Contribution of I-131 SPECT/CT uptake on the dynamic risk assessment of papillary thyroid cancer

Nowadays, dynamic risk stratification (DRS) is the chosen strategy for monitoring papillary thyroid cancer (PTC). However, other variables contribute information to optimise monitoring. The objective of this study was to analyse the quantitative parameters of the single-photon emission computed tomography/computed tomography (SPECT/CT) imaging after treatment with post-surgical I-131 uptake, and to analyse those that predict a non-excellent response (NER) at 1 year of monitoring as well as their predictive capacity.

A retrospective observational study was designed that included patients diagnosed with PTC and treatment with I-131. Monitoring was based on the DRS at 1 year comparing the patients that have an excellent response (ER) with those who have a response less than excellent (NER). For the study of the biokinetics of the I-131 in the post-surgical thyroid remnants, SPECT/CT images were acquired, as well as the following variables: the absorbed activity per the administered activity, percentage of remnant uptake at 2 and 7 days post-administration; and the accumulated activity over time.

Fifty-seven patients were studied and 103 remnants. Of them, 19.30% obtained NER. Among males, there was a higher percentage of NER, 45.45% while among females, 82.61% obtained ER (P=0.102). As for the surgical technique, the patients with ER, 78.26%, underwent total thyroidectomy (TT) with bilateral central lymph node dissection. Most of the patients who had TT, bilateral central and unilateral lateral lymph node dissection had NER (45.45% vs. 8.70%) (P=0.027). The I-131 absorbed activity per administered activity was greater for ER [13.8 (28.85) vs. 4.63 (5.66)], the accumulated activity 0.12 vs. 0.04 (P=0.017) and the percentage of administered activity at 2 days (P=0.003). The variables associated independently to a NER were the male gender (P=0.009), the percentage of administered activity at 7 days (P=0.043) and the absorbed activity per administered activity (P=0.014). The predictive capacity of the model was area under the curve (AUC) [95% confidence interval (CI)] of 0.816 (0.720-0.912).

In PTC follow-up, the male gender, the absorbed dose, and the percentage of absorbed dose at 7 d should be considered along with the DRS, as tools to support future decision-making.