SLN Biopsy Procedure
According to the ESGO guidelines, SLN mapping might be performed by a combined technique of blue dye (patent blue, isosulfan blue or lymphazurin) in association with radiocolloid Technetium 99m (99mTc) or by fluorescent dye alone [indocyanine green (ICG)]. Blue dye alone should be avoided and the association with isotopes may improve sensibility to detect SLN.[34,35]
The radioactive tracer colloidal rhenium sulfide labeled with technetium (99mTc) is mainly used. Two protocol injection are feasible: a long or a short. In case of long-protocol, a dose of 120-MBq is injected the day before surgery with a delay shorter than 15 hours. In case of short-protocol, a dose of 60-MBq is injected morning of surgery. Whatever the type of protocol, cooperation with the nuclear medicine department is essential. The injection is performed in healthy tissue at the periphery of the tumor or of the conisation scar. By using a 25-gauge needle, submucosal injections are performed into two (at 3 and 9 o'clock) or four points at 5–10 mm-depth (Figure 1). To guide the surgeons during the surgical procedure, a preoperative lymphoscintigraphy may be done after the isotope injection (1 to 3 hours in case of short protocol or 3 to 5 hours in case of long protocol). Detection rate and diagnostic performance are not affected by either the two protocols.[36,37] Due to a better resolution, the single photon emission computed tomography (SPECT-CT) provides more accurate information than lymphoscintingraphy alone on SLN topography and improves surgical staging.
Colorimetric tracer injection technique. (A) Syringe preparation; (B) Injection through vaginal access; (C) Different sites of injection.
Two types of colorimetric tracer can be used: blue dye or ICG. Using ICG require specific near-infrared detection system (for both open surgery and minimally invasive surgery) to visualize the fluorescence, whereas blue dye does not require any specific material since it can be seen with the naked eye. Both colorimetric tracers can be used at the same time. Usually, 2 mL of 2.5% blue dye are diluted in 2 mL of saline or 25 mg of ICG are diluted in 10 mL of glucose solution. Some authors suggested to inject concomitantly India ink to identify SLN in the parametrium at the final pathologic examination.
The injection of the colorimetric tracer is performed at the beginning of the surgical procedure following the same technique as described for radiocolloid injection. Practically, the injection can be done right after the induction of general anaesthesia, but due to the rapid migration of the tracer, this injection should be performed at the earliest after the surgical positioning of the patient in order to decrease the false negative results by identifying non-SLN instead the true SLN. In addition, late injection enhances to visualize intraoperatively dye migration until the true SLN. One millilitre of blue dye or ICG is injected into 2 or 4 points in the cervix (Figure 1). Higher injection volume should be avoided due to the risk of dye spillage in the surrounding tissue which may impair the surgical dissection. By contrast, a second injection can be performed during the surgery in case of unilateral detection.
A particular attention should be paid for a meticulous injection technique in order to decrease detection failure.[42,43] Some technical problems can occur with the cervical injection such as wrong injection site (too lateral or in the external cervical ostium), the presence of Nabothian cysts, tiny atrophic cervix in menopausal women, bleeding, distorted cervical anatomy, or lack of experience. In case of larger tumors and bulky cervix, tracer injection technique is more difficult due to the frequent presence of a dense and necrotic central core that may inhibit tracer diffusion. Appropriate modification of tracer injection technique may overcome this difficulty by using longer needle, bigger volume of blue dye and injection in the residual stroma around the tumor.
Surgery can be performed by minimally invasive approach (laparoscopy or robotic-assisted laparoscopy) or by open surgery. Most of published data are derived from series in which surgery was mainly performed by minimally invasive approach but the results of the LACC trial will certainly induce a paradigm shift in favour of open approach.
The intraoperative detection starts by exploring the abdominal and pelvic cavities over an intact peritoneum in order to look for stained and/or radioactive nodes detected by the Gamma probe. An adhesiolysis may be necessary in case of previous pelvic surgery. The absence of grossly suspicious nodes has to be verified even if this eventuality is less likely with an adequate preoperative imaging. The dye migration has to be followed in regard with the parametrium, internal iliac area, interiliac area, external iliac area, common iliac area and paraaortic area as described by Marnitz et al. (Figure 2).
Intraoperative view of blue tracer migration (A,B) and ICG migration (C). SLN, Sentinel lymph node; ICG, indocyanine green.
Second, the entire retroperitoneal space should be opened along the external iliac vessels with pararectal and paravesical fossa as well. Anatomical landmarks such as the umbilical artery, the ureter, the obturator nerve have to be identified (Figure 3). This approach permits to see the early drainage from the cervix through the parametrium by following the dye progression in the channels before any node is taken to ensure that the true draining SLN is identified and not missed (Figure 4). SLN are usually found in the external iliac and interiliac area. The SLN mapping may be guided by the information provided by the lymphoscintigraphy. Even if the false negative rate may be limited by increasing the number of SLNs sampled especially in teams with low experience, not all detected nodes (blue and/or hot SLN) should be removed but only the first draining node in the channel pathway has to be removed and labeled as SLN (Figure 5). This step is crucial to correctly identify the true SLN and not to confuse with non-SLN which correspond in fact to distal migration of tracer beyond the true SLN. Nonetheless, more SLNs can be sampled in case of truly separate channels which may correspond to distinct lymphatic drainage pathway (Figure 6). If no SLNs are found in the external iliac, interiliac and common iliac area, dissection of the promontory area and paraaortic area should be performed to avoid missing SLN in atypical topography. Whatever the type of technique used, a bilateral detection should be obtained (at least one SLN in each hemi-pelvis).
