A diagnosis of cancer in pets often brings with it some overwhelming emotions among pet parents, including a sense of loss of control and hopelessness. It is the number one natural cause of death in geriatric dogs and cats and it accounts for nearly 50 percent of deaths each year. Although cancer is the leading cause of death in geriatric patients, it’s also the most treatable disease when compared with congestive heart failure and renal failure. Let’s see how.
Cancer is the unrestrained growth of cells that destroy normal tissues and body parts. Majority of pet parents have common under standing that detection of cancer is the beginning of the end of a pet. When treating an animal patient with cancer, overcoming the pet parents’ fear is the first job for every veterinary team.
To understand cancer as a process, let’s look at the development of a tumour. Most cancers are believed to arise through a process called multistep carcinogenesis. This theory is based on the fact that in the majority of cancers, at least two genetic changes have occurred prior to the induction of malignancy. There are three basic steps in multistep carcinogenesis. These steps ultimately lead to the evolution of a cancer cell from a normal cell.
Initiation: Initiating agents induce a permanent and irreversible change in the DNA of the affected cell. In and of itself, the initiating event is not significant enough to induce neoplastic transformation. Initiated cells cannot be distinguished under the microscope from other cells in the surrounding environment.
Promotion: Promoting agents cause reversible tissue and cellular changes. Promoting agents can induce changes in the shape of a cell, its growth rate, and degree of terminal differentiation. Promotion serves to expand the initiated cell population and alter it in such a way as to increase the likelihood of occurrence of irreversible change.
Progression: Progressing agents are able to convert an initiated cell, or a cell undergoing promotion, into a cell exhibiting malignancy, capable of developing into a mature cancer. The process of progression is irreversible.
In order for a tumour to result, the affected cell must be irreversibly altered at least twice. The cell is altered once in the initiation phase and once in the progression phase.
Cancer can be treated with radiation therapy, which can be categorised broadly in two:
1. Systemic Radioisotope Therapy
The radionuclides with appropriate beta energy are administered orally, intravenously or locally in the lesion. This therapy is not very popular because of the requirement of dedicated facilities for housing animals administered with therapeutic radiopharmaceuticals. The most common therapies for animal cancers are Radioiodine Therapy and Bone Pain Palliation Therapy.
Radioactive Iodine Therapy: Radioactive iodine has become the treatment of choice for treating feline hyperthyroidism because of the single dose regimen and lack of side effects. This treatment is based on the fact that the thyroid gland is the only tissue in the body that actively accumulates iodine, which it uses to produce thyroid hormones. Radioactive iodine is rapidly absorbed by the hyperfunctioning thyroid tissue. The radioiodine emits a beta particle which selectively destroys the tumour while leaving normal tissue undamaged. Thyroid function typically returns to normal in 1-3 months. Before treatment with radioiodine, a thyroid scan is performed using a low energy radioisotope (technetium pertechnetate). The scan will confirm the diagnosis, identify the number of abnormal thyroid lobes, and aid in determining the radioactive dose required for effective treatment. The majority of cats (90 percent) need to be treated only once. Most cats are hospitalised from 5-7 days, depending on the dose of radioiodine administered and the excretion rate of the iodine. Once admitted, your cat cannot be discharged until her
radiation exposure rate is below permissible limits.
Therapy for Bone Primary and Secondary Lesions: Osteosarcoma and osteoblastic bone lesions due to secondary spread of cancers such as bone mammary gland cancer and prostate cancer are managed with systemic radionuclide therapy using 32P, 89Sr and 177Lu. Selectively these radiopharmaceuticals localise in osteoblastic lesions and the beta energy of the radionuclide kills the cells and controls the growth of the cancer. This therapy is used for treatment of skeletal micrometastasis. The haematological toxicity is one of the major concerns but this has been reduced to greater extent with new agents such as 153Sm and 177Lu labeled radiopharmaceuticals.
2. External Radiation Therapy
Radiation therapy is effective for control of certain types of cancer. It may be used alone or in combination with other forms of treatment. Radiation therapy is indicated for the treatment of tumours that would have unacceptable functional and/or cosmetic side effects if they were surgically removed. It can also be used to eliminate residual disease left behind when surgery could not remove the entire tumour without creating serious side effects. All of the side effects associated with radiation therapy will be limited only to the area where the radiation is applied. ‘Radiation sickness’, manifested by nausea, vomiting, and diarrhoea, is not seen in veterinary patients.
