Cancer in Dogs and Cats: Getting a Diagnosis

It is not possible to recommend treatment and offer a prognosis for a dog or cat with cancer withoutCancer in Dogs and Cats: Getting a Diagnosis a specific diagnosis.  cancer in animals is not a single disease process.  It consists of over 100 different types of diseases that can be grouped into several major categories.  The 2 main methods of obtaining a tissue diagnosis of cancer include cytology and histopathology.  Cytology and histology complement each other and reflect a trade off between a less invasive procedure with cytology and the increased amount of information obtained from histology.  This article discusses histological diagnosis of cancer.  Histology is gold standard method to obtain a diagnosis of cancer so that a prognosis can be determined and adequate treatment provided.

Histological diagnosis of cancer

Histopathology is the main stay of cancer diagnosis.  It is more accurate and provides more information of the cancer type than can be obtained with cytology.  It involves obtaining a piece of tissue and looking at very thin slices of the tissue under the microscope.  It provides information on the architectural structure of the tumor, which cytology cannot provide.  However, it is typically more invasive, has greater risks, and is more expensive than cytology.   Regardless, there are times where cytology does not provide a diagnosis and histopathology is the only way to do so.  Often times, the increased information obtained from a histological diagnosis of cancer is helpful in decisions regarding the best means of treating the disease.  Obtaining complete information may involve the need for additional tests that can be performed on tissue samples such as special stains, flow cytometry, or polymerase chain reaction (PCR).

In most tumors, differentiation between benign and malignant tumors can only be accomplished by evaluating the tumor with histology.   In some tumors, differentiation between a benign and malignant tumor is determined by the degree of invasion of the tumor into the surrounding normal tissue.  In others, features such as the mitotic index (number of dividing cells), amount of necrosis (dead cells), and variation in cell and nuclear size result in a determination of aggressiveness of the tumor, often referred to as the grade of the tumor.

Histological determination of tumor grade and completeness of excision in the diagnosis of cancer

Besides obtaining a diagnosis of the type of cancer, there are two questions that histology should answer that are essential for treatment and prognosis of a cancer, the grade of the tumor and the completeness of excision.  Grading systems can vary between tumor types, but typically involves a system in which tumors are graded I, II or III (one, two, or three) based on the degree of malignancy.  Grade I tumors are considered to be less aggressive and less likely to spread to other sites that Grade III tumors which are considered highly malignant.  Determining if the tumor has been completely excised is important.  If it is determined that the tumor is not completely excised, additional treatment is indicated, most commonly in the form of additional surgery or radiation therapy.

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Dr Stephen Atwater

Stephen W. Atwater, DVM, MS, Diplomate ACVIM (oncology) is a board-certified veterinary oncology specialist. His professional interests include utilizing emerging therapies for difficult to treat cancers.

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