Dosimetry of Electron Beams at Extremely Extended SSD: Monte Carlo Calculations and Measurements
W Xu, R Zhang, M Xu*, J Roeske, Loyola Univ Medical Center, Maywood, ILSU-E-T-316 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: We have calculated and measured the depth dose, dose distribution, and MU for 6 and 12 MeV electron beams at extremely extended SSD up to 350 cm. We report dosimetry characteristics based upon calculation and measurements at extremely extended SSD range.
Methods: An Eclipse TPS implements the electron Monte Carlo (eMC) algorithm for dose calculations. The eMC was used for dose distribution calculations at different electron energies, field sizes, and extremely extended SSDs. The isodose distribution on a plane and the profile along an axis at a certain depth were measured by MapCHECK, which has 445 diodes to form a two dimension array detector with a maximum measuring field size of 22 x 22 cm2.
Results: We obtained calculated and measured isodose distributions at extremely extended SSDs varied from 100 to 350 cm from the Eclipse workstation and MapCHECK, respectively. The isodose distributions demonstrate an excellent agreement between the measurement and eMC calculations. As we prescribed a certain dose as Dmax for each depth dmax at the central axis, and we found the MU value with the calculations. We then delivered this amount MU detected by MapCHECK. The calculated and measured output factors were obtained as a function of SSD factors at a given applicator size and an electron energy.
Conclusion: The depth dose, isodose distribution, and MU were calculated and delivered at SSDs from 100 to 350 cm for the electron beams of 6 and 12 MeV. The eMC can accurately predict depth doses, isodose distributions, and output factors for the energy of 12 MeV at a SSD ranging from 100 to 350 cm. However, for 6 MeV, the output factor deviates from the measurement starting at SSD of 130 cm with 10 x10 cm2 and 150 cm with 20 x20 cm2 fields respectively.