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Efficient Verification Method for Modulated Electron Radiotherapy Treatment Plans

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D Henzen

D Henzen1*, C Chatelain1 , P Manser1 , D Frei1 , W Volken1 , H Neuenschwander2 , A Joosten1 , K Loessl1 , D M Aebersold1 , M K Fix1 , (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


SU-E-T-148 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

For shallow tumors, modulated electron radiotherapy (MERT) promises a reduction of dose to distal organs at risk. At our institution a framework was developed in order to create treatment plans for MERT employing both forward and inverse optimization. In this work, an efficient quality assurance (QA) process is established.

Treatment plans for three different tumor sites were created using an inverse optimization. These plans consist of 6-12 segments and energies between 6 and 18 MeV. An already established QA process for photon IMRT plans is now extended to additionally handle MERT plans. First, the dose distributions are calculated in a homogenous water phantom. For this task a dedicated Monte Carlo (MC) framework for MERT is used. Second, the segments are applied on a stand-alone amorphous silicon electronic portal imaging device (EPID) using a source-to-surface distance of 70 cm. This device was calibrated for electron beams in a previous work. An in-house developed analysis software, is then utilized for comparisons and evaluation of the measured and calculated dose distributions.

For all three plans the calculated dose distributions agree well with the measured ones. Using a 2D gamma comparison (2% of dose max/2 mm and 10% dose threshold) passing rates >98% are achieved.
The dose calculation for each plan on the water phantom, using voxels of 0.2x0.2x0.2 cm³, takes at maximum 30 min on a single core Pentium 2.66 GHz system with 6 GB RAM, to reach a statistical uncertainty of 2% (1 std. dev.).

An already established QA procedure for IMRT photon plans was applied for MERT. The dedicated MC framework and the use of EPID measurements allow an efficient QA procedure in a clinical environment. 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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