Methods for sentinel lymph node mapping in oral cancer: a literature review

Oral cancers, excluding non-melanoma skin cancer, are the most common cancers of the head and neck. Of these, 90% are squamous cell carcinomas (SCC) [1, 2, 3]. Their most common locations are the tongue, the lip, and the floor of the mouth [1]. Oral squamous cell carcinoma (OSCC) is two to three times more common in men in most ethnicities. OSCC is one of the 10 most common cancers in the world; together with throat cancers, it ranks sixth in terms of incidence. The incidence of head and neck cancers is increasing. The most worrying numbers of OSCC incidence by World Health Organization (WHO) regions concern the areas of South and Southeast Asia (Sri Lanka, India, Pakistan, and Taiwan) as well as Western Europe (France) and Eastern Europe (Hungary, Slovakia, and Slovenia), Latin America and the Caribbean (Brazil, Uruguay, and Puerto Rico), and the Pacific regions (Papua New Guinea and Melanesia) [2]. According to WHO, mortality due to oral cancer is 1.9% [3]. At the turn of 2000–2009 there was a significant increase in OSCC incidence in Poland. In line with the predictions for the coming years, the scale of the problem will continue to increase [4].

One of the factors determining the choice of OSCC treatment method is the severity of the disease. In the clinical assessment of the stage of OSCC the TNM classification is used [2, 3, 5]. In the case of these neoplasms, surgical (dissection of the primary tumor, (feature T in the TNM classification) and regional lymph nodes (feature N in the TNM classification) is considered a radical method of treatment. There are several ranges of cervical lymphadenectomy: 1) selective neck dissection (SND) 2) modified radical neck dissection (MRND) 3) radical neck dissection (RND) [5]. The histopathological result of the surgical material determines the possible complementary radiotherapy. Chemoradiation therapy is offered to patients who are not eligible for surgery. Surgical treatment of the lymphatic system in head and neck cancers evolves from RND to MRND in order to reduce postoperative mortality and preserve the patient’s function and quality of life, with normal oncological outcomes [6]. Approximately half of the patients are diagnosed with OSCC in stage I/II of the disease [7]. In patients with stage T1N0 and T2N0 it is routine to resect all group I, II, and III lymph nodes in the tumor side of the neck. [5, 8] The issue of the purpose of the SND at the N0-defined nodes has been ongoing for many years [9]. The main argument in favor of such management is the reduction in the number of tumor recurrences in the area in patients with N0 characteristics (from 12% to 33%), who underwent prophylactic lymphadenectomy [10] and the detection of micrometastases in N0 nodes in approximately 23% to 43% of patients in the postoperative histopathological examination for OSCC and the oropharyngeal region [11]. The study by Shoaib et al. demonstrates that in 80% of patients with SCC of the head and neck region with clinically defined N0 features, no lymph node metastases are detected in the histopathological examination; they remain in the N0 group [12]. However, some believe that the probability of hidden or subclinical lymph node metastases in the early stages of OSCC reaches 3% [9]. Preoperative imaging studies (MRI –magnetic resonance imaging, CT – computed tomography) detect N0 node metastases in 40% to 60% of cases, but they also have a high rate of false-positive diagnoses [4]. Even when the neck lymphatic system is assessed using a combination of advanced clinical diagnostic imaging techniques, such as ultrasound-guided fine-needle aspiration (UsgFNA), MRI, and/or CT, in the early clinical stage of OSCC T1-2N0 latent lymph node metastases are present in 20%– and 30% of patients [13]. Other sources state that the reliability of combining palpation and additional imaging methods (CT, MRI, ultrasonography, or positron emission tomography [PET]) in the investigation of neck lymph node metastases does not exceed 80% [14]. The visualization and palpation of lymph nodes intraoperatively also cannot be considered a reliable predictor of metastases [10].

The sentinel lymph node (SLN) concept changes the approach to the extent of oncological surgery in the early stages of OSCC. It assumes that the metastasis of the primary focus to the lymph nodes has a strict sequence. Cabanas was the first to present this concept in 1977 based on studies in penile SCC [15]. Subsequently, Morton et al. in 1992 presented the possibility of marking metastatic lymph nodes, based on studies in patients with cutaneous melanoma [16]. In 1994, Giuliano et al., modifying the Morton et al. method, performed an intraoperative mapping of the lymphatic system in breast cancer patients using isosulfan blue [17]. SLN mapping, followed by intraoperative examination and a subsequent histopathological examination, in many cancers (breast cancer, melanoma, genital cancers) reveals the status of all regional lymph nodes, thus determining the needed extent of surgery [18]. The undoubted advantages of SLN mapping are reduced invasiveness of the procedure, better quality of life for the patient, and low health care costs in comparison to SND [13]. The SLN concept can be considered validated primarily for breast cancer and is in that case the standard of clinical practice [19]. The reliability of SLN mapping has also been studied for cutaneous melanoma, genitourinary cancer, colorectal cancer, gastric cancer, and ocular tumors [20].

