Application of indocyanine green in the management of oral cancer: a literature review

ICG

ICG is a commonly used fluorescent contrast agent to map the lymphatic system. It plays an important role in the diagnosis of lymphedema, which is currently diagnosed using magnetic resonance lymphography and Doppler ultrasound (DUS). While the former can display lymph nodes and lymph vessels using contrast agents combined with fat-suppressed T2-weighted images, the latter can determine the nature of the lymph nodes through the Doppler effect produced by human blood flow. However, both have certain drawbacks. The former cannot be repeatedly applied and real-time dynamic monitoring of lymph nodes is limited due to its high cost of use, and the exploration range of the latter is limited [20]. With no risk of radiation exposure and the ability to dynamically monitor lymph node drainage, ICG is beneficial in the diagnosis of lymphedema. ICG binds to the lymphatic proteins; as edema progresses, the reflux of ICG in the lymphatic vessels slows down, prolonging the flow time. This enables accurate diagnosing and grading of lymphedema using ICG angiography (ICGA) [21]. In Unno et al.’s [22] study, 12 patients with secondary lymphedema underwent ICG lymphangiography, and four abnormal lymphatic drainage patterns of lymphedema were observed. The findings confirmed that ICG lymphangiography can be used to detect secondary lymphedema lesions. Yamamoto [23] used ICG to diagnose primary lymphedema, with a sensitivity and a specificity of 100 % each. ICG fluorescence angiography is a minimally invasive and safe technique that can be used to assess the severity of lymphedema and determine its prognosis.

Cells and tissues often do not have autofluorescent properties. NIR fluorescent contrast agents, therefore, play a significant role in SLN visualization, as they not only maximize the signal-to-noise ratio but also enhance tissue contrast. ICG is a widely used fluorescent contrast agent to detect SLN in different cancers. When ICG is injected into normal tissues surrounding the tumor, it binds to plasma proteins in the interstitial space to form macromolecular compounds, which do not diffuse freely into the surrounding tissues. Once it enters the lymphatic tissue for enrichment, and under the irradiation of NIR light with a specific wavelength, ICG-enriched sites are stimulated to produce fluorescence signals. These signals are then transmitted to a computer system for processing and imaged through the fluorescence imaging system of the NIR camera for SLN localization.

In SLN biopsy of patients with early oral cancer, using ICG alone for SLN localization is reportedly not sufficient. For instance, Al-Dam et al. [24] used ICG to perform SLN biopsy on 20 patients with early oral squamous cell carcinoma and found that although sentinel nodes were detected in all patients (100 % specificity), the sensitivity was only 50 %. This was because 50 % of the patients had false negative results, suggesting that the reliability of SLN biopsy after ICG imaging could not be verified. On the contrary, a meta-analysis conducted by Chen et al. [25] found that the sensitivity and specificity of ICG alone in detecting SLNs in patients with oral cancer were 86 and 92 %, respectively, suggesting the effectiveness of ICG alone in SLN detection. However, relevant studies on the use of ICG alone for SLN biopsy are limited. Therefore, further studies should be conducted to validate the above findings. At present, ICG is mainly used in combination with other tracers or auxiliary equipment for SLN detection (Table 1).

