Page 157 - Sentinel lymph node biopsy in oral cavity cancer - Inne J. den Toom

lymphatic channels as well as blood capillaries. As stated by Ellner et al., [99mTc]Tc- tilmanocept showed a percentage of injected dose below 2.6% for liver, kidney, bladder and head.27 Although the background radioactivity for [99mTc]Tc-tilmanocept was still marginal (2.23%; SUV 0.132), it explains the residual distribution of [99mTc]Tc-tilmanocept in the presence of a lower radioactivity residing in both the injection site, as well as in the lymph nodes. One of our study limitations is the difference in amount of radioactivity between both tracers in the first 10 patients: 74 MBq [99mTc]Tc-tilmanocept vs. 120 MBq [99mTc]Tc- nanocolloid respectively. [99mTc]Tc-tilmanocept was approved by FDA (Food and Drug Administration) and EMA (European Medicines Agency) for identification of SLNs using 74 MBq in a two-day protocol. In our institution SLNB is routinely performed with 120 MBq [99mTc]Tc-nanocolloid. Because the first 10 patients were surgically treated based on [99mTc]Tc-nanocolloid, they received this routinely used amount of radioactivity to safely perform SLNB. This difference was corrected during quantitative analysis by correlating measured radioactivity in the VOIs to the radioactive dose injected. In the second 10 patients, [99mTc]Tc-tilmanocept was leading for SLNB procedure and therefore the amount of radioactivity could be equalized for both tracers (74 MBq). Another limitation is the impossibility of comparing hotspots at different time points post injection. Due to the impossibility of performing attenuation correction on planar lymphoscintigraphy, we unfortunately could not reliably compare SLN visualization at different time points due to different imaging modalities. Intensity of hotspots could easily be under- or overestimated based on physiological structures in near surroundings (e.g. mandible). On planar lymphoscintigraphy only anterior-posterior or oblique images could be used. This impedes us from differentiating and analyzing hotspots located in the same plane. Therefore, we opted to perform only quantitative analysis based on SPECT-CT. In some patients for whom [99mTc]Tc-tilmanocept was leading to identify SLNs during surgery, it proved challenging to accurately locate SLNs due to a scarce of activity on the second day, which was considered a drawback by the surgeon. This may be due to the relatively low radioactive uptake in SLNs of [99mTc]Tc-tilmanocept that was seen 8 in our population. As the injected activity was lower than used in [99mTc]Tc-nanocolloid SLNB (74 vs. 120 MBq) with also lower uptake in SLNs (3.16% vs. 1.95%) this resulted in less activity in SLNs in SLNB with [99mTc]Tc-tilmanocept, on average 1.4 MBq vs. 3.8 MBq at time of SLN scintigraphy. Vidal-Sicart et al. faced similar challenges during intraoperative localisation of SLNs using [99mTc]Tc-tilmanocept, which can probably be overcome by a higher injection dose of [99mTc]Tc-tilmanocept.13 155