August 2010 "Just Watch that Mass" or Maybe Not

- M. Joy Weinstein, DVM, MS, DACVS

When is it acceptable to watch a mass on a pet? Too many times, we see pets with very large ulcerated masses that are now causing serious problems. Owners often report they were told to watch the mass at one time when it was smaller. Many of the masses that are quite large are malignant, and they were not aspirated to check for malignancy. Of course, these animals would have a better prognosis if the malignancy was removed sooner rather than later. Some very large ulcerated benign masses would have been much easier and less expensive to remove when they are small.

It is always best to know a mass is benign before the owners are told to watch the mass. A tiny sebaceous adenoma that is not ulcerated may not cause problems for a pet. It may not be practical for a pet to have 50 sebaceous adenomas surgically removed. Even benign sebaceous adenomas should be removed and biopsied if they are ulcerated, infected or growing rapidly. A fine needle aspirate of all dermal and subcutaneous masses would be recommended prior to making recommendations for treatment. Remember a mast cell tumor can be hiding in even the most benign looking lesion. We have seen several mast cell tumors that grossly resemble inflamed skin tags. Keep in mind that you can aspirate fluid from benign and malignant masses. If you aspirate a serous cyst and it is completely flat after aspiration, this is not as concerning as aspirating fluid out of a mass that is still firm and nodular once the fluid has been removed. A serous cyst that fills with fluid once yearly can probably be watched while one that is filling up frequently should be removed and biopsied. In summary, all malignant dermal masses and any benign dermal masses that are enlarging, ulcerated, or infected should be excised and biopsied.

Keep in mind there are several types of lipomatous masses. A benign freely moveable lipoma over the lateral aspect of the thorax that is growing slowly in an elderly pet may be watched. If a lipomatous mass is deep and firmly attached, this may be an infiltrative lipoma or a liposarcoma. Surgical removal of infiltrative and malignant lipomatous masses would be easiest when they are still small. Several times, we have seen fat aspirated from lesions that are actually soft tissue sarcomas. If you aspirate fat from a lesion that appears more aggressive than a routine lipoma, perform an actual tissue biopsy to be sure you are not misdiagnosing the lesion. This can be done with a tru-cut biopsy needle or a skin punch. If you diagnose a lipoma that is in the axilla, inguinal region, or within the muscle of a limb, recommend early removal before the mass causes problems with ambulation. Lipomas in other locations that are growing rapidly should always be removed before they become a "ball and chain" for the pet.