Systematic and random errors of PET-based ⁹⁰ Y 3D dose quantification

The objective was to characterize both systematic and random errors in Positron Emission Tomography (PET)-based ⁹⁰ Y three-dimensional (3D) dose quantification.

A modified NEMA-IEC phantom was used to emulate ⁹⁰ Y-microsphere PET imaging conditions: sphere activity concentrations of 1.6 and 4.8 MBq/cc, sphere-to-background ratios of 4 and 13, and sphere diameters of 13, 17, and 37 mm. PET data were acquired using a GE D690 PET/CT scanner for 300 min on days 0-11. The data were downsampled to 60-5 min for multiple realizations to evaluate the count starvation effect. The image reconstruction algorithm was 3D-OSEM with PSF + TOF modeling; the parameters were optimized for dose-volume histogram (DVH), as a ⁹⁰ Y 3D dose quantification. ⁹⁰ Y-PET images were converted to dose maps using the local deposition method, then the sphere DVHs were calculated. The ground truth for the DVH was calculated using convolution method. Dose linearity was evaluated in decaying ⁹⁰ Y activity (reduced count rate and total count) and decreasing acquisition durations (reduced total count only). Finally, the impacts of the low 32-ppm positron yield on PET-based 3D ⁹⁰ Y-dose quantification were evaluated; the bias and variability of resulting DVHs were characterized.

We observed nonlinear errors that depended on the ⁹⁰ Y activity (count rate) and not on the total true prompt counts. These nonlinear errors in mean dose underestimated the measured mean dose by> 20% for a measured dose range of 40-230 Gy; although the shapes of the DVH were not altered. Compensation based on empirical models reduced the nonlinearity errors to be within 5% for measured dose range of 40-230 Gy. In contrast, the errors due to nonuniformity introduced by image noise dominated the systematic errors in the DVH and stretched the DVH on both tails. For the 37-mm sphere, the magnitude of errors in D₈₀ increased from -25% to -36% when acquisition duration was decreased from 300 to 10 min. The effect of image noise on DVH was more extensive in smaller spheres; for the 17-mm sphere, the magnitude of errors in D₈₀ increased from -29% to -45% acquisition duration was decreased from 300 to 10 min. For the 37-mm sphere, the errors in D₂₀ increased from +3.5% to only +10.5% when the acquisition duration was decreased from 300 to 10 min; in the 17-mm sphere, the errors in D₂₀ were 6.5% for both 300- and 10-min sphere images.

Count-starved ⁹⁰ Y-PET data introduce both systematic and random errors. The systematic error increases the apparent nonuniformity of the DVH, while the random error increases the uncertainty in the DVH. The systematic errors were larger than the random errors. Lower count rate of ⁹⁰ Y-PET also introduces systematic bias, which is scanner specific. The errors of bias-compensated mean tumor dose were <10% when ⁹⁰ Y-PET scan time was >15 min/bed for tumors >37 mm. D_median and D_mean were the most stable dose metrics. An acquisition duration of 30 min is recommended to keep the random errors < 10% for a typical tumor with sphere equivalent diameter >17 mm and average tumor dose >40 Gy.