When discussing cancer treatment in companion animals, owners often ask, “Isn’t it over once the tumor is properly removed?” Surgically removing the visible mass is indeed an important starting point of treatment. However, oncology poses a more critical question: “Has this cancer already begun to spread somewhere else in the body?”

The part of the body that provides the earliest answer to this question is the “sentinel lymph node (SLN).” A sentinel lymph node is the first lymph node to which cancer cells are likely to spread from the primary site along the lymphatic vessels. In human medicine, particularly in breast cancer and cutaneous melanoma, it has already been established as a standard evaluation method and is regarded as a major innovation that enables accurate staging while reducing the extent of lymph node dissection.

In veterinary medicine, by contrast, this approach has not yet been standardized across all tumor types. However, related research has been increasing rapidly in recent years, and data demonstrating the clinical relevance of sentinel lymph node evaluation are steadily accumulating in mast cell tumors, apocrine gland anal sac adenocarcinoma (AGASACA), mammary tumors, and oral melanoma. For some tumors, it has been confirmed that the results of sentinel lymph node evaluation directly affect staging decisions, treatment planning, and prognosis, making this a key area of interest in veterinary oncology.

In the past, clinicians often simply palpated or used ultrasound to check the size of the “anatomical regional lymph nodes” located near the tumor. Recent studies, however, show that actual lymphatic drainage can differ substantially from the directions traditionally assumed. In canine mast cell tumors, for example, reports indicate that in 30–60% of cases the sentinel lymph node is located in a different position from the lymph node initially expected. This implies that microscopic metastasis may already be progressing in a lymph node that appears normal externally.

This difference is not merely of academic interest; it changes the actual course of treatment. For instance, if sentinel lymph node evaluation confirms micrometastasis in a patient with a mast cell tumor or oral melanoma, the surgical field, chemotherapy plan, and explanation of prognosis all change completely. Studies in oral melanoma have reported that lymph node metastasis is confirmed in a significant proportion of patients even when the primary tumor is not large or the lymph nodes appear to be of normal size.

An important point here is that “the lymph node is not enlarged” does not automatically mean “there is no metastasis.” In clinical practice, microscopic metastasis is frequently detected in lymph nodes of normal size. Consequently, veterinary oncology is moving away from relying solely on palpation to assess lymph node status and toward combining imaging-based lymph node mapping with cytological and histopathological evaluation.

Methods for identifying sentinel lymph nodes are also advancing rapidly. Techniques such as CT lymphangiography, lymphoscintigraphy, fluorescent imaging using indocyanine green, and methylene blue staining are being applied, and more recently, intraoperative real-time fluorescence imaging to directly visualize lymphatic flow has been introduced. This change represents more than simply “adding another test”; it reflects a fundamental shift in how cancer is viewed.

Whereas tumor treatment in the past focused on “removal of visible tumors,” contemporary oncology is evolving into “a discipline that tracks the pathways along which cancer cells travel.” Sentinel lymph node evaluation is at the center of this shift. As the life expectancy of companion animals increases, cancer has become a more common disease, and owners’ expectations have risen accordingly. Owners now ask not only whether a tumor can be removed, but also what the risk of recurrence is, whether metastasis is present, and how long and how comfortably their animals can live.

To answer these questions with greater precision, veterinary oncology is becoming increasingly sophisticated. Sentinel lymph node evaluation is one of the most important starting points in this transformation.

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