A Prospective Study of Quantitative SPECT/CT for Evaluation of Lung Shunt Fraction Before SIRT of Liver Tumors

Abstract

The lung shunt fraction (LSF) is estimated using Tc-macroaggregated albumin (Tc-MAA) imaging before selective internal radiotherapy (SIRT) of the liver to reduce the risk of pulmonary irradiation. Generally, planar scans are acquired after injection of Tc-MAA into the hepatic artery. However, the validity of this approach is limited by differences in attenuation between liver and lung tissue as well as inaccurate segmentation of the organs. The aim of this study was to evaluate quantitative SPECT/CT for LSF assessment in a prospective clinical cohort. Fifty consecutive patients intended to undergo SIRT were imaged within 1 h after injection of Tc-MAA using a SPECT/CT γ-camera. Planar scans of the lung and liver region were acquired in anterior and posterior views, followed by SPECT/CT scans of the thorax and abdomen. Emission data were corrected for scatter, attenuation, and resolution recovery using dedicated software. To quantify the radioactivity concentration in the lung, liver, urinary bladder and remainder of the thoracoabdominal body, volumes of interest were defined on the SPECT/CT images.Tc-MAA concentrations were calculated as percentage injected dose (%ID). MeanTc-MAA uptake in liver and lung accounted for only 79 %ID, whereas 13.1 %ID was present in the remainder of the body. In all patients, LSF as calculated from planar scans accounted for a median of 6.8% (range, 3.4%-32.3%), whereas the SPECT/CT quantitation revealed significantly lower LSF estimates, at a median of 1.9% (range, 0.8%-15.7%) ( < 0.0001, Wilcoxon test). On the basis of planar imaging, dose reduction or even contraindications to SIRT had to be considered in 10 of 50 patients, as their LSF was calculated at 10% or more. In contrast, SPECT/CT quantitation showed substantial shunting in only 2 of the 50 patients. Quantitative SPECT/CT reveals that the LSF is considerably lower than shown on planar imaging. Thus, the resulting dose to the lung parenchyma may be less than conventionally assumed. However, the safety of the SPECT/CT-derived dose range will have to be evaluated.

Calculation of lung mean dose and quantification of error for 90Y‐microsphere radioembolization using 99mTc‐MAA SPECT/CT and diagnostic chest CT

“…The derived uncertainties in LSF and LMD for our methodology were 10% and 13%, respectively. We found that both LSF new and LMD new were significantly lower than the values estimated using the microsphere package‐insert instructions, which is consistent with previous publications . LSF new and LMD new were always lower than the planar scintigraphy‐based values, with an average difference of −0.05 Gy (−63%) and −5 Gy (−53%), respectively.…”

“…In our methodology, total lung mass was estimated from diagnostic chest CT contours drawn using a well‐established lung region‐growing algorithm . Previous studies of 90 Y‐microsphere lung dosimetry have used upper HU thresholds ranging from −150 to −600 HU to derive the lung contours . Our empirically calculated upper HU thresholds of −280 HU for CT maa and −340 HU for CT diag fall within the range of these previous reports.…”

“…Previously published SPECT/CT‐based lung dosimetry studies estimated lung mass by using manual or automatic contouring methods on CT images to define the lung volume, which was multiplied by either a fixed 0.3 g/mL or by a patient‐specific estimate of lung density . In this work, we followed the method proposed by Busse et al for the determination of lung mass.…”

“…Previous studies in SPECT/CT‐based lung dosimetry have proposed estimating the liver 99m Tc‐MAA distribution using either the SPECT data to guide the liver contour using a percent threshold algorithm or the CT data to manually or semi‐automatically contour the liver . In this work, all whole livers (not just treated segments) were manually contoured on the CT maa (VOI liver ) and propagated to the rigidly registered SPECT maa using MIM software.…”

“…Alternatively, use of 99m Tc‐MAA single‐photon emission computed tomography (SPECT)/computed tomography (CT) has been proposed to improve the estimation of lung and liver 90 Y distribution. This modality can include scatter and attenuation corrections and facilitate more accurate three‐dimensional segmentation of the lungs and liver . Additionally, CT images can be used to personalize the lung mass estimation via densitovolumetry instead of assuming a 1‐kg mass in all patients.…”