HDRMC, an accelerated Monte Carlo dose calculator for high dose rate brachytherapy with CT-compatible applicators.
OBJECTIVE
To present a new accelerated Monte Carlo code for CT-based dose calculations in high dose rate (HDR) brachytherapy. The new code (HDRMC) accounts for both tissue and nontissue heterogeneities (applicator and contrast medium).


METHODS
HDRMC uses a fast ray-tracing technique and detailed physics algorithms to transport photons through a 3D mesh of voxels representing the patient anatomy with applicator and contrast medium included. A precalculated phase space file for the(192)Ir source is used as source term. HDRM is calibrated to calculated absolute dose for real plans. A postprocessing technique is used to include the exact density and composition of nontissue heterogeneities in the 3D phantom. Dwell positions and angular orientations of the source are reconstructed using data from the treatment planning system (TPS). Structure contours are also imported from the TPS to recalculate dose-volume histograms.


RESULTS
HDRMC was first benchmarked against the MCNP5 code for a single source in homogenous water and for a loaded gynecologic applicator in water. The accuracy of the voxel-based applicator model used in HDRMC was also verified by comparing 3D dose distributions and dose-volume parameters obtained using 1-mm(3) versus 2-mm(3) phantom resolutions. HDRMC can calculate the 3D dose distribution for a typical HDR cervix case with 2-mm resolution in 5 min on a single CPU. Examples of heterogeneity effects for two clinical cases (cervix and esophagus) were demonstrated using HDRMC. The neglect of tissue heterogeneity for the esophageal case leads to the overestimate of CTV D90, CTV D100, and spinal cord maximum dose by 3.2%, 3.9%, and 3.6%, respectively.


CONCLUSIONS
A fast Monte Carlo code for CT-based dose calculations which does not require a prebuilt applicator model is developed for those HDR brachytherapy treatments that use CT-compatible applicators. Tissue and nontissue heterogeneities should be taken into account in modern HDR brachytherapy planning.
Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111.
Omar
O
Chibani
Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111.
Charlie
C
C-M Ma
D000465:Algorithms; D001918:Brachytherapy / Q000379:methods*; D002138:Calibration; D004938:Esophageal Neoplasms / Q000532:radiotherapy; D005260:Female; D006801:Humans; D007496:Iridium Radioisotopes / Q000627:therapeutic use; D008954:Models, Biological; D009010:Monte Carlo Method; D017785:Photons; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D013116:Spinal Cord / Q000528:radiation effects; D002583:Uterine Cervical Neoplasms / Q000532:radiotherapy; D014867:Water
D007496:Iridium Radioisotopes; D014867:Water