Laparoscopic intraoperative view of the right ilio-obturator fossa with anatomical landmarks.
Laparoscopic intraoperative view of the right ilio-obturator fossa with fluorescent afferent lymphatic channel, fluorescent SLN and fluorescent efferent lymphatic channel. SLN, Sentinel lymph node.
Intraoperative view of the left ilio-obturator fossa with two distinct lymphatic channels (yellow and red arrows).
In case of unilateral detection, extensive dissection of the retroperitoneal space has to be performed carefully due to the risk of diffuse smearing induced by an excessive manipulation. A second injection of colorimetric tracer might be helpful. According to MSKCC algorithm, if no SLNs are found, an ipsilateral pelvic lymphadenectomy should be performed. Caution should be paid to swollen lymphatics channel which might be mistaken with SLN and are responsible of 40% of detection failure. Detected SLNs are removed selectively and sent for frozen section examination (FSE) after ex vivo palpation.
Finally, the absence of residual in vivo radioactivity and the absence of residual fluorescence are checked. The opening of the paravesical and pararectal space enhance to individualize the parametrium and to begin the radical hysterectomy. At this step, SLNs in parametrium can be found and have to be reached.
These different steps have to be respected to ensure the reproducibility and the reliability of the technique. This surgical methodology may be implemented in a check-list of "quality-assurance" (Table 1).
Experience and learning-curve
Efficient cervical injection, intraoperative detection and accurate node sampling require a learning-curve which may be done by performing systematic SLN biopsy before each pelvic lymphadenectomy (for cervical or endometrial cancer) under the control of an experienced operator.
This concept of learning-curve has been widely described in the literature.[15,45,50–52] In their cohort of 55 patients, Plante et al. have shown a better SLN identification rate in the last 15 cases compared to their 28 first cases (93% vs. 51%, P<0.01) and concluded that experience in SLN detection increased significantly with time. This acquired experience had an impact in the improvement of SLN detection in subgroup of patients with higher risk of SLN detection failure such as patients with larger tumor size and obese patients. According to Khoury-Collado et al., the cut-off for learning curve in SLN biopsy in endometrial cancer should be fixed at 30 cases. In an ancillary analysis of SENTICOL I and SENTICOL II cohorts, bilateral detection rate was significantly higher in SENTICOL II cohort (2009–2012) than in SENTICOL I cohort (2005–2007), respectively 83.5% vs. 75%, P=0.04. Moreover, centre with low surgical skills (≤1 case/year) had significantly lower bilateral detection rate than intermediate-skill centre (1–5 cases/year) and high-centre (>5 cases/year), 56.3% versus 78.3% and 83.1% respectively (P=0.02).
As demonstrated in ovarian cancer, treatment in expert centre with a high-volume of surgical procedures is a quality indicator with better outcomes.[53,54] Surgery should be performed by gynecologic oncologist and trained surgeon in a specialized team dedicated to management of gynecologic cancers.
Impact of ICG
As previously mentioned, the ESGO guidelines recommend for SLN mapping might be performed by a combined technique of blue dye with radiocolloid 99mTc or by ICG alone. SLN detection using ICG and near-infrared fluorescence has been introduced in 2012 by Rossi et al. in cervical and endometrial cancer.
In a meta-analysis including 538 patients with cervical or endometrial cancer and comparing ICG to blue dyes with or without 99mTc, Ruscito et al. have shown that ICG improved significantly overall detection rate (OR =0.27, 95% CI: 0.15–0.50. P<0.0001) and bilateral detection rate (OR =0.27, 95% CI: 0.19–0.40. P<0.00001) compared to blue dye alone. There were no differences in overall and bilateral detection rates between ICG and association of blue dyes and Technetium-99m. Near-infrared imaging with ICG enhances deeper visualization of SLN through adipose tissue with a penetration of 1 cm which may explain that ICG provides better SLN detection rate compared to blue dye in obese patients.[57,58] In a randomized controlled trial comparing ICG to methylene blue in 132 patients with endometrial cancer, ICG increased significantly the SLN detection rate per hemipelvis of 26.5% compared to methylene blue. Recently, the prospective randomized FILM trial has shown better SLN detection rates with ICG compared to blue dye in patients with endometrial and cervical cancer (97% vs. 47%, P<0.0001).
ICG have several advantages. This tracer is easier to use and does not require lymphoscintigraphy and no human or technical resources linked to a nuclear medicine department. In terms of cost, an ICG vial is cheaper than using Technetium-99m. Preoperative radiocolloid tracer injection may be painful whereas ICG is injected after the induction of general anaesthesia. Risk of allergies and anaphylactic shocks are higher with blue dye than with ICG.
Based on these results, this technique is of growing interest and will surely become soon the agent of choice for a worldwide implementation because of its safety profile, its ease of use, its affordable low cost and its diagnostic performance. Although ICG is a fluorescent dye approved by the FDA (Food and Drug Administration) and the European Medicines Agency, it seems important to precise that ICG is used off-label for intratissular injection to date.
Chin Clin Oncol. 2021;10(2):18 © 2021 AME Publishing Company