High SLN Detection in Canine Oral Tumors with CT & Methylene Blue

Sentinel lymph node mapping with computed tomography lymphangiography and intraoperative methylene blue peritumoral injection has a high detection rate with moderate agreement in dogs with oral neoplasms

Sohee Bae, Charly Mckenna, Stephanie Goldschmidt, Owen T. Skinner, Judith Bertran, Debbie Reynolds, Michelle L. Oblak

Background

Sentinel lymph node (SLN) mapping improves cancer staging by identifying the primary lymph node likely to harbor metastases. This approach reduces unnecessary lymphadenectomy and associated morbidity. In veterinary medicine, reliable SLN detection remains challenging, especially in canine head and neck cancers due to complex lymphatic drainage and variability. CT lymphangiography (CTL) and intraoperative methylene blue (IOL-MB) are emerging alternatives to radioactive tracer-based techniques, but comparative data are limited.

Methods

This was a prospective study of 38 client-owned dogs with oral neoplasms (macroscopic or incompletely excised malignant tumors). Dogs underwent preoperative CTL with iodinated contrast and intraoperative SLN mapping with peritumoral methylene blue injection. Bilateral mandibular and medial retropharyngeal lymph nodes were surgically removed and analyzed. Agreement between CTL and IOL-MB was assessed using Cohen’s kappa (κ), and detection accuracy was evaluated based on histopathological confirmation of metastases.

Results

The combined CTL and IOL-MB approach achieved a 97.4% SLN detection rate. Moderate agreement was observed between the two modalities (76.8%, κ = 0.485). All 6 metastatic LNs (from 4 dogs) were identified using CTL, and 5 of 6 with IOL-MB. Nine dogs had discrepancies between the two mapping techniques, including one where IOL-MB failed to detect a metastatic node. Bilateral SLNs and contralateral drainage patterns were noted in several cases, emphasizing the anatomical variability.

Limitations

The study had a small sample size and low rate of lymph node metastasis (2.7%), partly due to exclusion of cases with overt nodal involvement. Different institutions used varying contrast agents, and there was no centralized pathology review. The timing of IOL-MB administration and node removal was variable and may have influenced SLN identification. Prior excisional biopsies and patient body size may have also affected lymphatic drainage.

Conclusions

CTL and IOL-MB provide a high SLN detection rate with moderate inter-modality agreement. The techniques are complementary and should be used together, especially in the absence of clinically evident nodal metastasis. Anatomical variability and lymphatic flow alterations from metastasis can influence SLN identification, underlining the importance of using multiple modalities and evaluating all suspicious nodes. Further research is needed in larger populations with higher metastatic burden to validate and refine SLN mapping protocols.

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