Radiation therapy cannot be given in a single large dose sufficient to control tumours without causing severe complications. Giving small doses over a period of time gives the best chance of controlling the tumour with minimal damage to surrounding normal tissues. Radiation therapy is usually given in 12 or more small doses of radiation over a 3-4 week period. Each treatment requires 10-30 minutes. The patient must remain perfectly still during the treatment so that the radiation only goes where it is needed. A short-acting anaesthetic is given to immobilise the patients. There is a small but definite risk associated with repeated anaesthesia; therefore, patients are monitored closely.
Many factors affect the response of tumour to radiation therapy. Larger tumours require larger doses than smaller ones. The anatomical location may mandate that sensitive normal tissues, such as the eye, be included in the treatment field. The type of tumour is also important as well as the type of radiation therapy used. The effects of the radiation therapy are not instantaneous. Most tumours will not have any visible changes for several weeks. Some will not change in size, but stop growing. The most important thing to remember is that while the side effects will be manageable and transient, tumour control should be permanent.
Computerised treatment planning systems are used to improve the localisation and distribution of the therapeutic beam within the patient. This limits the dose to normal tissues and also increases the dose to the neoplastic tissue being treated, increasing cure or control rates and reducing the severity of normal tissue complications. These programmes are best used in conjunction with CT or MRI images, which determine the position and extent of the tumour within the body and its relative position to normal structures. Many hours of planning may be required to generate a treatment plan for a large, complex tumour.
Whenever possible, elimination of a tumour by surgery is preferred. However, in many instances, large neoplasms, or those in critical areas such as the brain, are not amenable to complete or even partial surgical removal. Even when a tumour is grossly removed, microscopic foci of neoplastic cells may extend beyond the limits of the surgical field. In all of these instances, radiation therapy, often in combination with chemotherapy, is useful in treating the remaining cancer. Radiation therapy is often the treatment of choice for brain tumours, nasal tumours and other neoplasms of the head and neck. Radiotherapy is also used to treat soft tissue tumours of the skin and subcutis either before or after surgery. It is seldom used in the treatment of lung neoplasia or in the treatment of neoplastic disease of the abdominal cavity, due to the mobility of tumours in these areas. As the sophistication of radiotherapy techniques increases, more and more types of neoplasia are being treated at least in part by radiation therapy.
In the course of radiation treatment of cancer, some surrounding normal tissues will be affected. The radiation-induced effects to normal tissues usually do not begin until the end of the therapy period and they continue for several weeks after the treatments have ended. These are called the acute side effects. They usually resolve within a few weeks to a month after radiation has been completed. Other adverse effects associated with radiation therapy may occur many months or years after radiation is complete. These are called delayed adverse effects. While the adverse effects of radiation therapy are difficult to predict, a few of the most common possible effects are listed below.
Skin: Radiation reactions that may appear toward the end of radiation therapy include loss of hair and a sunburn-like effect to the skin that may become itchy, dry or moist. Most pets develop a change in the colour of the skin and hair in the area being treated and, occasionally, hair will fall out and not re-grow in that area. Other changes to the skin that are much less common include formation of a non-healing wound and the formation of thickened scar tissue in the area being treated.
Mouth: If tumour in or around the mouth is being treated, injury to this area can result in a sunburn-like effect to the tongue and the tissues lining the mouth. This can result in loss of appetite, altered tongue function and tenderness to the lining of the mouth.
Large intestine and rectum: Occasionally, the colon and rectum are affected when tumours in that area of the body are being treated. Most pets have only mild, transient side effects that can include loose stool (bowel movement) that may contain blood, and perhaps some discomfort passing stool.
Eye: The eye is often unavoidable in the treatment field when tumours of the facial skin, sinuses or nasal cavity are treated. While most pets do not show any adverse effects associated with damage to the eye itself, side effects can include cataract formation months to years after therapy is finished, damage to the retina, decreased tear production, and irritation to the tissues around the eye. Occasionally, an ulcer of the cornea may be noted.
More than fifty percent of cancers are treated alone with radiation or in adjunct to other modalities. So, if your pooch is detected with cancer, do not lose hope!
(Dr PR Chaudhari, MVSc, DMRIT PhD, is stationed at Comparative Oncology & Experimental Nuclear Medicine, Advance Centre for Treatment, Research and Education in Cancer, Tata Memorial Centre, Navi Mumbai).