Evaluation of Surgical Aid of Methylene Blue in Addition to Intraoperative Gamma Probe for Sentinel Lymph Node Extirpation in 116 Canine Mast Cell Tumors (2017–2022)

Abstract

Methylene Blue (MB) is combined with radiopharmaceutical for intraoperative sentinel lymph node (SLN) mapping, but its role during SLN extirpation has not been investigated yet in veterinary medicine. The aim of this study was to assess whether MB increased surgical detection of SLN beyond the use of intraoperative gamma-probe (IGP) alone in clinically node-negative dogs with mast cell tumors (MCTs) following the detection of sentinel lymphocentrums (SLCs) via preoperative planar lymphoscintigraphy. Dogs enrolled underwent MCT excision and SLC exploration guided by both MB and IGP. Data recorded for each SLN were staining (blue/non-blue), radioactivity (hot/non-hot), and histopathological status (HN0-1 vs. HN2-3). A total of 103 dogs bearing 80 cutaneous, 35 subcutaneous, and 1 mucocutaneous MCTs were included; 140 SLCs were explored, for a total of 196 SLNs removed. Associating MB with IGP raised the SLNs detection rate from 90% to 95%. A total of 44% of SLNs were metastatic: 86% were blue/hot, 7% were only blue, 5% were only hot, and 2% were non-blue/non-hot. All HN3 SLNs were hot. Combining MB with IGP can increase the rate of SLN detection in dogs with MCTs; nonetheless, all lymph nodes identified during dissection should be removed, as they might be unstained but metastatic.

Weishaar's classification system for nodal metastasis in sentinel lymph nodes: Clinical outcome in 94 dogs with mast cell tumor

“…In fact, in Guerra et al 21 the majority of cases were mapped by only intraoperative direct visualization of lymph nodes with methylene blue. In contrast, in the present paper, the combined use of planar lymphoscintigraphy for preoperative identification of the SLC and the intraoperative gamma probe and blue dye for guiding the removal of SLN led to the successful removal of all SLN within each identified SLC 11,19,50 . Indeed, the number of SLC identified and of SLN removed is correlated with an increase in the HN class, suggesting that a single MCT could be engaged with 1 or more SLC, similarly to what is reported in humans bearing melanoma 51 .…”

“…In addition, intraoperative gamma probe use allowed the measurement of residual radioactivity that led to straight‐forward decision‐making during the dissection of the SLCs, since its absence indicates that there are no more SLN to be removed within the SLC. On the other hand, the inability to directly visualize a node, either blue or nonblue, does not exclude the presence of other “hot” SLNs within the same SLC, 11,50 and the use of intraoperative gamma probe allows avoiding missing any lymph node 52 that could belong to a different HN class 53 . For these reasons, the surgeons might be enticed in exploring more SLCs and remove more SLNs when using a high‐performance tracer that allows for an intraoperative guidance such as Technetium‐99 or indocyanine green in near‐infrared fluorescence 19,54,55 …”

“…21 51 In fact, while preoperative mapping might identify 1 or more SLC, the employment of intraoperative gamma probe in association with blue dye ensures an accurate exploration of the SLC and removal of multiple SLN hidden within the adipose tissue or unstained, with a detection rate that ranged among 91%-98%. 11,50 In addition, intraoperative gamma probe use allowed the measurement of residual radioactivity that led to straight-forward decisionmaking during the dissection of the SLCs, since its absence indicates that there are no more SLN to be removed within the SLC.…”

“…On the other hand, the inability to directly visualize a node, either blue or nonblue, does not exclude the presence of other "hot" SLNs within the same SLC, 11,50 and the use of intraoperative gamma probe allows avoiding missing any lymph node 52 that could belong to a different HN class. 53 For these reasons, the surgeons might be enticed in exploring more SLCs and remove more SLNs when using a high-performance tracer that allows for an intraoperative guidance such as Technetium-99 or indocyanine green in near-infrared fluorescence.…”