Sentinel lymph node in veterinary medicine, where are we in 2019?

Sentinel lymph node (SLN) identification and lymph node mapping is a well-known concept in human oncology. Each area in the body has a dedicated lymphatic drainage pathway, which can be associated with multiple lymph nodes organised in clusters (lymphatic basin). Veterinary clinical experience shows that the draining lymph node will not always be necessary the regional anatomic lymph node. The SLN is defined as the first lymph node to drain a specific body area. Consequently, if an area is affected by a tumour, and since tumour staging implies systematic nodal assessment, the SLN should be the first one to show metastases and therefore should be the lymph node to be assessed in priority. In human oncology, especially breast cancer and cutaneous melanoma, the specific identification and sampling of one lymph node within a lymphatic basin, for staging purpose, prevent potentially unnecessary extensive lymphadenectomy and its associated morbidity. Should the SLN being positive for tumoural spread however; further local radical lymphadenectomy might be recommended although clinical value of radical lympadenectomy in these indication remains unclear.

Human standard of care for SLN identification is peritumoural injection of radioactive marker and subsequent scintigraphy. SLN identification in surgery is performed with a portable gamma camera in combination with vital dye injection as a direct visual help.

Worley et al. reported the application of a similar protocol in a cohort of 19 dogs affected with mast cell tumours (MCT). Technetium was used for the scintigraphy and methylene blue (MB) was used for vital colouration. In this study, 42% of dogs diagnosed with nodal metastases were advised to received adjuvant therapy, which would have otherwise not been advised had SLN mapping not been performed. All but two of the “hot” extirpated lymph nodes had positive uptake of MB. Unfortunately, scintigraphy is not widely accessible in veterinary hospitals. Therefore, efforts were made to look for alternative solutions. [...]