The study aimed to explore the relationship of different exposure measures with I therapy response in patients with benign thyroid disease, estimate the variability in the response, investigate possible covariates, and discuss dosing implications of the results.
A population exposure-response analysis was performed using nonlinear mixed-effects modelling. Data from 95 adult patients with benign thyroid disease were analysed. Evaluated exposure parameters were: administered radioactivity dose (Aa) [MBq], total absorbed dose (ABD) [Gy], maximum of absorbed dose-rate (MXR) [Gy/h] and biologically effective dose (BED) [Gy]. The response was modelled as ordered categorical data: hyper-, eu- and hypothyroidism. The final model performance was evaluated by a visual predictive check.
The probability of the outcome following I therapy was best described by a proportional-odds model, including the log-linear model of I effect and the exponential model of the response-time relationship. All exposure measures were statistically significant with p<0.001, with BED and ABD being statistically better than the other two. Nevertheless, as BED resulted in the lowest AIC value, it was included in the final model. Accordingly, BED value of 289.7 Gy is associated with 80% probability of successful treatment outcome 12 months after I application in patients with median thyroid volume (32.28 mL). The target thyroid volume was a statistically significant covariate. The visual predictive check of the final model showed good model performance.
Our results imply that BED formalism could aid in therapy individualisation. The larger thyroid volume is associated with a lower probability of a successful outcome.

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