors involved are, in any case, involved in exactly the same way with any other therapy. However, radiotherapy for dogs is not available everywhere.

Dr. Stephen Withrow, chief of the Comparative Oncology Unit at the Colorado State University Veterinary Teaching Hospital started a new treatment protocol (reported in the Cornell Newsletter of April/May, 1987) for osteosarcoma using a combination of radiation, chemotherapy, and surgery in an effort to avoid amputation of affected limbs. The procedure was to treat the tumour with chemotherapy and radiation to halt the spread of the tumour at its margins, then the cancerous bone was removed and replaced with a bone graft. After this procedure, limb function was good to excellent, especially when the forelimb was involved. Cancer control was achieved in about 90 per cent of treated dogs and the incidence of tumour spread greatly reduced. One year after treatment, fewer than half the dogs treated had died of metastatic tumours compared with the 85 per cent that die within one year after being treated by amputation alone.

At a Cornell seminar, Dr. Claudia Barton of Texas A & M University reported that remission of lymphosarcoma can be achieved in 9 out of 10 cases that are detected early - that is, before the bone marrow, blood, and liver are affected. Even when the disease has progressed to the stage where these sites are involved and the dog has stopped eating and become lethargic and depressed, remission can usually still be obtained, although the length and cost of chemotherapy and the risk of adverse side effects are greatly increased. A dog in an advanced stage of the disease before treatment begins tolerates the drugs poorly. Untreated dogs live on average only 30 to 60 days.

In Dog World (the American version) of March 1987 there was a report on a "glimmer of hope for dogs afflicted with osteosarcoma". Dr. Gregory MacEwen, oncologist at the University of Wisconsin-Madison Veterinary Medical Teaching Hospital, Dr. Isaiah J. Fidler of the Tumor Metastasis Branch of the National Cancer Institute, and the Swiss pharmaceutical company Ciba-Geigy had been collaborating on a project to produce multilamellar vesicles (MLV) to stimulate the body's immune system to fight cancer. The idea was to utilise the cells called macrophages, which are located in the walls of blood vessels and loose connective tissue, and which act as "garbage men" by clearing the system of foreign substances. The macrophages can be stimulated by immuno-stimulant drugs such as muramyl dipeptide and muramyl tripeptide, which are derived from a tuberculosis vaccine, combined with liposomes to form a MLV which keeps the drug from floating freely in the body and, as a foreign body, attracting the attention of the macrophages.

This technique was combined with amputation of the affected limb and only increased the survival time from 3 or 4 months to 6 or 7 months but it had no side effects and possibly could be used in a combination treatment with radiation, chemotherapy, or surgical techniques.