Program Information
The Accuracy of the Out-Of-Field Dose Calculations of a Commercial Treatment Planning System
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 HallPurpose:
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.
Methods:
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.
Results:
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.
Conclusion:
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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