The penetration range of ICG is <5 mm. Therefore, radioactive signals can penetrate through the skin but not directly through the sternocleidomastoid and platysma muscles. This makes it difficult to locate SLNs directly through the neck skin. When SLNs are detected in patients with early oral cancer, a neck flap surgery is often performed. After the flap is opened, ICG (<2 mg/kg) is injected around the tumor at five different sites, and the SLNs are then located using an infrared camera. However, this method causes great trauma to patients, especially to those with negative SLNs. In computed tomography lymphography (CTL), the iodine contrast agent is directly injected into the patient’s lymphatic vessels. Once the agent accumulates in the vessels, the image of the lymphatic drainage is obtained by CT, which can be used for preoperative SLN localization. In recent years, using CTL with ICG for SLN detection has gained increased attention from researchers [26, 27, 37]. ICG fluorescence imaging-guided SLN biopsy has developed into a new non-radioimmunoassay technique. Honda et al. [26] investigated the feasibility of using CTL with ICG for SLN detection in patients with oral cancer. The study included 18 oral cancer patients who underwent CTL a day before surgery. During surgery, ICG fluorescent dye was injected around the primary tumor according to the imaging results. The NIR fluorescence camera was used to search for fluorescence signals. Based on the signals observed, the SLNs were labeled and resected. Of 18, the SLNs of 16 patients (89 %) were located during preoperative CTL examination and by ICG fluorescence signals. Five of the 16 patients were found to have metastasis in the postoperative pathological examination, suggesting that this technique has a high application potential in neck dissection of early oral squamous cell carcinoma. As the SLNs were located preoperatively, the SLN biopsy was performed using a small incision according to the preoperative localization and fluorescence signal. Most importantly, this technique reduces trauma to patients. Sugiyama et al. [27] compared the effects of using CTL at different time points after injection of iodine contrast agent to detect SLNs and found that CTL had the best detection effect on early SLNs after 2 and 5 min of injection of the agent. In addition, three-dimensional (3D) imaging was performed on the patient’s imaging results before surgery, and it was found that the difficult-to-detect tongue lymph nodes could be well identified by analyzing the 3D reconstruction images. Timely intervention of metastatic lymph nodes can avoid poor prognosis. These findings indicated that CTL combined with ICG can effectively assist SLN detection in patients with early oral cancer. However, CTL has some limitations, such as its lack of applicability in patients with iodine allergy. Cone-beam CT (CBCT) can avoid the effects caused by iodine allergy. Muhanna et al. [38] successfully used CBCT preoperative 3D imaging and intraoperative near-external red light-guided ICG fluorescence imaging in rabbit SLN biopsy. CBCT and micro-CT showed comparable performance in lymph node detection, with a sensitivity and specificity of 87.5 and 100 %, respectively. This indicated that 3D reconstruction technology combined with fluorescence imaging is a potential new research direction in the future.

Owing to their high accuracy, minimal invasiveness, and less postoperative complications, surgical robots are increasingly used in the treatment of oral cancer. The fourth generation of the da Vinci robotic surgical system is equipped with NIR fluorescence imaging. During surgery, the surgeon no longer needs a hand-held NIR camera to find fluorescence signals but instead can switch between the normal mode and fluorescence mode through the robot console operation. In the fluorescence mode, surgeons can evaluate the range of lymph node metastasis in real-time through the fluorescence signal image. The da Vinci surgical robot can help surgeons accurately and rapidly perform SLN biopsies [39]. Chow et al. [28] combined ICG and robotic NIR fluorescence imaging (Firefy^®) for SLN biopsy in 10 patients with cN0 oral cancer. SLNs were detected in all patients, with an average detection rate of 1.2 SLNs per person. The average detection time was 171.0 (68.0–312.0) s, and the average ratio of fluorescence signal to background signal was 5.62 (3.51–7.91). None of the patients had adverse reactions postoperatively. Similarly, Kim et al. [29] also evaluated the feasibility of ICG-guided SLN biopsy in nine patients with oral cancer. ICG was injected around the tumor during the surgery, and the robot firefly system was used to detect the fluorescence signal. The results showed that the positive rate, sensitivity, specificity, and negative predictive value (NPV) of ICG-guided SLN biopsy were 22.2, 100, 91.5, and 100 %, respectively. The findings confirmed that ICG and robotic surgery could accurately identify and locate SLNs.