The research is promising for OSCC and SCC of the oropharynx. A meta-analysis by Govers et al. shows that SLN biopsy appears to be a sensitive method for detecting metastases to the neck lymphatic system [11]. At the First International Conference on SLN biopsy in head and neck SCC it was shown that in 95% of the 316 patients collected from 22 research centers, it was possible to detect SLN with a 90% diagnostic accuracy [20].

To date, these are the known conventional and available methods for SLN mapping (Table 1):

Mapping with methylene blue dye (MBD)

This is a technique for intraoperative mapping of the lymphatic system. Under general anesthesia, 10–15 minutes before the start of the planned oncological operation, MBD is injected around the primary tumor in its four quadrants. The lymph nodes are then observed to be dyed blue and removed for intraoperative and histopathological examination [9, 21]. A meta-analysis by Li et al. for breast cancer patients demonstrated that mapping the location of SLNs using MBD alone yields an acceptable identification rate but an excessive false-negative rate. The combined use of dye and radioisotopes was found to be significantly better [22]. The latest study by Vishnoi et al. reports that SLN biopsy using only MBD in early-stage OSCC can be successfully used with good sensitivity and negative predictive value in countries with limited financial resources and limited access to nuclear medicine. They achieved an identification rate of 93.61%. Immunohistochemical examination increases the diagnostic potential of the method. However, the need for a larger study group is highlighted [21]. Similar results were previously achieved by Shivakumar et al. (97.7% identification rate) [23] and Rajaraman et al. (90.6% identification rate) [9]. Initially, only sulfan blue was used for SLN mapping. However, it is less available [14]. Simon et al. first reported that MBD can serve as an alternative to isosulfan blue in the detection of SLNs in breast cancer, obtaining a method efficacy in 90% of patients – similar to that of isosulfan blue [24]. MBD, despite advantages such as ease of dye availability, low cost, and a very low rate of allergic reactions, also has its drawbacks. The main disadvantage is the possibility of rapid loss of visibility of the marker in the presence of large amounts of adipose tissue, significant intraoperative bleeding, or rapid transmission of the dye in the lymphatic pathways. Compared to other methods, it is not possible to minimize the skin incision in the operated area. In addition, deeper-lying dyed lymph nodes can be missed [9, 25]. The blue dye stains the primary focus area and alters the absorption of the laser energy, which can make the oncologic procedure more difficult. There are reports of possible extravasation of the dye in the surgical field [6].

The metastable radioactive isotope technetium (99mTc) is used as the marker in this method. In contrast to the method using a dye, the radioactive substance is not administered intraoperatively. The marking procedure – the injection of 99mTc in the area of the primary tumor – is performed one day before surgery [7, 26] or on the morning of the day of the surgery [9]. The patient is then assessed for lymphatic drainage pathways and sites of radioisotope accumulation by lymphoscintigraphy [7, 26]. According to the imaging examination, the location of the SLN is marked on the patient’s skin. There are reports of the possibility of using SPECT-CT (single photon emission computed tomography-computed tomography) to more precisely determine the sites of technetium (99mTc) marker accumulation preoperatively [13, 18]. The next step in the described method is an intraoperative confirmation of SLN sites with a gamma camera, which is sensitive to isotope activity. The node with the highest technetium uptake (99mTc) is referred to as the SLN [7, 26]. The gamma camera does not provide visual information to the surgeon, only audio information [25]. Krag was the first to study the use of radioisotopes for SLN marking in breast cancer [27]. The first mapping with a radioactive substance of an SLN in a patient with head and neck SCC was performed by Alexa and Krag in 1996 [6]. In 2010, a multicenter study by Civantos et al. using only a radioactive marker in patients with OSCC evaluated 106 SLNs, yielding a negative predictive rate of 94%, and 96% with additional immunohistochemical examination [10]. In 2016, a multicenter study was performed by Miura et al. in which they identified SLNs in all patients, 196 in total, of which 35 (17.8%) had metastases detected [28]. Kaya et al. found, after the final SLN histopathological examination, that the negative predictive value, the positive predictive value, the accuracy of SLN biopsy, and the accuracy of the frozen sections were equal to 100%. It should be noted that the study group consisted of 18 patients [29]. A limitation of the method arises when the tumor and SLN are in close proximity. The hot spot of the primary focus may hide the SLN that clinicians are looking for. This is called the shine-through phenomenon. It is particularly evident in patients with squamous cell carcinoma of the floor of the mouth – in these cases the sensitivity of the method decreases by up to 63% [13]. It is considered that radiation exposure during SLN biopsy using radioisotopes is limited and the method is safe for pregnant surgeons and patients [30, 31]. However, some have expressed concerns about radiation exposure to both the patient and the surgeon [25]. The SENT (Sentinel European Node Trial) study shows that the SLN marking technique is effective for OSCC; the radioisotope can detect SLNs in 99% of patients [7].

