Quantification performance of silicon photomultiplier-based PET for small ¹⁸F-, ⁶⁸Ga- and ¹²⁴I-avid lesions in the context of radionuclide therapy planning

Purpose: The aim of this study was to investigate conditions for reliable quantification of sub-centimeter lesions with low F-18, Ga-68, and I-124 uptake using a silicon photomultiplier-based PET/CT system. Methods: A small tumor phantom was investigated under challenging but clinically realistic conditions resembling prostate and thyroid cancer lymph node metastases (6 spheres with 3.7-9.7 mm in diameter, 9 different activity concentrations ranging from about 0.25-25 kBq/mL, and a signal-to-background ratio of 20). Radionuclides with different positron branching ratios and prompt gamma coincidence contributions were investigated. Maximum-, contour-, and oversize-based partial volume effect (PVE) correction approaches were applied. Detection and quantification performance were estimated, considering a +/- 30 % deviation between imaged-derived and true activity concentrations as acceptable. A standard and a prolonged acquisition time and two image reconstruction algorithms (time-of-flight with/without point spread function modelling) were analyzed. Clinical data were evaluated to assess agreement of PVE-correction approaches indicating lesion quantification validity. Results: The smallest 3.7-mm sphere was not visible. If the lesions were clearly observed, quantification was, except for a few cases, acceptable using contour- or oversized-based PVE-corrections. Quantification accuracy did not substantially differ between F-18, Ga-68, and I-124. No systematic differences between the analyzed reconstruction algorithms or shorter and larger acquisition times were observed. In the clinical evaluation of 20 lesions, an excellent statistical agreement between oversize- and contour-based PVE-corrections was observed. Conclusions: At the lower end of size (<10 mm) and activity concentration ranges of lymph-node metastases, quantification with reasonable accuracy is possible for F-18, Ga-68, and I-124, possibly allowing pre-therapeutic lesion dosimetry and individualized radionuclide therapy planning.