Intraoperative near-infrared imaging can identify canine mammary tumors, a spontaneously occurring, large animal model of human breast cancer.

Introduction: Current methods of intraoperative margin assessment in breast conserving surgery are impractical, unreliable, or time consuming. We hypothesized that intraoperative near-infrared (NIR) imaging with an FDA-approved NIR optical contrast agent could identify canine mammary tumors, a spontaneous large animal model of human breast cancer, during surgery.

Methods: Dogs with mammary tumors underwent a standard of care lumpectomy or mastectomy with wide surgical margins 20 hours after indocyanine green administration (3 mg/kg IV). During surgery, NIR imaging was performed on tumors and wound margins in situ and tumors and lymph nodes ex vivo. Following resection, the wound bed was examined for residual fluorescence. Fluorescence intensity was determined by signal-to-background ratio (SBR). All tumors, areas of residual fluorescence, and lymph nodes underwent histopathologic analysis.

Results: There were 41 mammary tumors in 16 female dogs. Twenty tumors were malignant and 21 were benign. Twenty-eight tumors were fluorescent (mean SBR 1.5±0.2). Sensitivity of fluorescence for all malignant tumors was 80% (16/20) and 93.3% (14/15) for malignant tumors > 2 cm. Specificity for malignancy was low (< 2cm = 55%; > 2cm = 30%). Tumors > 2 cm were more likely to be fluorescent (OR 6.05, 95% CI 1.50-24.44, P = 0.011) but not more likely to be malignant (OR 3.09, 95% CI 0.86-11.14, P = 0.085) than tumors ≤ 2 cm. Four out of seven inguinal lymph nodes excised in the mastectomy specimen fluoresced. All four drained malignant tumors; however only 2/4 contained metastatic disease.

Conclusion: Systemic ICG accumulates reliably in malignant canine mammary tumors > 2 cm. Although no tumor margins fluoresced, a wider margin of normal tissue is removed in canine mastectomy, making direct comparisons with breast conserving surgery difficult. Targeted NIR imaging agents are likely required to improve detection of smaller tumors and improve the specificity of NIR imaging for residual disease and metastatic lymph node detection.