ICG and methylene blue (MB)

The effectiveness of ICG combined with MB in identifying SLNs in various types of tumors has been validated previously [40], [41], [42]. Although MB was first synthesized by Caro, a German scientist, in 1876 [43], it was not until 1994 that it was first used for SLN detection in Lymphatic mapping and sentinel lymphadenectomy for breast cancer [44]. The basement membrane of tissues is often incomplete or discontinuous, and the capillary lymphatic vessels are highly permeable. When MB is injected into the interstitial space, it binds to plasma proteins to form macromolecular compounds, which enter the lymphatic vessels and are then drained into the lymph nodes. These are then ingested by antigen-presenting cells in the lymph nodes and are shown in blue. The blue dye helps surgeons map the path of the lymphatic drainage and ultimately locate the SLNs. Because the visible light emission spectrum of MB is 400–700 nm, in clinical application, surgeons can identify blue-stained lymph nodes with the naked eye [43]. Some scholars have also used MB to detect SLNs in patients with oral cancer, such as Vishnoi et al. [45]. The researchers conducted a prospective experimental study including 94 patients with early oral cancer and found that the sensitivity, specificity, positive PV (PPV), NPV, and accuracy of MB in SLN tracing were 84.6, 100, 100, 93.9, and 95.5 %, respectively. The findings indicated MB’s reliability in SLN detection in early oral cancer. However, compared to using ICG or MB alone, ICG and MB combined has better clinical application as they visually complement each other (fluorescence imaging and blue). The combination of the fluorescence method and the dye method has higher sensitivity and specificity in SLN detection. In Peng et al.’s [30] study, 26 patients with oral/oropharyngeal cancer were injected with 1 mL ICG (5 mg/mL) and 1.5 mL MB (1 mg/mL) around the primary tumor in a four-quadrant mode, sentinel lymph nodes were detected in all 26 patients, the number of sentinel nodes (SNs) per case varied from 1 to 9, with an average of 3.4, consistent with pathological results. The above findings suggested that ICG combined with MB could be a new potential method for SLN detection in patients with oral cancer. In addition, compared with radioisotope detection, ICG combined with MB has better operability with no risk of radiation exposure. However, given the limited studies in this regard, further studies should be conducted to validate these findings.

ICG and 99m Tc nanocolloids

Despite the advantage that MB has no risk of radiation exposure, its effectiveness in SLN detection is not as good as ICG combined with radiocolloid (Tc^99m nanocolloid or Tc sulfur^99m colloid) [46]. Frontado et al. [31] used ICG-^99mTc-nanocolloid and blue dye to detect SLN in 20 patients and found that while 100 % of SLNs were radioactive and fluorescent, however, of the 14 patients injected with blue dye, only 39.2 % of sentinel lymph nodes were detected In head and neck surgery, MB can also contaminate the surgical field, hindering the surgery [47]. Therefore, compared with ICG alone or ICG combined with MB, an alternative most studies use for SLN localization in oral cancer is ICG-99mtc nanocolloid. Murase et al. [32] used ICG-^99mTc-nanocolloid double tracer to locate SLNs in 16 patients with oral cancer. They found that ICG detected 35 lymph nodes, of which 28 were identified by the gamma-ray probe, indicating the efficacy of the combined application of fluorescence signal and radiation signal in detecting SLNs. ICG-^99mTc-nanocolloid is a new conjugate obtained by combining ICG and ^99mTc-nanocolloid with two compounds of different molecular structures. It is composed of radioactive and NIR fluorescent components, which have the advantages of accurate localization of the radionuclide method and real-time fluorescence-guided SLN visualization [48]. When ^99mTc-nanocolloid is injected into the body, lymphatic scintigraphy and single photon emission CT (SPECT)/CT can be used to observe the anatomical distribution of the lymphatic system and draining lymph nodes preoperatively [49]. A gamma detector can then be used to detect the radioactive count of lymph nodes during operation, and the location of SLNs can be accurately located according to the radioactive count value. In Meershoek et al.’s [33] study, which included 19 patients with early tongue squamous cell carcinoma, ICG-^99mTc-nanocolloid was used as a tracer to determine the edge of the tumor and mark the SLNs. Lymph scintigraphy and SPECT/CT were used for SLN mapping, and the pathological results were verified. The results showed that ICG-^99mTc nanocolloids can be used in combination with tumor boundary delineation and SLN detection, which has potential clinical application. Similarly, Berg et al. [34] enrolled 14 patients with oral cancer and injected ICG-^99mTc-nanocolloid mixed with tracer for SLN localization. Forty-three SLNs were located and resected under combined guidance. The high radiation signal background around the primary tumor may hinder the localization of SLNs [50]; however, the addition of an ICG fluorescence signal can sufficiently compensate for this defect. Overall, ICG-^99mTc-nanocolloid is effective for SLN detection in patients with oral cancer.

