Combining lipiodol-based indirect radiography and intraoperative methylene blue for sentinel lymph node mapping in canine cutaneous mast cell tumors

Canine cutaneous mast cell tumor (cMCT) is one of the most common skin neoplasms in dogs, with lymph nodes representing the primary route of metastasis. Accurate sentinel lymph node (SLN) identification is therefore essential for staging and treatment planning. This study evaluated indirect lipiodol lymphography (IL) and intraoperative methylene blue (MB) mapping in 21 dogs, including 26 tumors and 38 excised lymph nodes, to assess SLN detection and its association with tumor characteristics and nodal status. MB identified 34 SLNs, whereas IL detected 29. The two techniques were concordant for 21 SLNs. In five tumors in which IL failed to identify a SLN, MB subsequently detected six axillary SLNs. Concordance with the expected regional lymph node (RLN) was higher for MB (76.9%) than for IL (61.5%) and increased to 84.6% when both techniques were combined, highlighting their complementary value for SLN mapping. MB detected additional axillary nodes, whereas IL aided in identifying some inguinal nodes. Larger tumors were more frequently associated with incomplete surgical margins and higher nodal histologic grades, indicating an increased risk of metastasis. Importantly, 19% of tumors drained to at least one SLN outside the expected regional basin, highlighting lymphatic anatomical variability. The absence of IL opacification did not exclude clinically relevant nodes, as MB identified all such cases, including HN0-HN2 nodes. Although MB stained some non-metastatic nodes, it provided useful intraoperative guidance. Adverse events were uncommon, with two transient reactions after IL and none after MB, and postoperative morbidity was low (13.2%; 5/38). Overall, MB offered higher detection rates, while IL improved anatomical delineation. Combining lipiodol-based indirect radiography and intraoperative MB staining offers a highly effective, complementary approach for identifying the SLN in dogs with cMCTs.