The study was approved by the institutional review board and Medical Ethics Committee UZ Brussel. Need for written informed consent was waived. A retrospective analysis was conducted for all patients over the age of 18 who:
- had undergone a total thyroidectomy, with diagnosis of DTC on pathological examination.
- received subsequent 131I remnant ablation.
- had their complete follow-up care (blood analyses of bTg and sTg, US of the neck, 131I remnant ablation and 123I WBS) in our institution from October 2013 onwards, after which a more sensitive Tg assay with a functional sensitivity of 0.1 ng/ml was introduced.
Subjects without 131I remnant ablation, data of histopathological diagnosis, bTg and sTg measurement, or 123I WBS results were excluded. Subjects with possible TgAbs interference were excluded as well.
Data were retrieved from the medical records and included histopathological results, blood analyses (bTg, sTg, TSH, free Thyroxine (fT4), TgAbs), US of the neck, 131I and 123I WBS results over a period of 1 year after thyroidectomy.
bTg was measured at a 3 to 6 month interval using the Elecsys Tg II immunoassay with functional sensitivity up to 0.1 ng/mL. A value below 0.1 ng/mL was reported as undetectable. sTg is defined as the thyroglobulin concentration after stimulation. It was measured 5 days after 2 rhTSH (thyrotropin alfa, Sanofi Genzyme) injections, or 3 weeks after hormone withdrawal. A value below 1 ng/mL was reported as a low sTg. TgAbs were measured at a 3 to 6 months interval using the Elecsys Anti-Tg Assay. The lower detection limit was 10.0 mU/L. The threshold value for positivity was the proposed manufacturer’s cut-off (115 mU/L).
TSH and fT4 were measured at a 3 to 6 months interval. In accordance with the ATA guidelines, levothyroxine dosage after surgery was modified to maintain a suppressive low TSH. The specific target serum TSH was risk-dependent.
A post-operative US of the neck was performed at a 6 to 12 months interval by a certified radiologist. The ultrasound was performed on a Canon Aplio i800 system, using a dedicated thyroid program on a multi-frequency ultra-wideband linear 18 Mhz probe. The neck was scanned in the axial and sagittal plane for remnant thyroid tissue. Different colour Doppler modes were used to assess the vascularity of a suspected remnant. The different anatomical regions of the neck were scanned for atypical or enlarged lymphnodes.
Indeterminate or inconclusive results were discussed interdisciplinary and, when necessary, followed by fine needle aspiration (FNA), Computed Tomography (CT) or Magnetic Resonance imaging (MRI) of the neck.
Conventional planar WBS (131I and 123I) was combined with Single Photon Emission Computed Tomography (SPECT) to enable precise localization and characterization of abnormal foci of Radioactive iodine (RAI) accumulation.
Remnant ablation with 131I was performed after rhTSH stimulation in low to intermediate risk patients within 2 months after surgery. Hormone withdrawal was preferred in high risk patients. Dosage was decided multidisciplinary and ranged from 30 mCi to 150 mCi.
A diagnostic 123I WBS was performed 1 year after the total thyroidectomy: rhTSH (thyrotropin alfa, Sanofi Genzyme) was injected (intramuscular) on day one and two. On day five, 2 h after injection of 123I, planar scintigraphy was performed. Optional SPECT/CT was performed on day four.
The study population was first assessed for detectable vs. undetectable bTg, 6 months after total thyroidectomy. The 2 groups were correlated to the sTg at 12 months, the US of the neck after 6 to 12 months and the diagnostic 123I WBS after 1 year. Both groups were subsequently classified into 3 groups: a low-, intermediate-, or high-risk group depending on the tumour histology, locoregional or distant metastases, lymphovascular invasion and avidity on post therapy scan as proposed by the ATA 2015 guidelines.
Descriptive statistics were applied to all collected variables expressed as frequencies for categorical data or mean values ± standard deviations for continuous data. Group comparisons were carried out using the chi-square test. A p value less than 0.05 was considered significant. Statistical analysis was performed using the IBM SPSS® Statistics 24 software package.