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The Accuracy of the Out-Of-Field Dose Calculations of a Commercial Treatment Planning System

L Wang

L Wang1*, G Ding2, (1) Stanford University, Stanford, CA, (2) Vanderbilt University , Nashville, TN

SU-E-T-545 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

The accuracy of calculated out-of-field dose in a commercial radiotherapy treatment planning system receives less attention as the out-of-field dose is significantly lower compared to the prescribed target dose. However, it can be important as it relates to the dose to the organ-at-risk as well as the risk of a radiation-induced secondary malignancy. This study evaluates out-of-field dose calculation accuracy of a commercial treatment planning system by using Monte Carlo techniques.

The EGSnrc Monte Carlo codes was used to calculate dose distributions resulting from IMRT fields from a Varian Clinac accelerator delivered using sliding window with Millennium 120 multi-leaf collimator. The out-of-field incident beams were simulated accurately in the Monte Carlo calculations. The dose distributions were calculated using real patient IMRT plans of a 6MV beam on a water phantom with different inhomogeneity geometry. The commercial treatment planning system evaluated is Varian Eclipse system (v.10) with AAA dose calculation algorithm.

The discrepancy of calculated dose profiles between Eclipse and the Monte Carlo showed a small dependency with depth. At depth of 10 cm, Eclipse calculations under-estimates the out-of-field dose by 35% of local dose at the off-axis location where the dose is only 1% of dose maximum. The maximum deviation occurred between 2 to 3 cm depths, where the discrepancy was up to 60% of local dose at 1% dose level.

Based on Monte Carlo calculations, the uncertainty of out-of-field dose distributions predicted by the Eclipse system is less than 1% of maximum dose for highly modulated IMRT beams in regions where the dose relative to the maximum dose is less than 5%, although the uncertainties can be much larger relative to local doses.

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