Impact of SPECT corrections on 3D‐dosimetry for liver transarterial radioembolization using the patient relative calibration methodology

Abstract

Purpose: Many centers aim to plan liver transarterial radioembolization (TARE) with dosimetry, even without CT-based attenuation correction (AC), or with unoptimized scatter correction (SC) methods. This work investigates the impact of presence vs absence of such corrections, and limited spatial resolution, on 3D dosimetry for TARE. Methods: Three voxelized phantoms were derived from CT images of real patients with different body sizes. Simulations of 99m Tc-SPECT projections were performed with the SIMIND code, assuming three activity distributions in the liver: uniform, inside a "liver's segment," or distributing multiple uptaking nodules ("nonuniform liver"), with a tumoral liver/healthy parenchyma ratio of 5:1. Projection data were reconstructed by a commercial workstation, with OSEM protocol not specifically optimized for dosimetry (spatial resolution of 12.6 mm), with/without SC (optimized, or with parameters predefined by the manufacturer; dual energy window), and with/without AC. Activity in voxels was calculated by a relative calibration, assuming identical microspheres and 99m Tc-SPECT counts spatial distribution. 3D dose distributions were calculated by convolution with lesions and healthy parenchyma from different reconstructions were compared with those obtained from the reference biodistribution (the "gold standard," GS), assessing differences for D95%, D70%, and D50% (i.e., minimum value of the absorbed dose to a percentage of the irradiated volume). γ tool analysis with tolerance of 3%/13 mm was used to evaluate the agreement between GS and simulated cases. The influence of deep-breathing was studied, blurring the reference biodistributions with a 3D anisotropic gaussian kernel, and performing the simulations once again. Results: Differences of the dosimetric indicators were noticeable in some cases, always negative for lesions and distributed around zero for parenchyma. Application of AC and SC reduced systematically the differences for lesions by 5%-14% for a liver segment, and by 7%-12% for a nonuniform liver. For parenchyma, the data trend was less clear, but the overall range of variability passed from −10%/40% for a liver segment, and −10%/20% for a nonuniform liver, to −13%/6% in both cases. Applying AC, SC with preset parameters gave similar results to optimized SC, as confirmed by γ tool analysis. Moreover, γ analysis confirmed that solely AC and SC are not sufficient to obtain accurate 3D dose distribution. With breathing, the accuracy worsened severely for all dosimetric indicators, above all for lesions: with AC and optimized SC, −38%/−13% in liver's segment, −61%/−40% in the nonuniform liver. For parenchyma, D50% resulted always less sensitive to breathing and sub-optimal correction methods (difference overall range: −7%/13%). Conclusions: Reconstruction protocol optimization, AC, SC, PVE and respiratory motion corrections should be implemented to obtain the best possible dosimetric accuracy. On the other side, thanks to the relative calibration, D50% inaccuracy for the healthy paren...

Improved quantitative 90Y bremsstrahlung SPECT/CT reconstruction with Monte Carlo scatter modeling

“…Although these values are typical for RE, there is a large range in lesion size and uptake ratios and these are well worth investigating in a future study. In addition, the quantification approach of the present study was limited to the relative calibration approach based on the assumption that the activity in the liver is known . This has the advantage that a separate acquisition of a calibration phantom or source with known activity is not required to determine a CF that relates detected counts to activity.…”

“…To convert counts to activity the SPECT voxel counts were multiplied by this CF. The liver relative calibration approach used here and in past dosimetry studies is considered to be suitable for RE because the infused microspheres become permanently trapped in the micro capillaries of the treated liver and do not redistribute.…”

Quantitative Bildgebung für die Dosimetrie mit SPECT/CT

“…Auch im Bereich der Therapie-assoziierten Bildgebung gibt es Berichte über die Auswirkungen von Bewegung. Pacilio et al [47] zeigten in einer Simulationsstudie den Einfluss der Atembewegung bei der prätherapeutischen SIRT-Evaluation auf und fanden insbesondere in fokalen Läsionen eine Unterschätzung der Aktivitätsmenge und absorbierten Dosis aufgrund der Bewegung. Sanders et al [48] finden eine mittlere Atemverschieblichkeit der Nieren von 5 -7 mm in axialer Richtung, daraus resultiert alleinig aufgrund der Bewegung eine Unterschätzung der Gesamtmenge des Radiopharmakons von 10 -15 %, falls dies nicht durch entsprechende Anpassung der VOI kompensiert wird.…”

Multi-modal image analysis for semi-automatic segmentation of the total liver and liver arterial perfusion territories for radioembolization

“…For this reason, using a more detailed dosimetry is vital for treatment optimization [8,26]. The role of MAAbased dose estimation for a safe normal parenchyma absorbed dose and efficient tumor absorbed dose has been investigated in several studies [27][28][29]. Post-treatment dosimetry has also been studied to verify the treatment planning [30][31][32][33].…”