Three-dimensional radiobiological dosimetry (3D-RD) with 124I PET for 131I therapy of thyroid cancer

Abstract

Radioiodine therapy of thyroid cancer was the first and remains among the most successful radiopharmaceutical (RPT) treatments of cancer although its clinical use is based on imprecise dosimetry. The positron emitting radioiodine, (124)I, in combination with positron emission tomography (PET)/CT has made it possible to measure the spatial distribution of radioiodine in tumors and normal organs at high resolution and sensitivity. The CT component of PET/CT has made it simpler to match the activity distribution to the corresponding anatomy. These developments have facilitated patient-specific dosimetry (PSD), utilizing software packages such as three-dimensional radiobiological dosimetry (3D-RD), which can account for individual patient differences in pharmacokinetics and anatomy. We highlight specific examples of such calculations and discuss the potential impact of (124)I PET/CT on thyroid cancer therapy.

Fig. 1

Representative coronal slices of absorbed dose (D) maps of two different data sets: torso (measured) (a) and head (modeled) (b). In modeled calculation, average tumor activity concentration was placed in two tumor-associated volumes of interest defined using CT; voxels representing normal brain were assigned average (background) brain activity concentration. 3D-RD was then executed using these two as source regions of uniform activity irradiating normal brain. In this way, possible calculation artifacts associated with high tumor count–density gradients were avoided. Both images are viewed anteriorly

Fig. 2

Dose-volume histograms (and BED histograms) for the lung volume of interest voxels. The blue histograms represent normal lung tissue, while the red lines are tumor tissue. The thick vertical lines show the voxel averages

Fig. 3

a Dose-volume histograms (DVHs) for normal organs are shown with the voxel averaged absorbed dose (blue dotted line) and the mean absorbed dose obtained from a whole-organ contour (red dotted line) superimposed on the DVHs. b The DVH for two tumors receiving a mean absorbed dose of 110 Gy with an EUD of 16.8 Gy and a mean absorbed dose of 61.5 Gy with an EUD of 28.4 Gy. c Transverse and coronal cross sections depicting absorbed dose images of the two tumors in b

Fig. 3

a Dose-volume histograms (DVHs) for normal organs are shown with the voxel averaged absorbed dose (blue dotted line) and the mean absorbed dose obtained from a whole-organ contour (red dotted line) superimposed on the DVHs. b The DVH for two tumors receiving a mean absorbed dose of 110 Gy with an EUD of 16.8 Gy and a mean absorbed dose of 61.5 Gy with an EUD of 28.4 Gy. c Transverse and coronal cross sections depicting absorbed dose images of the two tumors in b