Effect of dose-delivery time for flattened and flattening filter-free photon beams based on microdosimetric kinetic model.

Hisashi Nakano,Yukio Akagi,Yutaka Hirokawa,Daisuke Kawahara,Kaoru Ono,Qinghui Zhang

doi:10.1371/journal.pone.0206673

Hisashi Nakano, Yukio Akagi + Show 4 more

Open Access

https://doi.org/10.1371/journal.pone.0206673

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Abstract

The effect of dose-delivery time with flattening filter (FF) and flattening filter-free (FFF) photon beams based on microdosimetric kinetic model (MKM) was investigated in this study. Monte Carlo simulation with the particle and heavy ion transport code system (PHITS) was performed to calculate the dose-mean lineal energy yD (keV/μm) of FF and FFF 6 MV photon beams using the IAEA phase-space files of Varian TrueBeam linear accelerator. Human non-small cell lung cancer NCI-H460 cells were used to determine the MKM parameters under the condition that dose-delivery times with continuous irradiation were 1, 5, 10, 30, and 60 min, and the adsorbed dose was 2, 4, and 8 Gy in this study. In addition, the relative biological effectiveness (RBE) of FF and FFF photon beams were calculated for evaluating the effect of dose delivery time. The RBE of FF decreased to 99.8% and 97.5% with 5 and 60 min for 2 Gy in comparison to 99.6% and 95.1% for 8 Gy, respectively. Meanwhile, that of FFF decreased to 99.5% and 94.9% with 5 and 60 min for 2 Gy in comparison to 99.5% and 94.9% for 8 Gy, respectively. Dose-delivery time has an effect on the RBE with photon beams. In other words, the dose-delivery time should be considered during radiation therapy. Furthermore, FFF photon beams were an effective irradiation method compared to FF in dose-delivery time on account of improving clinic throughput.

Highlights

Flattening filter-free (FFF) photon beams provide an increased instantaneous dose of X-ray pulses compared with a conventional flattening filter (FF) photon beams by removing the flattening filter
In this study, using the microdosimetric kinetic model (MKM), we investigated the effect of dose-delivery time for both the FF and FFF beams
Rprd2 where z is the specific energy deposited in the domain [Gy], N is the number of domains, A and B are coefficients, rd is the radius of the domain (0.5 μm), ρ is the density of the domain (1.0 g/cm3), D is the absorbed dose [Gy], and yD is the dose-mean lineal energy [keV/μm]

Summary

Introduction

Flattening filter-free (FFF) photon beams provide an increased instantaneous dose of X-ray pulses compared with a conventional flattening filter (FF) photon beams by removing the flattening filter. The high dose rate provided by the FFF photon beams decreases the beam-ontime and improves clinical throughput [1,2,3]. These features can be effectively used for minimization of intrafraction motion of lung tumors [4]. The dependence of the SF as a function of dose-delivery time was evaluated in the high dose range using the modified MKM. The dependence of the RBE of the FFF beams as a function of dose-delivery time has not been reported. In this study, using the MKM, we investigated the effect of dose-delivery time for both the FF and FFF beams

Materials and methods

Results

Discussion