In recent years, fluorescent markers have begun to be used in clinical trials. One of them is indocyanine green (ICG), which emits radiation with a wavelength close to infrared 700–900 nm (NIR – near infrared). ICG is able to penetrate human tissue to a depth of 1–2 cm. It is injected in the area of the primary tumor. The lymphatic system is characteristically dyed, and light is produced, which can be detected with a special camera and displayed on a monitor (NIR fluorescence imaging system). This makes percutaneous visualization of the SLN possible [25]. SLN mapping by this method has been successfully used in breast cancer, melanoma, cervical cancer, and vulvar cancer [18, 32]. A meta-analysis of breast cancer patients showed that intraoperative ICG fluorescent marking of SLNs yielded a detection rate of 98%, a sensitivity of 92%, and a specificity of 100% [33]. The first promising study using NIR fluorescence to mark SLNs in patients with head and neck cancers was performed by Bredell et al. [34]. The human eye is insensitive to NIR wavelengths, which is an advantage for primary tumor preparation [35]. ICG travels rapidly in the lymphatic system and SLN can be identified after only 5 min. This limits the diagnostic time window significantly and may lead to false positive diagnoses of SLNs [32, 36]. There are a very limited number of publications on the NIR fluorescence method for OSCC. All studies to date are performed on small study groups. The authors emphasize the validity of using ICG for SLN mapping, but the method requires further research. ICG may also constitute an alternative to MBD in a combined radioisotope method [25, 32, 37, 38]. A recent study by Xia et al. on a group of 29 patients with head and neck SCC who received ICG in the area of the primary tumor showed a sensitivity and a negative predictive value of the method of 75% and 99.4%, respectively [39].

The method of marking with both MBD and ICG dyes can be combined with the method of mapping SLNs with a radioactive substance to achieve greater efficiency [20, 25, 36, 37]. Many studies suggest that SLNs do not always dye blue using the dye method and are not always radioactive, so there is a growing opinion that the standard of care should be to mark the lymphatic system using a combined method. Morton et al. were the first to combine a blue dye and radioisotope technique, identifying SLNs in 237 patients with melanoma. The detection rate was 82% [20]. Albertini et al. refined the combined method first for breast cancer patients [22].

Rutger et al. reviewed the latest developments in preoperative SLN imaging in early-stage OSCC in 2020, which could replace the previously known methods and offer greater efficacy. According to the review they presented, human studies are currently underway on the use of a gadolinium-based paramagnetic contrast agent [Gd3+] and a superparamagnetic iron oxide nanoparticle (SPIO). The researchers inject the area around the primary tumor with the contrast agents, which are usually used intravenously in magnetic resonance lymphography. Gadolinium-based contrast agents cannot be detected intraoperatively. SPIO can be detected preoperatively via MRI and intraoperatively using a hand-held magnetometer [13]. There is one publication to date by Hernando et. al. on 11 patients with OSCC, in which SLN was identified using SPIO [40]. There are also attempts to use contrast agents used in lymphography with CT scans, which are more available and less expensive. There are reports of injecting iodine-based contrast agents into the area of the primary OSCC tumor lesion (Saito et al.). The limitation once again lies with the inability to detect the agents in question intraoperatively. Some researchers have turned to substances used in the latest radiological imaging technologies. Heuveling et al., while marking the primary tumor with [89Zr] nanocolloid in 5 patients with OSCC, achieved SLN detection using PET lymphoscintigraphy, as well as intraoperatively using a hand-held PET probe [41].

Fig. 1

In conclusion, head and neck tumors, due to the anatomical structure of the aforementioned region, pose a diagnostic and surgical challenge to the surgical team. Difficult surgical access to the neck lymphatic system, rapid transmission of the marker to the neck lymph nodes, and thus difficulty in finding SLNs in a timely manner, do not favor the standardization of the surgical procedure with the concept of SLNs in the early stages of OSCC, as is the case of breast cancer and melanoma of the skin. There is a need for extensive, long-term studies on a substantial study group using available SLN mapping methods to select the most beneficial therapy for the patient.