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Experimental Validation of Monaco Electron Monte Carlo Dose Calculation for Small Fields


Varadhan1*, S Way2 , L Arentsen3 , B Gerbi4 , (1) Minneapolis Radiation Oncology, Fridley, MN, (2) Minneapolis Radiation Oncology, Robbinsdale, MN, (3) University of Minnesota, Minneapolis, MN, (4) University of Minnesota, Minneapolis, MN


SU-F-T-74 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

To verify experimentally the accuracy of Monaco (Elekta) electron Monte Carlo (eMC) algorithm to calculate small field size depth doses, monitor units and isodose distributions.

Beam modeling of eMC algorithm was performed for electron energies of 6, 9, 12 , 15 and 18 Mev for a Elekta Infinity Linac and all available ( 6, 10, 14 , 20 and 25 cone) applicator sizes. Electron cutouts of incrementally smaller field sizes (20, 40, 60 and 80% blocked from open cone) were fabricated. Dose calculation was performed using a grid size smaller than one-tenth of the R₈₀₋₂₀ electron distal falloff distance and number of particle histories was set at 500,000 per cm². Percent depth dose scans and beam profiles at dmax, d₉₀ and d₈₀ depths were measured for each cutout and energy with Wellhoffer (IBA) Blue Phantom² scanning system and compared against eMC calculated doses.

The measured dose and output factors of incrementally reduced cutout sizes (to 3cm diameter) agreed with eMC calculated doses within ± 2.5%. The profile comparisons at dmax, d₉₀ and d₈₀ depths and percent depth doses at reduced field sizes agreed within 2.5% or 2mm.

Our results indicate that the Monaco eMC algorithm can accurately predict depth doses, isodose distributions, and monitor units in homogeneous water phantom for field sizes as small as 3.0 cm diameter for energies in the 6 to 18 MeV range at 100 cm SSD. Consequently, the old rule of thumb to approximate limiting cutout size for an electron field determined by the lateral scatter equilibrium (E (MeV)/2.5 in centimeters of water) does not apply to Monaco eMC algorithm.

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