Superficial and peripheral dose in compensator‐based FFF beam IMRT

Abstract

Flattening filter‐free (FFF) beams produce higher dose rates. Combined with compensator‐based intensity modulated radiotherapy (IMRT) techniques, the dose delivery for each beam can be much shorter compared to the flattened beam MLC‐based or flattened beam compensator‐based IMRT. This ‘snap shot’ IMRT delivery is beneficial to patients for tumor motion management. Due to softer energy, superficial doses in FFF beam treatment are usually higher than those from flattened beams. Due to no flattening filter, thus less photon scattering, peripheral doses are usually lower in FFF beam treatment. However, in compensator‐based IMRT using FFF beams, the compensator is in the beam pathway. Does it introduce beam hardening effects and scattering such that the superficial dose is lower and peripheral dose is higher compared to FFF beam MLC‐based IMRT? This study applied Monte Carlo techniques to investigate the superficial and peripheral doses in compensator‐based IMRT using FFF beams and compared it to the MLC‐based IMRT using FFF beams and flattened beams. Besides varying thicknesses of brass slabs to simulate varying thicknesses of compensators, a simple cone‐shaped compensator was simulated to mimic a clinical application. The dose distribution in water phantom by the cone‐shaped compensator was then simulated by multiple MLC‐defined FFF and flattened beams with varying apertures. After normalization to the maximum dose, Dmax, the superficial and peripheral doses were compared between the FFF beam compensator‐based IMRT and FFF/flattened beam MLC‐based IMRT. The superficial dose at the central 0.5 mm depth was about 1% (of Dmax) lower in the compensator‐based 6 MV FFF (6FFF) IMRT compared to the MLC‐based 6FFF IMRT, and about 8% higher than the flattened 6 MV MLC‐based IMRT dose. At 8 cm off‐axis at depth of central maximum dose, dmax, the peripheral dose between the 6FFF and flattened 6 MV MLC demonstrated similar doses, while the compensator dose was about 1% (of Dmax) higher. Compensators reduce the superficial doses slightly compared to open FFF beams, but increases the peripheral doses due to scatter in the compensator.

Comparison of the dose distribution of the VMAT radiotherapy technique depending on the beam used: FFF-X10MV and FFF-X15MV

“…Many dosimetry studies have been conducted to analyze photon beams [25][26][27][28][29]. The use of high-energy (> 10MeV) photon beams for treatment and its legitimacy in the case of modern radiotherapy using the dynamic VMAT technique focuses on an interesting aspect that has not been thoroughly investigated, and it is generally assumed that low energies are sufficient to implement radiotherapy with dynamic VMAT methods.…”

“…Extreme values are theoretical, where 0 means that OARs get a homogeneous maximal dose and 1 means that OARs get no dose [11,12]. Therefore, a comparison of dose distributions for two different VMAT beams applied to different locations of irradiated volumes associated with the PTV dimension can be useful in making clinical practice decisions related to treatment planning [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29].…”