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All regional lymph nodes should be assessed by palpation to assess size, firmness, and adherence to underlying structures and FNA cytology regardless of size as part of the evaluation of a cutaneous mass. The sensitivity and specificity of FNA cytology for diagnosis of metastatic disease in lymph nodes in solid neoplasms is 91–100% and 91–96%, respectively, compared to histopathology of the entire lymph node (Langenbach et al. 2001; Ku et al. 2017). Factors reported contributing to discrepancies between cytology and histology include focal distribution of metastases and poorly defined criteria for metastatic mast cell tumors (Ku et al. 2017). False‐positives results with cytology were more common with mast cell tumors and melanomas (Ku et al. 2017). Carcinomas are reported to metastasize to regional lymph nodes more frequently than sarcomas (Langenbach et al. 2001). Lymph node size is not predictive for metastatic status. Incisional or excisional biopsy and histologic assessment of the regional lymph node is the optimal approach to lymph node assessment.

Identification and biopsy of the first draining regional lymph node, the sentinel lymph node (SLN), is important in the prediction of survival for a variety of cancers in human and veterinary oncology (Tuohy et al. 2009; Beer et al. 2018). The anatomically closest regional LN is not necessarily the SLN, so SLN mapping is recommended. The sentinel lymph node can be identified using a variety of techniques including lymphoscintigraphy (Worley 2014), CT lymphography (Brissot and Edery 2017; Grimes et al. 2017; Majeski et al. 2017; Rossi et al. 2018), and methylene blue. SLN mapping and sampling allows identification of microscopic metastatic disease that would otherwise have been undetected. In such circumstances, clinical stage changes and consequently additional therapy is recommended that would have otherwise not been offered. This can lead to an improved oncologic outcome (Worley 2014).