Three-Dimensional Radiobiologic Dosimetry: Application of Radiobiologic Modeling to Patient-Specific 3-Dimensional Imaging–Based Internal Dosimetry

Density distribution (uniform) (A) for uniform activity distribution model (B) and nonuniform activity distribution model (C). In nonuniform distribution, same total activity as shown in Figure 3B is concentrated into half the volume (outer shell). Assuming a uniform density sphere (A), 2 activity distributions are depicted: uniform (B) and nonuniform (C). In C, the same total activity as in B is concentrated into the outer shell of the sphere.

(A) Spheric nonuniform density model in which inner sphere is twice unit density (2.0 g/cm³) and outer shell is at unit density (1.0 g/cm³). (B) Uniform activity distribution for density model in Figure 2A. (C) Cross-sectional slice of 3D-RD output for spheric nonuniform density model.

(A) Clinical CT portion of a SPECT/CT scan of patient showing nonuniform density distribution in lungs. (B) Clinical SPECT scan of patient showing nonuniform activity distribution. (C) Rate map generated from 3 longitudinally aligned SPECT images; regions with effective half-life greater than physical half-life of ¹³¹I reflect tumor uptake. (D) Cumulative activity generated from rate map and SPECT.

Comparison between MCNP-based dose volume histogram of Song et al. (32) over lung and tumor regions and results from EGS using same inputs. Mean value of MCNP method is 3.01 × 10⁻⁵ mGy/MBq-s per pixel, whereas EGS mean is 2.88 × 10⁻⁵ mGy/MBq-s per pixel.

BED map resulting from 3D-RD using full patient-specific data. Although values of absorbed dose and BED are different, their relative changes from voxel to voxel are so similar that it is nearly impossible to visually differentiate the two.