Perforator positioning

Accurate perforator localization is critical in preoperative flap design. Preoperative detection of the number, diameter, length, and course of perforator vessels can improve the success rate of flap repair, shorten the operation time, and reduce the occurrence of surgical complications. Some of the perforator localization techniques include hand-held DUS, color DUS, laser Doppler flowmetry, CT angiography, magnetic resonance angiography, and infrared imaging, with each having their own strengths and limitations. Owing to its real-time dynamic monitoring ability of perforator vessels and less adverse reactions, ICGA has been increasingly used in perforator vessel localization and flap design in recent years [14]. When ICG is injected into the body, it binds to plasma proteins to form macromolecular compounds, which do not freely diffuse into the surrounding tissues, thus enabling ICG to flow according to the blood vessel. When an NIR light source is irradiated to the observation site, it emits green fluorescence, and the fluorescence signal is imaged using the NIR camera. ICGA helps determine the number of blood vessels, the course of the blood vessels, and blood perfusion, among others. Using this principle, tissue blood perfusion after vascular anastomosis can be monitored and flap necrosis can be detected at an early stage [60]. Azuma et al. [61] located 2–6 perforator vessels in 14 patients using ICGA. Onoda et al. [62] used multidetector-row CT (MDCT), Doppler flowmetry, and ICGA to locate perforating vessels in 50 patients preoperatively. The results showed that MDCT had a 100 % (35/35) PPV and a 70 % (35/50) sensitivity in detecting perforators. Doppler flowmeter had an 80 % (40/50) PPV and a 100 % (50/50) sensitivity. The PPV and sensitivity of ICGA for detecting perforators were 84 % (42/50) and 76 % (38/50), respectively. For patients with a flap thickness of <8 mm and perforating vessels located in the superficial layer, it is difficult to identify the vessels using MDCT. For patients with a flap thickness of >20 mm and perforating vessels located in the deep layer, ICGA is not effective in identifying the vessels. Therefore, in clinical application, the detection method should be selected according to the characteristics of the flaps. At present, using ICGA alone to label perforating vessels is not the best method, but ICGA can achieve good results in assisting in identifying the optimal perforating vessels of pedicled skin flaps. Padula et al. [63] determined the best perforators for anterolateral thigh flap in 10 of 13 patients (77 %) using ICGA. In addition, it shortened the operation time and reduced the incidence of flap necrosis.

