Monte Carlo simulations with time-dependent geometries to investigate effects of organ motion with high temporal resolution.
OBJECTIVE
To calculate the dose in time-dependent geometry, the results of three-dimensional calculations are usually performed separately and combined. This approach becomes cumbersome when high temporal resolution is required, if the geometry is complex, or if interplay effects between different, independently moving systems are to be studied. The purpose of this project was the implementation of continuous (four-dimensional [4D]) Monte Carlo simulation to study the irradiation of tumors under respiratory motion.


METHODS
In taking advantage of object-oriented programming, we implemented 4D Monte Carlo dose calculation. Local dose depositions in the patient are calculated while beam configuration and organ positions are changed continuously. Deformable image registration is used to describe the CT voxel displacement over time.


RESULTS
The 4D Monte Carlo technique is applied to a lung cancer case planned for proton therapy. We show that the effect of motion on the dose distribution can be simulated effectively based on statistical motion parameterizations acting on the geometry or based on patient-specific 4D CT information.


CONCLUSIONS
We present a novel method able to calculate dose with underlying time-dependent geometry. The technique allows 4D dose calculation in arbitrary time scales in a single simulation even for double-dynamic systems (e.g., time-dependent beam delivery under organ motion).
Paganetti
Harald
H
Department of Radiation Oncology, Massachusetts General Hospital, Northeast Proton Therapy Center, Boston, MA 02114, USA. hpaganetti@partners.org
Jiang
Hongyu
H
Adams
Judith A
JA
Chen
George T
GT
Rietzel
Eike
E
D005240:Feasibility Studies; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D009010:Monte Carlo Method; D009068:Movement; D011859:Radiography; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods; D012119:Respiration