Abstract
Background:
Mammary tumors are the most common neoplasms in intact female dogs and have significant metastatic potential. Accurate oncologic staging often requires the assessment of regional and deep lymph nodes, including the medial iliac lymph node (MILN), which can be difficult to identify intraoperatively. Lymphatic mapping techniques may facilitate minimally invasive lymphadenectomy.

Aim:
This study aimed to evaluate the effectiveness of intradermal lymphatic marking using methylene blue and hemosiderin for laparoscopic identification of the medial iliac lymph node in female dogs with mammary tumors.

Methods:
Thirty-two female dogs with mammary neoplasms were prospectively allocated into four groups (n = 8 per group) according to dye type (methylene blue or hemosiderin) and injection site (peritumoral at the fifth mammary gland or dorsal torso). No formal sample size calculation was performed, and the study was considered exploratory. All dogs underwent laparoscopic medial iliac lymphadenectomy followed by ovariohysterectomy and unilateral radical mastectomy. The primary outcome was the intraoperative identification of MILN, defined as visible lymphatic marking before removal. The secondary outcomes included time to lymph node visualization, staining intensity, and perioperative complications.

Results:
Methylene blue enabled the intraoperative identification of MILN in all cases (100%), with rapid lymphatic visualization typically within 1 min and strong staining contrast that facilitated laparoscopic dissection. In contrast, hemosiderin resulted in less consistent intraoperative identification (62.5%) and weaker staining, with lymph node visualization occurring only after adipolymphatic tissue removal. The identification time differed significantly between the groups (p = 0.016), with faster detection observed in dogs receiving methylene blue. No intraoperative complications or adverse reactions related to dye administration were observed. Histopathological evaluation was performed per lesion (n = 36), revealing carcinomas (75%), adenomas (16.7%), and lobular mammary hyperplasia (8.3%). No metastases were detected in the medial iliac lymph nodes.

Conclusion:
Intradermal methylene blue is a practical, safe, and effective dye for the laparoscopic identification of the medial iliac lymph node in dogs with mammary tumors. Although hemosiderin is a low-cost alternative, its lower staining contrast limits its intraoperative applicability. These findings demonstrate the feasibility of lymphatic mapping to assist minimally invasive lymphadenectomy; however, no conclusions regarding oncologic outcomes or staging accuracy have been drawn.

Key words: Canine; Lymphatic mapping; Mammary neoplasms; Veterinary oncology; Vital dyes.