Impact of SPECT corrections on 3D‐dosimetry for liver transarterial radioembolization using the patient relative calibration methodology
2016
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 99mTc-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 99mTc-SPECT counts spatial distribution.
3D
dose
distributions were calculated by convolution with 90Y voxel
S-values, assuming permanent trapping of microspheres.
Cumulative dose-volume histograms in 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 parenchyma from absence of AC was
less than expected, while the optimization of SC was scarcely influent. The
relative calibration therefore allows to perform TARE planning, basing
on D50% for the healthy parenchyma, even without AC or with
suboptimal corrections, rather than rely on nondosimetric methods.
Keywords:
- Correction
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