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Mitigation Strategies to Reduce the Relative Extrafocal Dose (RED)

M Constantin

M Constantin*, H Hsu, S Mansfield, M Svatos, Varian Medical Systems, Palo Alto, CA

SU-E-T-510 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Several studies [1,2] indicate that IMRT may lead to peripheral (extrafocal) dose outside the intended treatment-field. The purpose of this study is to: (a) obtain an accurate assessment of the relationship between the relative extrafocal dose (RED) and beam energy, modulation factor, treatment modality, and Y-jaw settings in connection with the high-definition HD120-leaf MLC from Varian; and (b) identify the most efficient mitigation strategies to reduce RED when significant.

Methods: IMRT fields with varying modulation were simulated by using an accurate representation of the TrueBeam linac equipped with HD120-MLC in Geant4 (v9.4.p02). Dose to water phantom calculations were performed relative to a 10x10cm² field. RED distributions were normalized to dmax for the 6MV, 6FFF, 10MV, 10FFF and 15MV beams. Inline profiles indicated the spatial location of the RED maximum, while simulations with and without the flattening filter revealed its origin. The effect of reducing the Y-jaw position was quantified for different incident energies.

Results: RED is mainly due to photons scattered by the flattening filter, therefore it is not a concern (~0.7%) when the filter is removed (i.e. 6FFF and 10FFF). The RED maximum is located 4cm outside the outermost boundary leaf (i.e. 15cm off-axis for HD120-MLC). Its magnitude can be reduced below 1% by decreasing the Y-jaw setting, lowering the photon beam energy, lowering the modulation factor of the plan or by using FFF beams, if available. At 15MV, the RED magnitude was reduced from 2.6% to 0.9% by setting the Y-jaw limit to 8cm.

Conclusion: Methodologies to effectively lower the relative extrafocal dose were identified for various photon beams on a Varian linac with HD120-MLC, including using FFF modes, choosing lower-energy beams, limiting the Y-field size and lowering the modulation.

[1]E.E. Klein et al., Med.Phys.33 (2006), 2525-1531
[2]D.S. Sharma et al., B.J.R.79 (2006), 331-335

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