Changing Attitudes about Cancer: Prevention, Screening, and Early Diagnosis
Rodney L. Page MS, DVM Diplomate ACVIM
Major advances in cancer management have occurred over the last decade. Improvements in diagnosis, imaging and staging have identified strategies leading to better management of cancers that are clinically detectable. In addition, significant improvement in supportive care and in particular palliative management of cancer is companion animals has resulted in many more options for owners considering treatment for their pets with cancer. However, it is still clear that too many dogs and cats die from cancer.
Cancer control, until recently, has been focused on treatment of already existing, clinically detectable disease. However, this strategy is not optimal for control of most types of cancer since even if the tumor is diagnosed when it is small (1 cm diameter) the process is already well advanced and may be beyond our capability of complete eradication of all cancer cells. The goal of cancer research is to decrease the morbidity and mortality associated with this disease and in order to achieve this goal we must: 1) cure existing and invasive cancer, 2) control preinvasive cancers and, 3) prevent new cancer.
Early diagnosis and prevention could do more to reduce the impact of cancer on the lives of companion animals than cancer treatment programs are able to accomplish. The key to early diagnosis and prevention of cancer is to identify patients at increased risk of cancer development at a stage when interventions will be most successful. For some tumors, treatment is not curative even if a relatively early diagnosis is made. In dogs, tumor types that fall into this category include oral melanomas, hemangiosarcoma, osteosarcoma, bladder cancer, nasal tumors, brain tumors and most tumors arising from internal organs. In cats, oral squamous cell carcinoma, vaccine-associated tumors and mammary gland tumors are difficult to control once they have developed. If risk factors for development of these tumors could be identified, specific screening programs could be developed which would allow detection at stages where better outcomes may be offered with conventional therapy. More importantly, detection of pre-invasive forms of malignant tumors affords the opportunity to initiate strategies to keep the cancer contained. New compounds specifically developed to keep tumors "differentiated" rather than eradicate them is a rational method of cancer control since these strategies are much less toxic than conventional therapy and may be just as useful. This new field is called chemoprevention. Chemoprevention of cancer is conceptually similar to prevention of cardiovascular diseases in patients that are at high risk by administering medication to lower cholesterol and blood pressure before any symptoms occur. An example of chemoprevention for cancer in women is the use of tamoxifen to reduce the incidence of recurrent breast cancer.
Conventional risk factors (signalment, family history, carcinogen exposure) can identify some general populations of dogs and cats at risk of developing certain cancers. Although not available for veterinary medicine yet, new technology is making pre-malignant changes easier to detect in some populations of humans and surrogate markers for cancer development are being used to screen for tumors of internal organs more easily. It is currently possible to assign risk factors in companion animals for some of the more common and most difficult tumors to control. From this information, specific screening programs and owner recommendations for prevention and surveillance of these cancers can be developed as described below.
General Recommendations for Cancer Screening
As in humans, screening tests for cancer will evolve as new technology is developed and information is acquired to suggest improved outcome or quality of life. Colonoscopy, mammography and tissue specific protein assays (Prostate, Ovarian cancer proteins) have become mainstream screening tools for humans. In companion animals, it is well known that certain breeds are prone to develop cancer. In general, because cancer is a common disorder of older dogs and cats, animals beyond the age of 7 or 8 years of age should be considered "at risk" for cancer. General screening recommendations such as bi-annual physicals, screening laboratory bloodwork and urinalysis are becoming more common for geriatric animals. In addition, the use of cutaneous maps to chart the location, size and diagnosis of all skin masses will help to determine rapid changes in growth or any new masses to be concerned about.
Owners should be encouraged to take responsibility for prevention of cancer and frequent screening. Early neutering of male and female dogs is the best example of cancer prevention. Dogs are now known to develop upper respiratory and lung cancer when exposed to second-hand smoke and should be removed from a passive smoke environment. Observation of bowel or urinary habits should be strongly encouraged. Owners should also be able to accurately evaluate mammary glands, peripheral lymph nodes, oral cavity structures, examine interdigital spaces and external ear canals.
Recommendations for more elaborate screening will depend on client concern for early detection, financial considerations and validation of the benefits for early screening. Recommendations for thoracic radiographs, abdominal ultrasound, colonoscopy and even CT/MR of the nasal cavity and brain may not be too extreme if early diagnosis means improved outcomes for serious forms of cancer.
Specific Prevention and Screening for Cancer in Dogs
Specific Prevention and Screening for Cancer in Cats
Cancer screening recommendations for humans have become more thorough recently. Screening programs are focused on specific tumor types (colon, prostate, breast) and on specific populations of people. There is isn't any reason such screening programs shouldn't be developed for dogs and cats as well. Such programs will obviously make a major impact on cancer control in companion animals just as they have in people.