Principles of Oncology

Chapter 26 Principles of Oncology



Stephen D. Gilson, Rodney L. Page, Rance M. Gamblin


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



Signalment


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.


Table 26-1 SOME FACTORS PREDISPOSING DOGS AND CATS TO SPECIFIC NEOPLASMS






























































Factor Predilection For
Age
Histiocytoma Young dogs
Viral papilloma Young dogs
Sex
Malignant melanoma Males
Perianal adenoma Males
Anal Sac AdenoCA Females
Adrenal tumor Females
Meningiomas Females (dog), males (cat)
Color
Squamous cell carcinoma Nonpigmented regions
Malignant melanoma Darkly pigmented regions
Breed
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


History


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.



Physical Examination


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.



Client Counseling


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:


• Listen to the needs of the client and mutually determine a set of realistic goals for treatment.

• Explain treatment procedures, discussing risks, benefits, toxicity, and costs. It is helpful to provide written material for home review of information.

• Provide a realistic and unbiased view of all treatment options available and the animal’s likely prognosis.

• Beware of the tendency for clinicians untrained in oncology to prematurely recommend euthanasia.

• Consider consultation with or referral to a specialist when appropriate.


DIAGNOSIS



Laboratory Evaluation


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.).



Diagnostic Imaging



• Obtain survey radiographs to detect metastases (on the thorax, abdomen, or skeleton), to evaluate orthopedic soundness before amputation or limb-sparing surgery for dogs with osteosarcoma, and to determine bone margins of oral or nasal masses. When taking thoracic radiographs, remember to take both right and left laterals as well as a ventrodorsal/dorsoventral view, to avoid missing metastatic lesions (see Chapter 4).

• Contrast radiography may be helpful for defining the extent of disease in hollow viscera (see Chapter 4).

• Computed tomography and magnetic resonance imaging are useful to delineate deep masses of the body cavities and axial skeleton (see Chapter 4).

• Ultrasound is useful to image abdominal viscera. It can determine accurately the parenchymal nature of a mass and tumor proximity to large vessels, and evaluate for intra-abdominal metastases to lymph nodes or organs. Additionally, it can be used for direction of fine needle aspiration or minimally invasive biopsy procedures (see Chapter 4).


Cytology


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).



Biopsy


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.



General Considerations



• In general, larger tissue samples are more likely to provide an accurate diagnosis. Avoid traumatic tissue handling, electrocoagulation, and other tissue damage to provide a histologically useful sample.

• Fix tissue samples for routine histopathologic analysis in 10% buffered formalin (10:1 formalin-to-tissue ratio).

• Obtain samples for culture or other special analyses at the same time to avoid a second biopsy procedure if histologic analysis identifies an inflammatory or other nonneoplastic process.

• Include a complete history and description of the clinical and surgical findings with the biopsy specimen submission.

• If unsure of the most appropriate biopsy technique, consult with or refer the animal to a specialist.


Excisional Biopsy



• Complete removal of a tumor and submission for histologic diagnosis is most useful for cutaneous, mammary, and central nervous system (CNS) tumors and easily resectable masses found during laparotomy or thoracotomy.

• Ideally, an excisable mass is small and freely movable without significant adjacent tissue invasion.

• Excise the mass with a histologically confirmed complete resection margin of normal tissue. Use suture tags to identify areas of possible inadequate resection that require closer histologic scrutiny. Inking of all surgical margins can also aid the pathologist in determination of the completeness of the excision.


Nonexcisional/Incisional Biopsy



• Remove only a portion of a tumor (e.g., cutting needle, endoscopic, punch, or incisional biopsy) when definitive diagnosis or grading would influence treatment decisions.

• Take deep biopsies of superficial ulcerated masses to avoid sampling only overlying necrotic debris on the surface.

• Incisional and needle biopsy tracts are considered contaminated by tumor cells and must be completely within the future field of surgical resection or radiation treatment.

• Ideal histologic samples contain a margin of normal and neoplastic tissue. This allows assessment of tumor invasiveness, and margins are frequently the site of greatest tumor cell activity. Biopsy at the margin is often undesirable, however, because it disrupts the natural tumor margin and effectively extends it. This necessitates a wider resection or larger radiation field for definitive treatment. Therefore, if biopsies include marginal tissues, collect from areas where there is adequate tissue for wider excision.



Key Point


Improper biopsy technique can significantly alter treatment, increase treatment morbidity, and sometimes adversely affect prognosis.



Evaluation of Pathology Reports


The pathology report should include:


• Histologic diagnosis and, where appropriate, histologic grade (e.g., mast cell tumor, soft tissue sarcoma, osteosarcoma)

• Complete description of cellular characteristics and degree of anaplasia

• Mitotic index

• Identification of lymphatic or vascular invasion

• Evaluation of resection margins

If aspects of the report are questionable or do not match your clinical judgment, always discuss the results with the pathologist.



Tumor Staging


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:


• Determine the extent of neoplastic disease

• Provide a framework for rational treatment planning

• Facilitate communication between clinicians

• Allow for uniform comparison and evaluation of treatment results

• Aid in establishing a prognosis

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.



M = Metastasis


M0 or M1 indicates absence or presence of distant metastasis.


Example—TNM Classification of Tumors of the Oral Cavity in Dogs or Cats:


T: Primary Tumor


Tis = Preinvasive tumor (in situ)

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Principles of Oncology
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