Maureen A Griffin 1, Deanna R Worley 2, Brian K Flesner 1, Jennifer Reetz 1, David E Holt 1, Amy Durham 3, Nimar Gill 1, JoAnne Winget 1, Wilfried Mai 1
Background
Canine thyroid carcinomas, predominantly malignant, often metastasize to regional lymph nodes (LNs) and lungs. Identifying metastatic LNs is critical for accurate staging, prognosis, and therapeutic planning. However, current methods for lymph node evaluation (palpation, imaging, and non-targeted excision) have limitations in sensitivity and specificity. In human oncology, sentinel lymph node (SLN) mapping has improved staging and outcomes for thyroid and other cancers. This study aimed to describe the feasibility of combining preoperative and intraoperative SLN mapping techniques in dogs with thyroid carcinoma, a method not previously reported in veterinary medicine.
Methods
A prospective pilot study included six client-owned dogs with unilateral thyroid carcinoma and no overt nodal metastasis on imaging. Preoperative indirect CT lymphography (CTL) used peritumoral injection of iodinated contrast to detect SLNs. Intraoperatively, a combination of methylene blue (MB) and indocyanine green (ICG) was injected peritumorally, and SLNs were visualized using visible light and near-infrared (NIR) fluorescence imaging. SLNs and relevant regional LNs were excised and analyzed histologically. The protocol was adapted during the study to improve intraoperative dye localization.
Results
SLNs were identified in all dogs using both CTL and intraoperative techniques. A median of one SLN was detected by each method, with identified nodes including medial retropharyngeal, cranial deep cervical, and superficial cervical LNs. Concordance between CTL and intraoperative findings occurred in three of six dogs. Metastatic disease was confirmed histologically in 3 of 12 extirpated LNs (in 2 dogs). No adverse events from mapping techniques were noted. Variability in lymphatic drainage patterns and discrepancies between the mapping modalities were observed.
Limitations
The small sample size limits statistical power and generalizability. Protocols evolved during the study, introducing variability. Only selected LNs were excised, possibly missing metastatic nodes. Differences in injection techniques and personnel may have affected consistency. The study was not designed to assess long-term oncologic outcomes of SLN mapping and extirpation.
Conclusions
The study demonstrates the feasibility of combining pre- and intraoperative SLN mapping in dogs with thyroid carcinoma. These methods allowed targeted excision of LNs, including metastatic ones that were not apparent on conventional imaging. Larger, controlled studies are needed to refine techniques, assess diagnostic accuracy, and evaluate the impact on treatment outcomes in canine thyroid carcinoma.
Sentinel lymph node (SLN) mapping findings for dog 4 depicting a cranial deep cervical lymph node as sentinel via (A) preoperative indirect CT-lymphography, (B) in situ visible dye with methylene blue (MB), (C) in situ near-infrared (NIR) imaging with indocyanine green (ICG), (D) in situ NIR imaging with ICG overlaid on the visible light image, (E) ex vivo visible dye with MB, (F), ex vivo NIR imaging with ICG, and (G) ex vivo NIR imaging with ICG overlaid on the visible light image. (A) The yellow arrow depicts the SLN with contrast uptake. (B–D) Cranial is at the right of the image, the yellow arrow depicts the SLN, and the additional MB and ICG uptake is secondary to dispersion within the peritumoral tissues following injection.
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