Lung dosimetry for radioiodine treatment planning in the case of diffuse lung metastases

Abstract

The lungs are the most frequent sites of distant metastasis in differentiated thyroid carcinoma. Radioiodine treatment planning for these patients is usually performed following the Benua-Leeper method, which constrains the administered activity to 2.96 GBq (80 mCi) whole-body retention at 48 h after administration to prevent lung toxicity in the presence of iodine-avid lung metastases. This limit was derived from clinical experience, and a dosimetric analysis of lung and tumor absorbed dose would be useful to understand the implications of this limit on toxicity and tumor control. Because of highly nonuniform lung density and composition as well as the nonuniform activity distribution when the lungs contain tumor nodules, Monte Carlo dosimetry is required to estimate tumor and normal lung absorbed dose. Reassessment of this toxicity limit is also appropriate in light of the contemporary use of recombinant thyrotropin (thyroid-stimulating hormone) (rTSH) to prepare patients for radioiodine therapy. In this work we demonstrated the use of MCNP, a Monte Carlo electron and photon transport code, in a 3-dimensional (3D) imaging-based absorbed dose calculation for tumor and normal lungs.

Methods: A pediatric thyroid cancer patient with diffuse lung metastases was administered 37 MBq of (131)I after preparation with rTSH. SPECT/CT scans were performed over the chest at 27, 74, and 147 h after tracer administration. The time-activity curve for (131)I in the lungs was derived from the whole-body planar imaging and compared with that obtained from the quantitative SPECT methods. Reconstructed and coregistered SPECT/CT images were converted into 3D density and activity probability maps suitable for MCNP4b input. Absorbed dose maps were calculated using electron and photon transport in MCNP4b. Administered activity was estimated on the basis of the maximum tolerated dose (MTD) of 27.25 Gy to the normal lungs. Computational efficiency of the MCNP4b code was studied with a simple segmentation approach. In addition, the Benua-Leeper method was used to estimate the recommended administered activity. The standard dosing plan was modified to account for the weight of this pediatric patient, where the 2.96-GBq (80 mCi) whole-body retention was scaled to 2.44 GBq (66 mCi) to give the same dose rate of 43.6 rad/h in the lungs at 48 h.

Results: Using the MCNP4b code, both the spatial dose distribution and a dose-volume histogram were obtained for the lungs. An administered activity of 1.72 GBq (46.4 mCi) delivered the putative MTD of 27.25 Gy to the lungs with a tumor absorbed dose of 63.7 Gy. Directly applying the Benua-Leeper method, an administered activity of 3.89 GBq (105.0 mCi) was obtained, resulting in tumor and lung absorbed doses of 144.2 and 61.6 Gy, respectively, when the MCNP-based dosimetry was applied. The voxel-by-voxel calculation time of 4,642.3 h for photon transport was reduced to 16.8 h when the activity maps were segmented into 20 regions.

Conclusion: MCNP4b-based, patient-specific 3D dosimetry is feasible and important in the dosimetry of thyroid cancer patients with avid lung metastases that exhibit prolonged retention in the lungs.

FIGURE 1

Whole-body planar projection (anterior view) of radioactive iodine distribution 3 h (A), 26 h (B), and 146 h (C) after diagnostic administration of 37 MBq. A standard (~18.5 MBq) was placed by the patient’s right foot during scan. Stomach and bladder can be seen on 3-h scan. Activity is localized to both lungs and retained there 146 h after injection. Gray level intensity in part of left lung is weaker due to overlap of heart.

FIGURE 2

Selected transverse (A) and coronal (B) CT images (obtained from CT portion of SPECT/CT study and, therefore, of low resolution) and corresponding maximum-intensity-projection (MIP) images of transverse (C) and coronal (D) SPECT slices are shown. Loci with strong activity uptake are indicated by arrows. MIP images were generated using the MIAU software package.

FIGURE 3

(A) MIP slices of processed lung activity probability map obtained from corresponding SPECT study. Color bar indicates intensity for normalized activity probability. (B) Representative slice of lung density map obtained from 27-h SPECT/CT scan. Density map and activity probability maps are direct inputs for MCNP4b dose calculation. (C) ROIs of tumors in a representative transverse slice, generated by 3D-ID, are shown.

FIGURE 4

MIP slices of absorbed doses in lungs after MCNP4b electron and photon transports. Sagittal (A), coronal (B), and transverse (C) views of dose rates (mGy/MBq-s) calculated for activity and density maps at 27 h after tracer administration. Color bar indicates intensity for absorbed dose per disintegration.

FIGURE 5

(A) Absorbed dose–volume histogram corresponding to dose-rate images shown in Figure 4. Average absorbed dose rate per unit activity per voxel is 3.01 × 10⁻⁵ mGy/MBq-s (dashed line). (B) Relative errors after 10 million histories of electron transport are plotted against voxel energy deposited.

FIGURE 6

MIP coronal slice images of segmented activity maps. Activity was evenly divided into 10 (A) and 5 (B) levels. (C) Dose–volume histograms calculated from segmented activity maps: 20 (solid line), 10 (dashed line), and 5 (dotted line) levels; horizontal lines depict mean absorbed doses per disintegration.