Purpose: To retrospectively calculate three dimensional dose distributions in the lung and surrounding tissues using Monte Carlo (MC) simulations to evaluate the effect of including media heterogeneities for patients treated with I‐125 brachytherapy for resected non‐small cell lung cancer. Methods: The EGSnrc user‐code BrachyDose is used to perform simulations including media heterogeneities for patients treated with intraoperative I‐125 mesh brachytherapy for stage I non‐small cell lung cancer at Mayo Clinic. Heterogeneous simulation phantoms are generated using patient CT data: voxel medium (air, lung, soft tissue, bone) and density assignment is based on the densities in the CT images. Seeds are fully modeled. Three‐ dimensional dose distributions in the lung are calculated and are compared with the TG‐43 formalism via DVHs and dose metrics for normal tissue volumes and volumes consisting of various margins (5–20 mm) surrounding the surgical suture (PTV10 for 10 mm margin). Results: Differences in dose metrics for TG‐43 and heterogeneous MC calculations vary dramatically between patients; differences of up to 17% in V100, 26% in D90, and 20% in the mean dose are observed within PTV10. MC doses to voxels within PTV10 can differ by up to +/−20 Gy (20% of prescription dose) from TG‐43 doses. Correspondingly, large differences are observed in isodose contours for the two calculation methods, particularly in lung tissues; differences are smaller for soft tissue regions. Dose differences are highly sensitive to source positioning within the lung and on lung composition (i.e. media heterogeneities). Conclusions: Doses computed using full MC simulations differ considerably from TG‐43 calculated doses for I‐125 mesh brachytherapy. Accurate treatment planning and evaluation requires the inclusion of non‐water media based on imaging data in dose calculations. BrachyDose calculation times are sufficiently fast for clinical use.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging