Agreement between cytology and histology in the diagnosis of sentinel lymph node metastasis in canine cutaneous mast cell tumours: Preliminary results

Introduction: In canine cutaneous mast cell tumours (ccMCTs), metastasis to regional lymph node (LN) has been correlated with a poor clinical outcome. Published classification systems for either histological or cytological evaluation of ccMCT LN metastasis have been questioned due to false-negative and false-positive results. Moreover, the first draining LN (i.e. sentinel lymph node, SLN) does not always correspond to the regional one. Lymphoscintigraphy is used to overcome this problem. Our aim was to compare the agreement between cytology and histology in the diagnosis of SLN metastasis in ccMCTs. Materials and Methods: Cytological and histological specimens of SLNs obtained from dogs enrolled in a prospective ongoing clinical study using preoperative lymphoscintigraphy and intraoperative gamma probe detection were evaluated blindly. A modified version of published schemes for both cytological and histological assessment of ccMCT LN metastasis was applied. Consistency of morphological features between mast cells from SLNs and primary tumour masses was additionally evaluated. Results: Fourteen SLNs from seven dogs, with a total of nine ccMCTs, were included in the study. Cytology and histology results completely agreed in 78.57% of cases (11/14) and were in disagreement in 21.43% (3/14) of cases. Conclusions: To our knowledge, this is the first study that evaluates prospectively the agreement between cytology and histology in diagnosing SLN metastasis of ccMCTs. Despite the high agreement observed, our results and the lack of a gold standard test demonstrate that cytology and histology are complementary techniques that should be combined for a more precise evaluation of SLN metastasis of ccMCTs.