Mast cell tumors (MCT) are one of the most common skin tumors seen in dogs.1 These tumors can vary widely in appearance and biological behavior. MCTs are easily diagnosed with fine needle aspirate and cytology. Tumor grade, which is currently determined with histopathology, predicts behavior. The Patnaik three-tier grading scheme has largely been replaced with the Kiupel scheme, which designates these tumors as either low-grade or high-grade.2 Low-grade tumors have an excellent prognosis with effective local control and are unlikely to metastasize to other sites, whereas high-grade tumors are more difficult to control locally and have a higher chance of metastasis.2
Common sites of metastasis include the draining lymph node, spleen, and liver, with the draining lymph node being regarded as the most important site to evaluate, as the chance of spreading beyond the draining lymph node in the absence of a positive lymph node is extremely unlikely.3 Further, the treatment of a positive lymph node as part of local control with either surgical removal or radiation has been shown to improve outcome.4, 5
Prior to initiating a plan for local control, it is important to determine how much staging is indicated. If accessible, the draining lymph nodes should be aspirated for cytology, as this is the most likely site to be affect first. Although this seems straightforward, recent studies evaluating sentinel lymph nodes have shown the sentinel lymph node, or the lymph node that first drains the tumor region, may not be the closest regional lymph node.6 For initial staging, careful palpation and aspiration of regional lymph nodes is recommended.
