This study examined the feasibility of using two different intraoperative imaging modalities, cone beam CT and ICG with NIR fluorescence, to help with intraoperative mapping of the sentinel lymph nodes in a rabbit model of head and neck cancer. In addition, pre-operative imaging was performed and compared to the intraoperative NIR Fluorescence SLN yield rate.
Intraoperative imaging with cone beam CT
Cone beam CT is increasingly being used as an adjunct for improving treatment outcomes in head and neck cancer patients [13]. Recent publications have demonstrated the value of intraoperative cone beam CT scanning in facial trauma surgery, with the CBCT enabling rapid review of cases with revision if necessary [15]. Adaptive radiotherapy has been developed to improve the targeting of radiotherapy; CBCT scanning is done during radiotherapy treatment, with the gross target volume adjusted according to the tumor response of the radiotherapy [16]. The images obtained from the CBCT scanning in this study were of variable quality depending on the tissue being examined. The CBCT scans gave excellent bony resolution, allowing for very quick 3D reconstruction (15 seconds) and accurate identification of bony landmarks such as the angle of mandible. The use of IV contrast did help with identification of blood vessels within the head and neck region. Although gross soft tissue structures, including large lymph nodes and blood vessels, could be identified, the microCT scans had a much superior soft tissue definition [17]. Furthermore, our CBCT machine is one of the earliest generation scanners (built in 2002). It is likely that with advances in imaging software and technology, there is likely to be improvement in soft tissue imaging with CBCT in the future. It does, however, highlight the feasibility of using the CBCT in the OR setting for near real-time imaging feedback.
Intraoperative imaging with IGC and NIR fluorescence
SLNB in head and neck cancer has not translated into regular clinical practice in many centers, but given that it is now a recommendation in the NCCN guidelines [18] and a quality standard in the NICE guidelines, the technique might be adopted progressively. SLNB is a valuable technique as it enables the identification of the sentinel lymph nodes of the tumor, which can be processed for histology. Only if the SLN is positive for malignancy on histologic examination is a neck dissection subsequently performed, which should reduce operative morbidity in patients who have a negative SLN.
The current techniques used for SLNB include the use of methylene blue dye, radiocolloid and fluorescence guided surgery [19, 7, 8, 20, 21]. The current gold standard is the use of preoperative lymphoscintigraphy, intraoperative gamma probe detection with or without the use of blue dye [22]. However, there are a number of disadvantages with this gold standard technique. The preoperative lymphoscintigraphy requires an extra procedure for the patient, often the day before surgery. There are also reports that this stage of the SLNB process is painful for the patient due to the limited use of local anesthetic as it may interfere with results [23]. The use of any radioactive material requires a nuclear physician, with both staff and the patient being exposed to the radiation, although the dose is very low in comparison to the known safe limit. Furthermore, with head and neck cancer specifically, the often close proximity of the primary tumor to the sentinel node can lead to “shine through” from the primary tumor, making it difficult to identify primary tumor from metastasis, particularly in floor of mouth cancers[24].
In every rabbit used in this study, at least 1 SLN was identified, up to a maximum of 4 SLNs. Morton et al demonstrated, during the MSLT-I trial, that the SLN detection rate in the neck region was only 84.5%, in comparison to 99.3% in the inguinal region and 96.6% in the axillary region [24]. The lower incidence of success in the neck may reflect more complex lymphatic anatomy and smaller lymph nodes. One of the advantages that we have shown with the use of ICG with NIR fluorescence is that imaging for SLN identification can be done quickly after injection of the tumor. We identified the SLNs between 25-30 minutes after injection, which is comparable with van der Vorst et al, who observed that imaging time for identifying SLNs ranged from 5-30 minutes [25]. This shortened time frame is favorable compared to the use of radioactive material which may have to be injected as much as a day before the procedure. The volume of ICG that we injected was small (only 50 uL of 0.1 mg/ml). The dose of ICG injection we used in this study gave a high signal-to-background ratio (SBR) of fluorescence at 6.68 ± 3.00. This again is comparable to the study by van der Vorst et al, who reported a SBR of 8.7 ± 6.4. The small volume of ICG injected with good SBR is reassuring as previous results by Mieog et al have demonstrated that high doses of ICG can cause quenching, resulting in a decrease in NIR fluorescence signal [26]. A further advantage in using NIR fluorescence in the operating room is the immediate real time feedback obtained when performing the imaging, enabling the surgeon to tailor the procedure using real time imaging feedback. The equipment is small and portable enough that it is not a hindrance to clinical workflow. Toxicity from ICG is rare and primarily due to allergic reaction owing to the small amount of iodine content [27].
Pre-operative micro CT Vs Intraoperative NIR Fluorescence Imaging.
In addition to clinical examination, pre-operative imaging is commonly used to identify enlarged lymph nodes for further biopsy or resection. In the present study, microCT and DCE-MRI were used in order to preoperatively identify suspicious lymph nodes. DCE-MRI was discontinued after three rabbits because both the micro CT and DCE-MRI were identifying the same SLNs. Of the 21 SLNs identified by pre-operative microCT, only one was not highlighted by fluorescence after the topical injection of ICG, which could be attributed to the presence of multiple lymphatic drainage channels delaying or preventing adequate fluorescence. The intraoperative NIR fluorescence single node that was identified in Rabbit 5 was positive for malignant cells on histopathology. Our finding that only 12 of 24 enlarged lymph nodes identified by preoperative imaging were histologically positive for metastatic disease fits with findings by Langhans et al who demonstrated that positive nodes were not larger or different in shape than negative nodes in T1-T2 N0 M0 oral SCC[28].
There are limitations to this study. We did not compare this new approach for identification of SLNs directly with the current gold standard of using radioactive isotope.