Accurate staging information is an extremely important component of an overall oncologic plan. Although advanced preoperative diagnostic imaging studies have vastly improved our ability to define the disease stage for a given patient, noninvasive imaging can underestimate the tumor burden, especially in cases of peritoneal carcinomatosis or metastatic serosal hemangiosarcoma, smaller multifocal liver metastases from a variety of neoplasms, or micrometastatic disease to regional lymph nodes. Diffuse or multifocal smaller lesions are very difficult to define on noninvasive imaging modalities such as ultrasonography, computed tomography (CT), or magnetic resonance imaging (MRI). Diagnostic laparoscopy has a place in some patients, providing the staging benefit of identifying metastatic lesions missed by other imaging modalities, identifying locoregional lymph nodes for biopsy, and providing an option for a minimally invasive diagnostic biopsy of the primary lesion or as a preliminary evaluation of lesion resectability. Tissue diagnosis is an important component of an oncologic therapeutic plan, and although other image-guided techniques such as fine-needle aspiration or percutaneous biopsy may also provide minimally invasive options in diagnosis, laparoscopic biopsy provides excellent lesion visualization, the ability to manipulate and possibly minimize injury to nearby viscera, the potential for larger tissue samples, and an improved ability to provide hemostasis.

The function of the lymphatic system is to drain interstitial fluid back to the venous system. Lymph nodes are junctions throughout this system, between multiple afferent lymphatics and few efferent lymphatics. As such, they form barriers to lymphatic drainage, and in a neoplastic setting, they function as barriers to the lymphatic extension of cancer cells and a rational point of evaluation for metastasis. However, assessing lymph node involvement still remains a challenge in the human and veterinary medical fields.

In concept, locoregional lymph node staging is important in the therapeutic planning of most solid cancers, and identification of metastatic disease within the lymphatic system can significantly alter surgical and adjuvant treatment recommendations. A large body of literature exists evaluating the extent, timing, and methods of surgical lymphatic staging for different neoplastic diseases in humans,^6-10 and there is growing evidence in the veterinary literature for consideration of these techniques in our patients.^11-17 In general, removal of large, grossly abnormal, clearly metastatic lymph nodes is most commonly recommended in veterinary patients for the purpose of overall disease reduction and improved response to adjuvant therapy.¹⁸ The role of surgical sampling of nonenlarged regional lymph nodes in veterinary patients is less clear. Because of their intimate association with the terminal aorta or vena cava and other important blood vessels, abdominal and retroperitoneal lymph nodes can be challenging to sample preoperatively, especially if lymphadenomegaly is not present. However, as micrometastases may take many months to produce palpable lymphadenomegaly, it is conceivable that early histopathologic evaluation of normal-sized lymph nodes could allow for a more accurate assessment of stage. Increased sensitivity in the detection of micrometastasis by lymph node biopsy allows for upstaging of patients who might have been staged as negative for nodal metastasis based on aspiration alone. However, in the case of more superficial tumors in which the draining lymph nodes lie within the abdominal cavity or retroperitoneum, a separate open abdominal approach in addition to the tumor resection can be an unattractive option to many clinicians and clients. A minimally invasive option for biopsy of abdominal lymph nodes when indicated may encourage clients to pursue improved staging of various regional neoplastic diseases, even when lymph nodes are not measurably enlarged. As a profession, the question of which lymph nodes should be sampled and when is a bit of the-chicken-or-the-egg question. How do we justify an invasive procedure to remove a nonenlarged lymph node(s) if we have not demonstrated a reasonably high probability that it will benefit the patient? But how will we obtain that data if we do not remove nonenlarged lymph nodes, assess them for microscopic disease, and correlate that information with survival times and treatment recommendations? Although mechanisms of metastasis are likely to be similar among species, there are noteworthy anatomic differences in lymphatic basins in veterinary patients compared with humans, and lymph node biopsy practices in veterinary patients should be evaluated on their own merits. There is much work to be done in this area in veterinary medicine in general, and minimally invasive approaches may improve our opportunities to answer these questions.

