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Inverse Treatment Planning for MERT Using Monte Carlo Calculations

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D Henzen

D Henzen1*, P Manser1, D Frei1, W Volken1, H Neuenschwander2, A Joosten1, E J Born1, M K Fix1, M Stampanoni3, (1) Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital and University of Bern , Switzerland (2) Clinic for Radiation-Oncology, Lindenhofspital, Bern, Switzerland (3) Institut for Biomedical Engineering, ETH Zurich and Paul Scherrer Institut, Villigen, Switzerland

SU-E-T-659 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
The use of electron beams to treat shallow tumors could reduce the dose to the distal organs at risk.
To employ modulated electron radiotherapy (MERT) an inverse treatment planning strategy which bases on a Monte Carlo (MC) Electron Beam Model (EBM) has been developed.

Methods:
The inverse planning process starts with the selection of the SSD, gantry angle and collimator rotation for each electron field. These fields are then divided into a grid of beamlets. An MC EBM together with the macro MC (MMC) dose calculation algorithm is employed to pre-calculate the dose distribution for each beamlet. Afterwards a direct aperture optimization (DAO) using simulated annealing minimizes a cost function which is the sum of the squared dose differences in all voxels of the PTV and the OARs between the actual value and a given upper or lower limit. For each step either a weight change or an aperture change is initiated. As the beamlet dose calculation neglects the MLC interaction a post processing weight optimization and MLC aperture adjustment has been carried out after the DAO has been finished. To test the algorithm an inverse treatment plan has been created for an academic situation and compared to the obtained results using a previously developed forward planning strategy.

Results:
The implementation of an inverse optimization incorporating the home made EBM has been completed successfully. The time needed for the DAO of the academic situation is about 30 min. Compared to the forward planning the inverse optimization is able to reduce the dose to the OAR (mean dose from 41% to 38%) for the academic case.

Conclusion:
The developed inverse optimization employing the EBM and MMC can be used for treatment planning of MERT. This work was supported by Varian Medical Systems.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by Varian Medical Systems.

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