Purpose
To develop a detailed model of the internal vasculature of the adult liver for blood dosimetry in radiation therapy and demonstrate its application to allowing a more explicit differentiation of radiopharmaceutical decay sites within liver parenchyma separate from those within the organ’s blood content.
Method
Computer-generated models of hepatic arterial (HA), hepatic venous (HV), and hepatic portal venous (HPV) vascular trees were created within individual lobe segments of the ICRP adult female and male livers (AFL/AML) via an in-house algorithm based on the ConstrainedConstructive-Optimization (CCO) method. Hemodynamic and geometric parameters of the main vessels were used as inputs. For each iteration of the algorithm, pressure, blood flow, and vessel radii within each tree were updated as each new vessel was created and connected to the viable bifurcation site. The vascular networks created inside the AFL/AML were then tetrahedralized to perform radiation transport using Monte Carlo. Specific Absorbed Fractions (SAF) were computed for monoenergetic alpha particles, electrons, and photons. Dual-region liver models of the AFL/AML were proposed and particle-specific SAF values were computed assuming blood decays as modeled in two regions: (1) sites within explicitly modeled hepatic vessels, and (2) sites within the hepatic blood pool residing outside these vessels to include the liver capillaries and blood sinuses. S-values for 22 radionuclides commonly used in radiopharmaceutical therapy were computed using the proposed dual-region liver models which were then compared to S-values obtained in current dosimetric practice: a single-region liver model of homogenized liver parenchyma (LP) and liver blood (LB).
Results
Liver models with virtual vasculatures of ~6000 non-intersecting straight cylinders representing the HA, HPV, and HV circulations were created independently for the ICRP reference AFL and AML. SAF energy profiles were obtained using the single-region and dual-region models. For alpha emitting radionuclides, S-values using the single-region models were approximately 14% and 11% higher than the S-values obtained using the dual-region AFL and AML models, respectively. For beta and auger-electron emitters, S-values based on the single-region model were up to 13% and 11% higher than in the dual-region model for the AFL and AML, respectively.
Conclusions
The methodology employed for the liver can be applied to all major organs of the computational phantom for both improved dosimetry of organ parenchyma.