The question of which lymph node(s) to remove for a given tumor may be aided by the technique of sentinel lymph node (SLN) mapping. SLN mapping is based on the concept that lymphatic dissemination from a tumor is an orderly process, with initial involvement of the first lymph node receiving afferent lymphatics from the tumor before dissemination to the remainder of the nodes in the regional lymphatic basin.⁶ The SLN is the most likely lymph node to harbor metastatic deposits, and therefore if the pathology of the SLN is negative for metastasis, the nonsentinel nodes in the region should be at minimal risk of harboring metastases. Based on this concept, assessment of the regional lymphatic basin for microscopic metastatic disease via SLN biopsy (even when the lymph nodes in question are palpably unremarkable or normal in appearance on imaging) has become the standard of care in human breast cancer and melanoma patients and is increasing in use for a variety of other neoplasms.^6-9,19 SLN biopsy has been shown to be a highly sensitive and specific indicator of the patient’s true metastatic status.^20,21 Increased sensitivity in the detection of micrometastases in SLNs allows for upstaging of patients who might have been staged as negative for nodal metastasis (N0) based on regional lymph node aspirates alone. Human studies have demonstrated up to 30% of patients, previously staged as N0 before SLN evaluation, benefited from upstaging and subsequent adjustments in therapeutic plan after detection of micrometastases.^20,21 The clinical use of SLN mapping is in its infancy in veterinary medicine, but it has been described in a few cohorts of dogs.^13,15 In Worley’s study of dogs with mast cell tumor, 42% of patients had additional treatment recommended that would not have otherwise been offered based on findings of micrometastatic disease identified by SLN biopsy.¹³ Much experience still needs to be gained on the application of these techniques to veterinary patients, including application to different disease processes, tracer types, tracer doses, and timing.

The most common methods of intraoperative SLN mapping include the use of lymphoscintigraphy; intraoperative vital blue dyes (e.g., isosulfan blue, patent blue V, or methylene blue); or more recently, the use of intraoperative near-infrared fluorescent imaging.^{10,13,15,16,20-25} A small volume of the chosen tracer is injected peritumorally (intradermally or subcutaneously at the transition from palpable disease at the tumor margin). Assessment for tracer uptake in the locoregional lymph nodes is then performed. Lymphatic uptake is relatively quick, on the order of seconds to minutes, so SLN assessment must be performed very shortly after injection, or there is a higher likelihood of identification of second- or third-tier lymph nodes, and even possibly missing the true SLN because of tracer washout. In general, the use of scintigraphy and visual vital dyes is most commonly combined in human patients because the combination is more sensitive in SLN identification than using vital blue dyes alone.²⁶ Although scintigraphy does not discolor the dissection field around the primary tumor, it does not aid in visual identification of the lymph node at surgery. The blue dyes are widely available and do visually identify the lymph node at surgery, but they result in discoloration of the tissues around the primary lesion that can obscure regional anatomy and slow dissection. Near-infrared fluorescence imaging is an emerging modality that in early studies appears to have similar sensitivity and specificity to the combined use of scintigraphy and vital blue dyes and offers the benefit of avoiding patient and surgeon exposure to ionizing radiation.^10,22-25 The main downsides for near-infrared imaging are the need for specialized imaging systems that are not yet widely available and that the most commonly used near-infrared tracer, indocyanine green, is nonspecific and passes very quickly through the lymphatic system, making it prone to highlight second- and third-tier nodes if close attention to timing is not given.

Because, historically, biopsy of the abdominal and retroperitoneal lymph nodes could only be obtained by a major open abdominal procedure, excisional biopsy of these lymph nodes for staging purposes has not commonly been elected in the absence of overtly palpable or ultrasonographically identified lymphadenopathy. In Worley’s study of SLN mapping, the two SLNs identified by lymphoscintigraphy that were purposely not extirpated were medial iliac lymph nodes (with the implication that this was because of the intraabdominal location).¹³ In a canine experimental study evaluating the utility of various vital dyes for lymphatic mapping, a medial iliac lymph node identified by lymphography as the SLN, received afferent lymphatics from two anatomically divergent injection sites (a dorsal lumbar injection and a hock injection), confirming that the medial iliac lymph node has the potential to be sentinel for a wide variety of tumor locations.¹⁷ Although published clinical data on the minimally invasive removal of cavitary lymph nodes in clinical patients is lacking, a minimally invasive technique for excisional biopsy of the medial iliac lymph nodes using a lateral three-port laparoscopic approach has been described in a cohort of normal dogs.²⁷ Although this study was performed in a cohort of healthy dogs, anecdotally, this technique has been performed by the author in a clinical setting to obtain lymphatic staging information in dogs presenting for perineal neoplasia.