Anatomic SLN Mapping
One of the interests of SLN technique is to highlight the atypical lymphatic drainage pathway in secondary lymphatic area that are not systematically dissected during routine lymphadenectomies.[33,64] This technique is well appropriate to early-stage cervical cancer since the tracer injection is performed just around the tumor and reveal not only the cervical lymphatic drainage but the tumor lymphatic drainage as well.
Although most of SLNs are located in the interiliac and external ILAC area, SLNs have also been described in "unexpected" territories such as sacral, promontory and paraaortic areas.[33,46–48] To describe SLN topography, Marnitz et al. suggested the following area classification by: paraaortic (area 1), common iliac (from the bifurcation of the aorta to the bifurcation of the iliac vessels, area 2), external iliac (along the external iliac vessels, area 3), interiliac (nodes medial to the external iliac artery and vein, in the obturator fossa and over the bifurcation of the common iliac artery, area 4), internal iliac (medial to the internal iliac vessels down to the uterine artery bifurcation, including the presacral nodes, area 5), and parametrium (nodes medial to the bifurcation of the uterine artery, area 6). Areas 3 and 4 are considered to be located in typical sites whereas the other areas (areas 1, 2, 5, and 6) are defined as atypical sites. In a meta-analysis of 27 articles which included 1301 patients with 3012 SLNs, Ouldamer et al. reported that 83.7% of SLNs were located in classic areas of the pelvis (obturator, external iliac, and internal iliac), 6.6% in the common iliac area, 4.3% in the parametrial area, 2.0% in the paraaortic area, 1.3% in the presacral area, 0.2% in the hypogastric area, 0.07% in the inguinal area, and 0.07% in the cardinal ligament area. Takeshita et al. described also SLNs in the circumflex iliac area. Even if most of metastases are located in the pelvic area, it was also described that positive lymph nodes were also found in gluteal, presacral, and paraaortic areas. Although the prevalence of involved nodes in these anatomical regions is low, this raises the question of the efficiency of systematic extended lymphadenectomy.[30,66] Moreover, the knowledge of the pelvic lymphatic anatomy is fundamental to understand the extension of lymphatic involvement.
Several studies have described the uterine and cervical lymphatic drainage anatomy.[67–70] More recently, some authors used ICG in SLN mapping for endometrial cancer and provided a surgical anatomic classification.[41,71] They described: an upper paracervical pathway (UPP) following the uterine artery to draining medial external and/or obturator lymph nodes with a continued course lateral to the common iliac artery to the lateral precaval and paraaortic areas, a lower paracervical pathway (LPP) with through the sacrouterine ligament to the internal iliac and/or presacral draining nodes before continuing medial to the common iliac artery to the medial paraaortic and precaval areas and an Infundibulo-pelvic pathway (IPP) with a course along the fallopian tube and upper broad ligament via the Infundibulo-pelvic ligament to its origin. These pathways are linked to finer lymphatics at the level of the cardinal ligaments, thereafter, dividing into distinct non-communicating courses lateral and medial to the common iliac arteries with further drainage to the paraaortic area. The UPP and the LPP were the most frequent pathway. In patients with tumor size larger than 20 mm, usual lymphatic drainage may be altered. Lymphatic vessels may be obstructed by cancer cells and it may result in a modification of tumor lymphatic drainage. Furthermore, large tumors can have a central necrotic part that may change the diffusion of colorimetric or/and radiocolloid tracers. Tumor cells migration could use other lymphatic drainage pathway than that run to interiliac or external iliac area. More SLNs are found in atypical area in nulliparous women. During the pregnancy, endometrial stromal cell decidualization may induce some modifications of the repartition of lymphatic vessels which result in changing lymphatic network.[72,73]
Atypical pathway are present in 25% of patients and are exclusive in 10% of patients. These atypical pathways might contain some isolated positive nodal metastases that would not be removed during routine pelvic lymphadenectomy. This technique may reduce the false-negative case of lymph node staging and residual disease missing which leads to an undertreatment of patients. The frequency and the positive nodes rate of each lymphatic area are similar to those described in systematic pelvic and paraaortic lymphadenectomies, demonstrating that the same anatomic information could be obtained with a less invasive technique. Moreover, the SLN technique facilitates the sampling of informative and representative nodes especially in atypical locations not covered by conventional lymphadenectomy.
Chin Clin Oncol. 2021;10(2):18 © 2021 AME Publishing Company