Abstract
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Aim: PET/CT allows for accurate, post-therapeutic determination of the Y-90 distribution, which can be converted to maps of absorbed dose [1]. Yet, several parameters affect the accuracy of activity- and dose-maps. The aim of this work was to determine the effects of varying PET reconstruction parameters onto absorbed dose maps.
Material and Methods: After SIRT, a PET/CT scan (Siemens Biograph mCT 40, one bed-position, 30 minutes duration) was carried out in 10 patients in order to determine Y-90-resin microsphere distribution. From reconstructed PET data, dose-maps where derived by convolution with pre-calculated dose-voxel-kernels. An MR image was co-registered to PET/CT data and manually segmented for healthy liver (HL) and tumor (TU) regions, for which dose-volume-histograms (DVH) were calculated. DVHs were compared for six different PET reconstruction settings. All reconstructions had in common that corrections for decay, photon attenuation and scatter, and random coincidences were applied. The varied parameters were number of iterations, amount of post-reconstruction Gaussian smoothing, and whether point-spread-function (Siemens TrueX) was modelled or time-of-flight (ToF) information was applied. Evaluated reconstruction settings were: A) No PSF, No ToF, 2 iterations (i), 12 subsets (s), 6mm Gaussian B) PSF, No ToF, 2i, 12s, 6mm C) PSF, ToF, 1i, 21s, 0mm D) PSF, ToF, 1i, 21s, 3mm E) PSF, ToF, 1i, 21s, 6mm F) PSF, ToF, 2i, 21s, 6mm
Results: Reconstruction F) led to smallest deviations between injected and PET-derived activity (7.2±5.5%, ranging 0.5 - 18.8). For F), mean absorbed dose to HL was 26.8±9.2 Gy (15.3-40.1) and 81.8±48.2 Gy (29.2-157.5) for TU. Reconstructions A) and B) led to systematically lower (p<0.05) mean doses than C-F), for HL, as well as for TU. Mean dose differences between different reconstructions were on average 3.9±4.2 % (0.0-15.4) for HL and 6.1±5.5 % (0.0 - 22.4) for TU. Corresponding points on DVHs for different reconstructions showed higher deviations than in average case, especially in higher dose regions.
Conclusions: On general, the accuracy of PET-derived activity could be regarded as sufficient, although some outliers were found. The ToF-technique led to most accurate results. Differences in mean absorbed dose appeared to be minor. However, a detailed comparison of DVHs indicated that choice of reconstruction parameters has a strong influence on exact DVHs shape. References: D'Arienzo, M., et al. (2017). "Phantom validation of quantitative Y-90 PET/CT-based dosimetry in liver radioembolization." EJNMMI Res 7(1): 94.
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