Purpose: To establish a primary standard of absorbed-dose-to-water for targeted alpha therapy radionuclides by measuring surface absorbed-dose-to-water from a point-like Po-210 radionuclide using a planar, windowless extrapolation chamber. The measured ionization current at each airgap must be corrected to account for side-losses of charged particles at finite air-gaps ((kdiv)), the finite radius of the Po-210 source (k(point)), the difference in backscatter between the medium surrounding the air-cavity and water (k(backscatter)), and δ-ray disequilibrium due to the absence of build-up material (k(δ)). In this study, we determine the magnitude of these correction factors using Monte Carlo (MC) simulation.
Methods: GEANT4 was used to model the extrapolation chamber and circular, 3 mm diameter Po-210 source on a silver substrate along with a cylindrical air-cavity with the radius determined by COMSOL Multiphysics™ electrostatic simulations. The Po-210 source was simulated as a surface source and the absorbed dose to air was calculated for 100-250 um airgaps. k(div) was determined by extrapolating the calculated absorbed dose at zero airgap, k(point) was determined by simulating a point source, and k(backscatter) was determined by setting the material surrounding the cavity as water. The k(δ) correction factor was calculated by considering local absorption of δ-rays generated in the cavity.
Results: The radius of the air-cavity was calculated to be 2.02 mm according to the electrostatic simulations. The k(backscatter) and k(δ) correction factors were found to be in the 0.882-0.911 and 1.020-1.041 ranges, respectively, and their deviation from unity was observed to decrease with increasing airgap. In contrast, k(point) and k(div) were observed to linearly increase with increasing airgap with magnitudes in the 1.051-1.062 and 1.243-1.591 ranges, respectively. k(div) was determined to be the largest correction factor.
Conclusion: MC-based correction factors were calculated for an extrapolation chamber measuring surface absorbed-dose-to-water from an alpha-emitting radionuclide.