Vascular anastomosis

Vascular anastomosis is a key step in free flap repair of oral and maxillofacial defects, and high-quality vascular anastomosis can greatly improve the success rate of repair [64]. End-to-end anastomosis using suture remains the mainstream method of vascular anastomosis; however, it has some limitations, including high technical requirements, time-consuming, and foreign body response [65]. With the development of laser technology, laser-assisted vascular anastomosis (LAVA) gained increased attention due to its advantages, including simple operation, short anastomosis time, less inflammatory reaction, and reduced tissue trauma. In LAVA, when the laser is irradiated on the biological tissue, the heat generated causes the collagen in the tissue to form a triple helix structure reconstruction. The reconstructed triple helix collagen is polymerized together through covalent bonds, thus making the broken ends of blood vessels coagulated together. Although some scholars believed that it is the non-covalent binding of collagen fibers that causes vascular anastomosis, the principle of laser anastomosis remains controversial, and further research is warranted. The use of laser alone for vascular anastomosis can cause great damage to the tissue. Therefore, auxiliary materials have been developed to minimize this effect. When the laser is irradiated on the auxiliary materials, the materials dissolve, polymerize, and coagulate and finally realize the anastomosis of the broken ends of the blood vessels. Initially, auxiliary materials were liquid, and the thickness of the liquid film was difficult to master, making the surgical technique challenging. In recent years, semi-solid materials and new degradable materials have been introduced to assist vascular anastomosis [66]. However, repair and reinforcement materials often need to be combined with dyes. Dyes not only enhance energy absorption at the anastomosis more completely but also reduce the trauma caused by direct laser irradiation to vascular tissue. ICG fluorescent dye has also been used for laser-assisted vascular anastomosis. Schonfeld et al. [67] used a mixture of polycaprolactone (PCL) and ICG for vascular anastomosis. In the experiment, the mixture of PCL and ICG was electrospuned to make a patch, and then the patch was immersed in a 40 % weight bovine serum albumin (BSA) solution. The immersed patch was wrapped at the end of the blood vessel, and the balloon catheter was aligned at the end. The 810-nm diode laser was used to transfer welding energy from the intravascular cavity through the diffuser fiber to conduct vascular anastomosis. First, the in vitro test of the rabbit aorta was performed, followed by the in vivo test of the porcine aorta. The results showed that there was no blood leakage in the anastomosed vessels. The tensile strength of the vessels in vivo and in vitro was 138±52 mN/mm² and 117±30 mN/mm², respectively. Mbaidjol et al. [68] also used ICG and PCL-prepared patch to anastomosed blood vessels in the pig model and found that when the diode laser (emission wave 808 nm, power 2–2.2 W) irradiation time was 80–90 s, the immediate patency rate of blood vessels was the highest, and the patency rate was 83 % after 1 week of anastomosis. These results suggested the potential application value of ICG and PCL-composite patch for assisting vascular anastomosis using a laser. Schonfeld et al. [69] also prepared BSA blend microfibers using polyvinyl oxide, PCL, polyethylene, and gelatin as auxiliary proteins. ICG and PCL synthesized the light-absorbing layer and rotated the BSA/polymer layer to prepare layered patches. It was found that with the increase of BSA, the bonding strength of tissue fusion also increased. To validate the effectiveness and safety of ICG-assisted laser vascular anastomosis, future studies in animals and humans should be conducted.

Flap monitoring

Free flap transplantation plays a very important role in the repair of oral and maxillofacial defects. Free flap repair of defects solves postoperative aesthetic problems and reduces stigma in patients with oral cancer to a certain extent. In addition, it helps in the recovery of physiological functions such as swallowing [70]. In recent years, with the rapid development of microsurgery, the application of operating microscopes has increased the success rate of flap transplantation to as high as 95 %. However, postoperative vascular crisis and flap necrosis still occur in 10–30 % of patients [71]. Flap necrosis not only causes secondary injury to patients but also prolongs the length of hospital stay [72]. Therefore, early monitoring of flap blood perfusion is particularly crucial [73]. At present, the clinical methods for flap observation include visual observation, acupuncture experiments, and DUS [74], which are subjective. Owing to its advantageous characteristics, the application of ICG in flap crisis monitoring has proved helpful (Table 3) [75].

Holm et al. [81] used ICGA to evaluate the vascular anastomosis of 50 patients and found that compared with the traditional method of detecting vascular blood flow patency, ICGA was more sensitive and could accurately identify the vessels with anastomotic problems to avoid flap necrosis and secondary surgical exploration. Nagata et al. [76] conducted a study on ICG NIR angiography and acupuncture tests to monitor the blood flow irrigation of intraoral free flaps in 30 patients with oral cancer. The blood perfusion of all the patients was confirmed, and the results showed that ICG could effectively monitor the blood flow in flaps. Sacks et al. [77] used ICGA to intraoperatively identify the best perforator vessels of the anterolateral thigh flap in 15 patients and to monitor blood perfusion. All the patients underwent maxillofacial defect reconstruction, and only one flap failed to be reconstructed due to postoperative venous congestion. Taylor et al. [79] also used ICGA technology to evaluate the intraoperative perfusion of radial forearm free flaps and fibula free flaps, which was successful. No adverse reactions were observed. This proves that ICGA can monitor blood perfusion in real-time, which can help surgeons detect skin flap necrosis early, thereby avoiding harm caused by the second surgery.