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4D Monte Carlo Simulations of Beam and Patient Motion Using EGSnrc/BEAMnrc


E Heath

E Heath1*, I Badragan2, I Popescu3, (1) Ryerson University, Toronto, ,(2) BC Cancer Agency, Abbotsford, ,(3) British Columbia Cancer Agency, Vancouver, BC

TU-G-BRA-1 Tuesday 4:30:00 PM - 6:00:00 PM Room: Ballroom A

Purpose:
To develop a 4D Monte Carlo method that simulates continuous beam and patient motion using the EGSnrc Monte Carlo code.

Methods:
The simulation method links the collimator and gantry settings on a particle-by-particle basis to the respiratory phase of the patient at which it is delivered. This synchronization was achieved by implementing two novel particle sources into the defDOSXYZnrc Monte Carlo code which calculates dose in a deforming patient geometry. Validation in a virtual lung phantom was performed for static and tracking beam scenarios. Dose delivery of a RapidArc plan on a commercial dynamic thorax phantom was simulated using RPM and linac log files from the plan delivery. The effects of interplay between the beam and phantom motion were quantified by performing simulations with and without motion synchronization.

Results:
Simulations in the virtual phantom demonstrate recovery of the static dose profile when the MLC sequence tracks the tumor motion. Simulations of the RapidArc plan delivery showed dose variations of up to 9% of the prescribed dose between plans calculated with and without interplay effects.

Conclusions:
A 4D Monte Carlo method for EGSnrc has been developed which can calculate dose for combined beam and patient motion. Unlike discretized approaches, this method can calculate the dose distribution in a single simulation. Using treatment log files, this tool can reconstruct the dose actually delivered to a patient, accounting for respiratory motion and interplay effects.

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