Location and Causation of Residual Lymph Node Metastasis After Surgical Treatment of Regionally Advanced Differentiated Thyroid Cancer

Background: After initial surgical treatment of differentiated thyroid cancer (DTC) residual lymph node metastases are often found at the time of radioactive iodine (I) therapy. Recurrence of DTC is due to persistent disease not removed at initial surgery which also did not respond to I treatment. This study aimed at determining locations of, and reasons for, residual nodal metastases detected by I scans with single-photon emission computed tomography (SPECT/CT) obtained prior to radioiodine therapy following surgical treatment of DTC.

Methods: This is a retrospective study of 352 patients with intermediate and high-risk DTC treated with I therapy at the University of Michigan from 2007 to 2014. All patients underwent total thyroidectomy with or without lymph node dissection followed by radioiodine therapy. Pre-ablation diagnostic I scans with SPECT/CT were used postoperatively to localize nodal metastases, which were then compared with the cervical lymph node basins dissected at the time of surgery to determine the reason for residual nodal metastases: incomplete nodal dissection versus preoperative unrecognized nodal metastases.

Results: Of the 352 patients in the study, 146 (41.5%) had residual nodal metastases detected on I scans with SPECT/CT following initial surgery but prior to I therapy. Among the 146 patients with residual disease, there were a total of 218 distinct nodal metastases. Relative to the primary tumor, 71.6% (n = 156) of metastases were ipsilateral, 22.0% (n = 48) were contralateral, and 6.4% (n = 14) were non-sided in the central neck (level VI/VII). Cervical lymph node levels VI, III, and II had the greatest frequencies of residual metastases (33.9%, 22.9%, 18.8%, respectively). Residual metastases occurred because of incomplete nodal dissection (49.3%), lack of preoperative identification (37.7%), or a combination of both (13%).

Conclusion: Residual nodal metastasis following initial surgical treatment for regionally advanced differentiated thyroid cancer is rather common on highly sensitive I scans with SPECT/CT and is due to both unrecognized nodal involvement preoperatively and incomplete removal of metastatic lymph nodes during compartment-orientated nodal dissection. The surgical management of high-risk DTC should include preoperative imaging to evaluate for nodal metastases in the central and lateral neck and compartment-orientated nodal dissection of involved compartments. Attention should be given to complete dissection in levels VI, III, and II, particularly when dissecting compartments ipsilateral to the primary tumor.