Unexpected ^99mTc-pertechnetate avidity of lymph node metastases predicts better response to radioiodine therapy in differentiated thyroid cancer patients with lymph node metastases

Objective: To investigate the value of Tc-99m-pertechnetate scan in postoperative differentiated thyroid cancer (DTC) patients with lymph node (LN) metastases (LNM) uptake Tc-99m-pertechnetate, especially the predictive value to their response to radioiodine-131 ( I-131) therapy. Methods: This retrospective study collected 752 patients with DTC and LNM treated at Zhejiang Cancer Hospital between May 2012 and December 2017. Depending on the ability of LNM uptake Tc-99m-pertechnetate, the patients were grouped as the (99m)Tcpertechnetate-avid (n=88) vs. Tc-99m-pertechnetate-non-avid (n=664) groups. And Propensity score matching (PSM) was performed at a 1:4 ratio to reduce confounding bias. Results: In the PSM analysis, the 1:4 matched cohort comprised 752 patients (88 with (99m)Tcpertechnetate-avid LNM, 664 with Tc-99m-pertechnetate-non-avid LNM). Patients' age, initial I-131 activity and frequency of iodine therapy were included as covariates. After PSM analysis, 363 patients (Tc-99m-pertechnetate-avid group, n=83; Tc-99m-pertechnetate-non-avid group, n=280) were successfully matched. Among the 363 PSM-matched patients, 48/83 (57.8%) in the Tc-99m-pertechnetate-avid group and 158/280 (56.4%) in the Tc-99m-pertechnetate-non-avid group had two or more I-131 treatments. The nsTg and the percentage of changes in ssTg between the Tc-99m-pertechnetate-avid and Tc-99m-pertechnetate-non-avid groups were significantly different ([0.05 (0.04 to 0.90) vs. 0.40 (0.04 to 4.92), p=0.018] and [-88% (-98%, -50%) vs. -66% (-86%, -30%), p < 0.001], respectively). No significant differences were observed between the two groups in the other parameters (age, pathological type, distant metastasis, follow-up time, AJCC TNM stage, initial I-131 treatment activity, and I-131 treatment frequency) after PSM (all p > 0.05). Conclusion: In patients with DTC and LNM, LNM uptake of Tc-99m-pertechnetate is a rare phenomenon. Patients with Tc-99m-pertechnetate-avid LNMs were more likely to benefit from I-131 therapy, even after adjustment for age, I-131 treatment frequency, and initial I-131 activity.