Accessible Indirect Lymphography with Iohexol for Sentinel Lymph Node Mapping in a Dog with Auricular Melanoma: A Brief Report

Sentinel lymph node (SLN) mapping plays a pivotal role in oncological staging in veterinary medicine. However, conventional techniques often depend on radioactive tracers or specialized imaging equipment that is unavailable in many clinical settings. This brief report describes the use of indirect radiographic lymphography with the non-ionic iodinated contrast agent iohexol for SLN identification in a dog with auricular melanoma. A 12-year-old spayed female mixed-breed dog presented with an ulcerated cutaneous mass affecting the left auricle. Cytological evaluation was consistent with a melanocytic neoplasm. Immediately before surgery, iohexol was administered intradermally at four peritumoral sites (0.75 mL per cm2). Radiographic images acquired 3 and 4.5 min after injection demonstrated lymphatic drainage to the superficial ventral and superficial dorsal cervical lymph nodes, which were identified as sentinel lymph nodes. The patient underwent vertical ear canal ablation, conchectomy, partial auriculectomy, and excision of the mapped sentinel lymph nodes. Histopathological examination confirmed a mixed-type cutaneous melanoma with macrometastatic involvement of the superficial ventral cervical sentinel node and micrometastatic deposits in both the superficial dorsal cervical and left parotid lymph nodes. Although the parotid lymph node was not enlarged radiographically, afferent lymphatic channels indicated contrast uptake, and histopathology confirmed metastatic involvement. This finding supports considering elective regional lymphadenectomy alongside sentinel lymph node biopsy in similar cases. An incidental cutaneous hemangiosarcoma was identified at a distant abdominal site, excised with complete margins, and was unrelated to the primary tumor. No systemic adverse effects associated with iohexol administration were observed, and only transient mild-to-moderate localized edema developed at the injection site, resolving spontaneously without treatment. These findings suggest that iohexol-based indirect radiographic lymphography is a practical, safe, and accessible technique for sentinel lymph node identification and may represent a promising, though not yet validated, alternative for lymphatic staging in veterinary oncology, although further prospective studies are needed to establish its diagnostic accuracy.