Lili Duda, VMD
Ultima Vez Modificado: 5 de junio del 2013
Any new mass (lump, bump, or swelling) that is found on a companion animal should be evaluated in a systematic fashion. With very few exceptions, the diagnosis of malignancy (that is, determination of benign versus malignant) CANNOT be done by visual inspection or palpation (feeling the area) alone. Sometimes "watchful waiting" can be used for masses that are strongly suggestive of a benign process, such as a bug bite, hive, or pimple. However, any lesions that have not disappeared or significantly improved within a few weeks should be evaluated more thoroughly. Special caution should be used for masses located in areas where even a small increase in size could make subsequent surgical removal considerably more difficult or even impossible, such as the anus, or on the head and limbs.
There are two basic ways to evaluate a mass: fine-needle aspiration or tissue biopsy. There are advantages and disadvantages to each of these methods which are described below.
Fine-needle aspirates use a small needle (typically a 22 gauge needle which is the same size used for administering vaccinations). A needle attached to a syringe is "poked" into the tumor several times, and suction is applied to the needle to aspirate ("suck up") some cells. A small sample of the mass ends up in the needle, and the sample is then squirted onto glass slides which are processed using a series of "stains" which dye the various components of the sample. The slides are then evaluated under a microscope. This type of evaluation is called "cytology". Cytology evaluates the appearances of individual cells. A preliminary evaluation of the slides can often be done in-house by the general practitioner within a few minutes of obtaining a sample. However, a definitive diagnosis should be obtained by sending the slides to a veterinary clinical pathologist, who is a specialist trained in this diagnostic method.
The advantages of fine-needle aspirates are that the sampling is quick and easy, can usually be done with the patient awake using minimal restraint, and can provide an answer relatively quickly, usually within 1 or 2 days. In some cases, ultrasound guidance can be used to perform fine-needle aspirates of internal structures, such as the liver and spleen. This type of aspiration usually does require sedation or even anesthesia to insure the safety of the patient and prevent damage of adjacent internal structures. Sedation might also be required for aspiration of masses near sensitive structures such as the eye, ear, and anus, or in animals that are anxious or difficult to restrain.
The disadvantages are that many samples can be "non-diagnostic", which means that the nature of the mass cannot be ascertained by this method. Samples can also be equivocal or suspicious for, but not diagnostic of, cancer. In these cases, a tissue biopsy will still be needed for a definitive (more certain) diagnosis. Some cancer types are readily diagnosed by this method, especially the "round cell tumors" such as lymphoma and mast cell tumors. This is because these tumor types readily exfoliate ("give up") cells. Other cancer types can be difficult to diagnose this way, such as the general category of sarcomas, because they do not exfoliate cells readily. Fine-needle aspiration can be especially useful in evaluation of lymph nodes to help determine whether cancer cells have spread to a particular node that drains the area of a known cancerous mass. It should be clear that a "non-diagnostic" aspirate does not mean that a mass is not cancerous. Sometimes a fine-needle aspirate can "suck up" fat or blood that are within or around a cancerous mass without the cancer cells themselves getting "sucked up". If a fine-needle aspirate is non-diagnostic but there is a high suspicion that the mass is cancerous (for example, it is growing quickly, it is firm or hard, or it is attached to skin or underlying tissues), a tissue biopsy is recommended.
There are several ways of obtaining a tissue biopsy. These include needle or punch (core of tissue), pinch (grasping and pulling off a piece of tissue), incisional (slice of tissue from within a tumor), and excisional (removing the entire tissue mass) methods. In general, the smaller the tissue sample, the quicker and easier the procedure. However, the smaller the tissue sample, the less tissue available for evaluation. The tissue sample must be "fixed" or preserved in a liquid such as formaldehyde for about 24 to 48 hours. This tissue is then embedded in a block of paraffin wax and very thin slices are cut off the block, attached to glass slides, "stained", and then evaluated under a microscope by a veterinary histopathologist. This process typically takes 3 to 5 days, and might take considerably longer if additional specialized stains are required to help more fully characterize a tumor. Tissue biopsy provides information about the appearance of the cells as well as the three-dimensional structure of the tissue sample (such as whether cancer cells are invading into adjacent bone or blood vessels).
The advantages of tissue biopsy include the large amount of tissue that can be obtained for diagnosis. For small or benign tumors, an excisional biopsy can be a curative as well as a diagnostic procedure. The disadvantages are that tissue biopsies usually require anesthesia or at least sedation plus a local anesthetic. There are usually a few sutures or small incisions that will require wound care and healing time. Lastly, a tissue biopsy is generally considerably more expensive than fine-needle aspirates.
Once a mass has been evaluated and measured, this description should be recorded in the medical record. It is often helpful to have a "body map" which is a drawing of the patient with the locations, measurements, and mass descriptions recorded. Even a benign (non-cancerous) mass can continue to grow, become inflamed or ulcerated, or in some cases undergo "malignant transformation" (become cancerous over time). Therefore, it is helpful to know the previous size and location of a mass in order to know how it is changing over time.