Patient-specific dosimetry using deformable anthropomorphic models

Objectives Much progress has been made in patient-specific dosimetry for targeted radionuclide therapy including the development of Monte Carlo methods for dose calculations and methods for accurate quantification of activity uptake in tumors and body organs. The aim of performing these calculations is to optimize tumor dose while limiting toxicity to normal tissue during treatment. We propose a novel method for individualized dosimetry using deformable anthropomorphic phantoms as input for Monte Carlo dose calculations using the Geant4 simulation toolkit.

Methods We have established a method that uses non-uniform activity distributions in organs or tumors from quantitative SPECT or PET data, combined with either manually segmented CT data or the use of deformable (NURBS) anthropomorphic models as input into our established Geant4 transport codes to obtain 3D maps of absorbed dose. The code has been updated to model decay events for radionuclides used in radiopharmaceutical therapy. Corrections for partial volume effects previously studied on our imaging system were applied, and the effects of different reconstruction approaches were also studied.

Results We evaluated organ and tumor doses for several patient treatments including radioimmunotherapy for non-Hodgkin’s lymphoma and peptide receptor agents for neuroendocrine tumors. Average organ doses, 3D dose maps, and dose-volume histograms were developed. Calculations can be obtained by the Monte Carlo code in about 12 hours, with errors mostly less than 5%. Very small organs or tumors required more run time to improve accuracy. Organ doses were compared to results from established dosimetric codes (e.g. OLINDA/EXM) and dose maps and DVHs were compared to results reported by others (e.g. SIMIND).

Conclusions The application of this method to several patient studies demonstrates that use of deformable phantoms as input into Monte Carlo simulations for dose calculations can provide accurate, patient-specific 3D dosimetry in times that may be applied in routine clinical practice