Digital PET/CT improves the minimal detectable I-124 activity and dosimetry for small iodine-avid thyroid cancer metastases

Ziel/Aim I-124 positron emission tomography (PET) imaging allows accurate detection and dosimetry for differentiated thyroid cancer lesions. However, detection can be challenging at low thyroglobulin levels. The missing detectability may be associated with a small lesion size or/and a low radioiodine uptake. The aim of this study is to quantify the improvement of the minimal detectable I-124 activity (MDA) using a digital PET/CT system and to assess the possible impact on radioiodine treatment.

Methodik/Methods Phantom measurements were performed under challenging conditions (derived from almost 200 thyroid cancer metastases) using the following setup: an abdominal phantom containing small spheres (diameters from 3.7 to 9.7 mm), activity concentrations (ACs) from 0.25 to 25 kBq/ml and a low signal-to-background ratio of 20:1. PET data were acquired on a digital Siemens Biograph Vision and a conventional PET/CT system using a standard clinical acquisition protocol (4-min emission time per bed, TOF+PSF). The MDA was estimated using the contrast-to-noise ratio (CNR) at varying ACs. Following the Rose criterion, a lesion was regarded sufficiently visible at a CNR of 8. For each lesion size, the MDA was determined.

Ergebnisse/Results The MDA was increased by a factor of 2 for the digital compared to the conventional PET/CT system. The smallest sphere was not visible on both systems, whereas the 4.8-mm sphere was only detectable on the digital system at an AC of 25 kBq/ml. For this 4.8-mm lesion, dosimetry was calculated based on digital PET/CT data assuming a typical effective I-124 half-life of 60 h for lymph node metastases: the absorbed dose threshold of about 100 Gy could be achieved using a therapeutic activity of about 6 GBq I-131.

Schlussfolgerungen/Conclusions A substantial improvement in I-124 MDA was observed on a digital PET/CT that allows more accurate detection and dosimetry of small differentiated thyroid cancer metastases. Findings will be further evaluated in a clinical study.