Optimal timing of post-therapeutic [131I]NaI whole-body scintigraphy in predominantly low- and intermediate-risk differentiated thyroid cancer: a secondary analysis of prospectively acquired data and a structured literature review.

BACKGROUND: Post-therapeutic [131I]NaI whole-body scintigraphy (ptWBS) is mandatory after radioiodine therapy (RAIT), but the most suitable acquisition time remains uncertain. We compared image interpretability at early and delayed time points in patients with differentiated thyroid cancer (DTC). METHODS: This retrospective secondary analysis included prospectively acquired ptWBS data from 35 patients. Scans obtained at 6, 24, 48, 72, and 168 h after RAIT were independently ranked by three nuclear medicine physicians and evaluated with a 10-item criteria-based image-quality score. Rankings were analyzed using a Plackett-Luce model, and criteria-based scores were analyzed using generalized estimating equations. RESULTS: The analysis comprised 167 scans, 501 criteria-based ratings, and 500 intuitive rank observations. The 24-h acquisition had the highest estimated preference, followed by 48, 6, 72, and 168 h. After Holm adjustment, 24 h was favored over 6, 72, and 168 h, whereas the difference between 24 and 48 h was not significant (P = 0.091). The criteria-based score was highest at 24 h (39.52; 95% CI, 38.62-40.43) and next-highest at 48 h (34.79); all pairwise differences in scores were significant. Each 5-point increase in the criteria-based score was associated with higher odds of a more favorable intuitive ranking (OR, 2.01; 95% CI, 1.70-2.38). CONCLUSIONS: PtWBS acquired at 24 h provided the best overall interpretability of the images. A 48 h scan may be considered when practical or radiation-protection considerations favor later imaging, but equivalence to 24 h has not been established. Additional delayed imaging may still be useful when distant metastatic disease is suspected.