GAMMA-PROBE GUIDED SENTINEL LYMPH NODE EXTIRPATION TO ASSESS PATTERNS OF NODAL METASTASIS IN SPONTANEOUS HEAD AND NECK MALIGNANCIES OF THE DOG: A SECOND STEP BASED ON PREVIOUS EXPERIENCES ON MAST-CELL TUMORS

Canine oncology has recently reported interest in the detection of occult nodal metastases, and efforts have been made to identify the best strategy for accurate nodal staging. In this scenario, the present PhD thesis was aimed to improve knowledge on the current understanding of the feasibility and impact of sentinel lymph node (SLN) mapping with radiopharmaceutical and blue dye in cancer-bearing dogs. Initially, we evaluated the technique in canine mast cell tumors (MCT), given the high prevalence of this tumor type, the propensity for lymphatic spread and recognized prognostic impact of nodal metastases. Thirty client-owned dogs with 34 MCT were included. At least one SLN was identified in all but 3 dogs that had a scar from previously excised MCT, and in 30 cases (63%) the SLN did not correspond to the regional lymph node (RLN). As a second step, we explored the impact of SLN mapping and extirpation in dogs with head and neck tumors, given the well-accepted unpredictability of patterns of nodal metastases. Twenty-three dogs with tumors of the head and neck and absence of clinically evident nodal disease (cN0 neck) were prospectively included and underwent tumor excision and SLN extirpation. Reported detection rate was 83%, with at least one SLN identified in all but 4 dogs with thyroid tumors. In 52% of dogs, the SLN did not correspond to the RLN and at histopathology 42% of dogs had nodal metastases, of which 4 differed from the RLN. As the last step of this PhD project, we conducted an explorative study to identify clinical or pathological characteristics that could allow for identification of dogs with MCT at lower risk of SLN metastases, that could be excluded a priori from the SLN mapping and extirpation. Surprisingly, tumor grade was not able to predict the risk of having HN2-3 or HN3 SLN and the only variables that correlated with nodal metastases were tumor size, number or SLN and subcutaneous MCT. Given low to moderate discriminant ability, however, none of these variables could safely determine the exclusion of patients from the procedure. Results of the present project underscore the utility of implementation of SLN biopsy in the management of dogs with MCT and head and neck tumors, given the low correspondence between clinically expected RLN and SLN and the high rate of occult nodal metastases that could be detected with this procedure. Future research should be aimed at investigating the impact on long-term prognosis and disease free interval of the extirpation of metastatic SLN, and identify variables that could allow for exclusion of low-risk patients from the procedure.