A revised compartmental model for biokinetics and dosimetry of 2-[18F]FDG

Abstract

Background: The aim was to review available biokinetic data, collect own experimental data, and propose an updated compartmental model for 2-[¹⁸F]FDG in the frame of the revision of the ICRP report on dose coefficients for radiopharmaceuticals used in diagnostic nuclear medicine.

Methods: The compartmental model was developed based on published biokinetic data for 2-[¹⁸F]FDG. Additional data on urinary excretion in 23 patients (11 males, 12 females) undergoing whole-body PET/CT examinations were obtained within this study. The unknown biokinetic model parameters were derived using the software SAAM II and verified with a modified version of IDAC-Iodide. Dose coefficients for reference adults were calculated with the programme IDAC-Dose 2.1. A dynamic bladder model was employed for urinary bladder dosimetry.

Results: The proposed model consists of following compartments: blood, heart wall, brain, liver, lungs, pancreas, spleen, kidneys, urinary bladder content and a generic pool compartment "Other". The latter was introduced to account for 2-[¹⁸F]FDG in body organ and tissues besides the explicitly modelled ones. The model predictions showed a good agreement with experimental data. Urinary bladder wall received the highest absorbed dose coefficient of 7.5E-02 mGy/MBq under the assumption of initial urine volume of 100 ml, first voiding at 45 min p.i. and 3.75 h voiding intervals thereafter. The effective dose coefficient calculated according to the current dosimetry framework of ICRP amounted to 1.7E-02 mSv/MBq, compared to 1.9E-02 mSv/MBq in ICRP Publication 128.

Conclusion: A compartmental model for 2-[¹⁸F]FDG was proposed and will be used to replace the descriptive biokinetic model of ICRP Publication 128. The revised model and the provided dose coefficients are expected to improve reference dosimetry for patients administered with 2-[¹⁸F]FDG.

Keywords: Absorbed organ dose; Biodistribution; Effective dose; Fluorodeoxyglucose; Nuclear medicine.

Fig. 1

Structure of the proposed biokinetic model for 2-[¹⁸F]FDG

Fig. 2

Decay-corrected TACs in source regions predicted by the proposed biokinetic model along with the experimental data (scattered points; decay-corrected to the time of injection). For comparison, TACs reported in MIRD dose estimate report 19 [3] are given in dashed lines. For liver, lungs, pancreas and spleen, the TACs predicted by the proposed model for organ parenchyma only (i.e. without organ blood content) are shown in dotted lines. Activity of 2-[¹⁸F]FDG is given in % of administered activity

Fig. 3

TACs in source region urinary bladder contents. The blue dashed line gives the TAC under the assumption of no voiding, i.e. it corresponds to the cumulative excretion of 2-[¹⁸F]FDG in urine. The red and grey solid lines display the TACs as predicted by the dynamic bladder model with the first voiding at 45 min p.i. for male and female adults, respectively. The purple dashed-dotted line gives the prediction of the MIRD model [3] (cumulative excretion). The red star corresponds to the average value of the bladder activities assessed in this study (SUS: Skåne University Hospital); the green triangles and the yellow square—to the cumulative excretion reported by Mejia et al. [6] and Bach-Gansmo et al. [20], respectively. All values are expressed as % of administered activity, and not corrected for physical decay