Anatomical localization of radiocolloid tracer deposition affects outcome of sentinel node procedures in prostate cancer

Purpose Diagnostic imaging modalities have moderate sensitivity for the identification of lymph node (LN) metastases in prostate cancer (PCa) patients. Mapping the lymphatic drainage from the prostate can help to identify the LNs directly draining from the tumour (sentinel nodes (SNs)); the LNs stated to have the highest chance of containing metastatic cancer cells. Although the lymphatic drainage may differ between segments within the prostate, the location of the primary tumour is not routinely taken into account during peripheral zone-aimed tracer administration. This study evaluates whether linking the SN procedure to the primary cancer deposits increases the identification accuracy of lymphatic metastases. Methods Sixty-seven PCa patients, scheduled for robot-assisted laparoscopic prostatectomy (RALP) and extended lymph node dissection (ePLND) with subsequent SN biopsy, were included in this retrospective study. After injection of the hybrid tracer ICG-Tc-99m-nanocolloid in the prostate, SN mapping was performed based on lymphoscintigraphy and SPECT/CT. SNs were resected using a combination of radio- and fluorescence guidance. Pathology was used to determine the primary tumour location and metastatic spread. Fluorescence imaging of paraffin-embedded prostate tissue was used to determine the location of the tracer deposits in the prostate. This deposition was related to the primary tumour location, the lymphatic drainage pattern of the injected tracer, and the metastatic spread. Results In total 265 radioactive LNs (211 SNs and 54 higher-echelon nodes in 64 patients; 4.3 LNs per patient; IQR: 2-6) were identified. In three patients (4%) preoperative imaging did not allow identification of SNs. Tumour-positive SN visualization within the pelvis was shown to be influenced by intraprostatic location of tracer administration. This could be concluded from (1) a clear correlation between lymphatic drainage to the right or left side of the body and tracer deposition on the right or left side of the prostate, (2) visualization of a higher number of LNs after dorsal tracer deposition compared with ventral tracer deposition, (3) different drainage patterns observed for tracer deposition into the base or apex of the prostate, and (4) the indication that intratumoural tracer deposition increases the chance of visualizing nodal metastases compared with extratumoural tracer deposition. Conclusions The correlation between the location of the tracer deposits, the location of the primary tumour, and the visualization of the (tumour-positive) SNs indicated that placement of tracer deposits is of influence on the visualized lymphatic drainage pattern. This suggests that tracer injection near or into the primary tumour site is beneficial for the identification of metastatic spread.