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Penumbral Dose with Limited Scatter in Photon Beams with and Without Flattening Filter

S Srivastava

S Srivastava1,4*, Y Akino2, C Cheng3, I Das4, (1) Purude University,Lafayette, IN, (2) Osaka University Graduate School of Medicine, Suita, Osaka, (3) Indiana University- School of Medicine, Bloomington, IN, (4) Indiana University- School of Medicine, Indianapolis, IN

SU-E-T-358 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: Dose outside the active radiation beam plays an important role in estimation of critical organs complications and also an important contributing factor in IMRT plan optimization. In this study, we investigated the effect of scattering material and characteristics of penumbral dose between conventional accelerator and machine with flattening free filter (FFF).
Methods: A Varian True Beam was used that has FFF option for low energy beam. Penumbral dose was measured in a water phantom for all energies (6, 10, and 15 MV) with and without the flattening filter. Beam profiles were collected using a 0.125 cm³ ion chamber at various depths, from dmax-15cm for a set of field sizes. To study the effect of limited scatter in the penumbra, the water tank was shifted to reduce the scattering distance from the tank and field edge systematically from 5-20 cm including tank wall. The measured data were also compared with the Eclipse treatment planning data.
Results: For all beams with the flattening filter, penumbral dose vary from 5% to 12% from 5 cm to 15 cm depth. The variation is larger for lower energies and is a linear function of depth for all energies. It increases with depth linearly and decreases with beam energy. For FFF beam the slope is greater than that of a regular beam. The penumbral dose varies by <1% with scattering condition. For the same depth, the penumbra is significantly larger for FFF. There is significant difference between TPS FFF data compared to the measured values.
Conclusion: Penumbral dose is dependent on scattering distance, depth, beam energy and flattening filter. FFF provides relatively higher dose which is not properly modeled in treatment planning system. For high degree of precision, data should be collected with large scattering condition and beam modeling for FFF should be adequately performed.

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