Program Information
Monte Carlo Calculation of the Effect of An MRI Magnetic Field in HDR Brachytherapy Dose Distributions in the Lung Region
F Moreno Barbosa1*, E Moreno Barbosa1 , B de Celis Alonso1 , J Ramos-Mendez2 , (1) Benemerita Universidad Autonoma de Puebla, Puebla, Mexico,(2) University of California San Francisco, San Francisco, CA
Presentations
SU-E-205-7 (Sunday, July 30, 2017) 1:00 PM - 1:55 PM Room: 205
Purpose: The TOPAS Monte Carlo tool was used to quantify the effect on dose distributions that a concomitant 3T magnetic field has, if applied during a brachytherapy treatment of a patient with lung cancer.
Methods: A DICOM CT image from an anonymous patient with lung cancer was obtained from the database: https://wiki.cancerimagingarchive.net/display/Public/SPIE-AAPM+Lung+CT+Challenge. This database contains files used in an international contest for image registration. The HU information in the DICOM file was converted to density and material composition using the stoichiometric calibration curve from TOPAS. For dosimetry purposes three regions of interest were contoured: Tumor, planning target volume (PTV) and Lung (left lung). An array of brachytherapy sources of ¹³²Ir (VariSource, GammaMed Plus) was simulated within the tumor to achieve 120 Gy of prescribed dose in the PTV region. The decay of ¹³²Ir and the magnetic field were explicitly simulated in TOPAS. The physics included electromagnetic and radioactive decay processes from Geant4 modules G4EmPenelopePhysics and G4decay, respectively. A distance threshold for production of secondary electron of 0.1 mm was used, and the simulations ran up to achieve a statistical uncertainty of about 1% (1 standard deviation) in dose distributions. Dose distributions were calculated in absence and presence of the homogeneous magnetic field oriented on caudal-cephalic direction. The equivalent uniform dose (EUD) was used as comparative metric.
Results: The presence of the magnetic field reduced the EUD in 1.0%±0.3%, 2.3%±0.3% and 2.2%±0.2% in Tumor, PTV and Lung, respectively. The absolute differences in dose volume histograms were up to 3%, 6% and 7% for the Tumor, PTV and Lung, respectively.
Conclusion: We quantified the effect of magnetic fields on brachytherapy dose distributions with TOPAS simulations combining the complexity of patient geometry, magnetic field and radiation transport. Further investigation focuses now on proper quantification of the effect on a larger cohort of patients.
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