Chapter 26 Principles of Oncology
Cancer management in animals has evolved considerably over the past 3 decades as the result of several significant factors. Improved health care of animals has increased the age distribution of pets and hence their likelihood of developing cancer; clients are more aware of aggressive treatment choices; and there have been significant improvements in treatment success. There remains some controversy and confusion over the best course of treatment for many tumor types, and more studies are needed to provide the necessary data. However, clinicians can use a generic framework for evaluation and treatment management of many tumor types. This chapter provides an outline useful for clinical management of an animal with cancer.
INITIAL CLINICAL PRESENTATION
Many neoplasms more commonly affect animals of a certain age, sex, or breed, and such knowledge often aids diagnosis. Table 26-1 is a partial list of specific breeds and characteristics of dogs and cats predisposed to certain types of neoplasia.
|Viral papilloma||Young dogs|
|Anal Sac AdenoCA||Females|
|Meningiomas||Females (dog), males (cat)|
|Squamous cell carcinoma||Nonpigmented regions|
|Malignant melanoma||Darkly pigmented regions|
|Skin tumors||Basset, boxer, bull mastiff, Scottish terrier, weimaraner|
|Mast cell tumor||Brachycephalic breeds, Retrievers|
|Bone tumors||Large/giant breeds|
|Thyroid tumor||Boxer, beagle, golden retriever|
|Hemangiosarcoma||Retrievers, German shepherd|
|Lymphoma||Retrievers, boxers, mastiffs|
|Histiocytic malignancies||Burnese Mountain Dogs, retrievers, rottweillers|
The onset and duration of a mass, its growth rate, the presence of other masses, signs of paraneoplastic syndromes, and knowledge of prior treatments further help narrow diagnostic and treatment options and define the behavioral characteristics of a neoplasm.
Examination is used to define the extent of tumor burden and identify concurrent diseases that may limit treatment or affect survival. Tumor characteristics such as size (measure with calipers to determine objectively), location (e.g., oral melanoma is more malignant than cutaneous melanoma), invasiveness (is mass fixed to adjacent tissue), and presence of ulceration or necrosis of the tumor is important to determine tumor behavior and to plan adequately an appropriate biopsy and treatment regimen. Regional lymph nodes are evaluated for size, consistency, and fixation to adjacent tissues. To complete the clinical evaluation, a list of differential diagnoses is made and a diagnostic and staging plan is determined.
A diagnosis of cancer can evoke considerable emotional response from owners. For many clients, the diagnosis implies pain, discomfort, and impending death of their pet. Proper counseling on the part of the veterinarian should include the following:
Assess general health status to identify concurrent disease or paraneoplastic syndromes that may adversely affect prognosis and limit or alter therapy. After a thorough physical examination, perform screening laboratory evaluations, including a complete blood count, serum biochemistry panel, and urinalysis. Perform other special laboratory tests as indicated to aid diagnosis (e.g., feline leukemia virus, feline immunodeficiency virus, bone marrow aspirate, adrenal or thyroid function tests, etc.).
Cytology is useful to evaluate fine-needle aspirates of masses and lymph nodes, bone marrow aspirates, and buffy coat and peripheral blood smear preparations. It can provide rapid and inexpensive diagnostic and staging information. Do not over-interpret cytologic preparations. Base treatment decisions on cytologic diagnosis only when a definitive diagnosis can be made by a pathologist, as with lymphoma and mast cell tumors. Although a diagnosis may be accomplished with cytologic preparations, histologic assessment is usually necessary to determine prognosis (i.e., grade of malignancy).
Many techniques are available for tumor biopsy. The method selected should safely and simply procure an adequate tissue sample to provide an accurate diagnosis without making definitive treatment more difficult or invasive.
Evaluation of Pathology Reports
The pathology report should include:
If aspects of the report are questionable or do not match your clinical judgment, always discuss the results with the pathologist.
Accurate staging requires understanding the biologic behavior of different tumor types, combined with the results of a thorough diagnostic workup based on this expected behavior. Tumor staging is used to do the following:
Several staging systems are available. Most are based on assessment of local, regional, and distant disease involvement. Some systems include other factors, such as presence or absence of clinical signs (e.g., lymphomas), tumor histologic grade (e.g., mast cell tumors), or tumor location (e.g., squamous cell carcinoma of mouth, tonsil, pinna, or digit). The TNM staging system (T, tumor size or extent; N, lymph node involvement; M, metastasis) devised by the World Health Organization is the standard system for most tumors in veterinary medicine. Table 26-2 describes this staging scheme and gives an example. Staging systems should be revised as new prognostic information is acquired.
Table 26-2 WORLD HEALTH ORGANIZATION TNM CLASSIFICATION OF TUMORS
From Owen LN: Classification of Tumours in Domestic Animals. Geneva: WHO, 1980.
T = Tumor Size or Extent
T1–T4 represent specific size categories designated for each tumor type and define the extent of local tumor involvement.
N = Lymph Node Involvement
N1–N3 (± a, b) describes regional lymph node characteristics for presence or absence of neoplasia, number and location of enlarged lymph nodes, and occurrence of adjacent tissue adhesion.