Tracking Head Sentinel Lymph Nodes: Principles, Status, Use of Indirect Lymphangiography and Methylene Blue

Introduction
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. The SLN is defined as the first lymph node to drain the area. Consequently, if an area is affected by a neoplastic process, the SLN will be the first one to show metastases. Based on the TNM staging system, the sentinel lymph node should be the one to be assessed first, even if it is considered normal on palpation or on conventional imaging as microscopic disease can not be ruled out. A recent publication investigated the use of dye and technetium injection and surgical tracking for the treatment of cutaneous mast cell tumours (MCT) (Worley 2012), in this study 12/22 dogs showed lymph node metastasis and 8 had been advised to received adjuvant therapy which would have probably not been advised if SLN mapping had not been performed. Recently, Mayer et al. and Pastikas et al. reported the benefits of lipid-based iodinated contrast medium for direct and indirect lymphangiography in normal healthy dogs and cats in different locations. This allowed long term identification of SLN with contrast concentration within the lymphatic tissues for several weeks.

We hypothetised that identification and sampling for subsequent histological examination of the sentinel lymph node will improve the comprehension of the disease and allow better treatment for our patients due to earlier detection of metastatic disease. We discussed the principle with the different departments routinely involved in the oncology cases: imaging, medical, oncology and soft tissue surgery. Our aim was to establish a protocol where every solid tumour will have an additional CT acquisition or radiographs for identification of the sentinel lymph node after peritumoral iodine-based contrast medium injection. Perioperative injection of methylene blue will be performed to allow identification and resection of the sentinel lymph node identified after imaging.  [...]