Neoplastic skin disorders (Proceedings)
Some of the most common canine cutaneous tumors include: histiocytoma, mast cell tumor, and the adnexal tumors. Cutaneous T-cell lymphoma (mycosis fungoides) is a pruritic neoplasia of older dogs with a poor prognosis. Recent literature supports a novel therapeutic agent for this terrible disease to improve clinical remission and quality of life. The focus of this presentation is to review common canine dermatologic neoplastic diseases, comment on diagnostic procedures and evaluate treatment options. The intent is to review those diseases that private practitioners should be comfortable treating and to assess when a case might be better in the hands of a competent veterinary oncologist.
A mass is a mass. The three primary differential diagnoses for a skin mass are: tumor (neoplasia), granuloma (infectious and non-infectious) and cyst (thank you Dr. Michael Willard for this important concept). Nobody can tell for sure what is going on without aspiration cytology and biopsy with histopathology and culture. Histopathology is considered to be the gold-standard. However, in the hands of a skilled clinical pathologist, a tentative diagnosis may be reached. Biopsy for macerated tissue culture is also an important yet commonly overlooked procedure.
Direct impression cell cytology evaluation should be performed on all skin masses with an erosive or ulcerative surface. A clean glass slide should be pressed against the skin surface and allowed to air dry. The author believes that heat-fixation is an important process but some clinical pathologists will disagree. Nonetheless, direct impression skin samples should be prepared in triplicate. One slide should be prepared for modified Wright's stain (Diff-quik ®) for the clinician to evaluate. One slide should be saved for Gram Stain evaluation if rod or cocci bacteria are seen on evaluation or if the lesion is severely exudative or hemorrhagic. A Gram stain is beneficial in helping to "wash-out" the background cellular structures to better evaluate the infectious organisms. Most rod shaped bacteria are Gram negative but a Diff-quik stain will not confirm this information. The third and final sample should be reserved for laboratory submission. Slides are cheap. It is better to take too many samples than not enough.For all skin masses, aspirate cytology should be performed. The term aspirate implies that negative pressure will be applied to help exfoliate the skin mass and achieve a better diagnostic sample. Aspiration samples are best for sarcomas where exfoliation is difficult to achieve. For most skin samples, a needle core sample or "woodpecker technique" is recommended. Samples larger than 1 cm. should be collected with a 20 gauge needle. Smaller samples should be collected with smaller needles. The term "fine-needle aspiration" implies that a 25 gauge needle be employed. It is the author's experience that acquiring diagnostic samples with anything less than a 23 gauge needle is futile. The "woodpecker technique" is performed by stabilizing the mass and inserting the needle into the mass. For small skin masses, the needle can be inserted all the way through the mass if it is elevated above the skin surface. For larger masses, the needle should be inserted and withdrawn almost all the way out and then the angle should be changed for the next insertion. The idea is to collect a core sample of cells. Overzealous or unlucky sample technique can result in excessive hemorrhagic contamination. All samples should be evaluated regardless of the amount of hemorrhage. Once collected the needle should be attached to a syringe of at least 6cc size. A syringe of 12 cc will supply a greater pressure to remove exfoliated cells from the needle than a smaller needle. The needle should be aimed at a cleaned (yes, clean your slides with alcohol before sample technique) slide along the long axis of the slide. Slides should be prepared as described above.