Ex) Article Title, Author, Keywords
New Phys.: Sae Mulli 2021; 71: 885-889
Published online October 29, 2021 https://doi.org/10.3938/NPSM.71.885
Copyright © New Physics: Sae Mulli.
Yong-Cheol KWON1, Hyon-Suk JO2*, Se Byeong LEEY3, Wook-Geun SHIN4
1Department of Physics, Kyungpook National University, Daegu 41566, Korea Proton Therapy Center, National Cancer Center, Goyang 10408, Korea
2Department of Physics, Kyungpook National University, Daegu 41566, Korea
3Proton Therapy Center, National Cancer Center, Goyang 10408, Korea
4Department of Radiation Oncology, Seoul National University Hospital, Seoul 03080, Korea
In proton radiotherapy, the dosimetry protocol TRS-398 does not provide the beam quality correction factors kQ;Q0 for all areas of the spread out Bragg peak (SOBP). Monte Carlo simulations using the TOPAS simulation toolkit were performed to calculate the beam quality correction factors at various depths of the SOBP to observe any variations. The SOBP of the generated proton beam had a range of 15 cm and a width of 15 cm. The beam quality correction factors kQ;Q0 were calculated not only at the reference depth of 7.5 g/cm2 recommended by TRS-398 but also at depths of 4 g/cm2 and 13 g/cm2. The comparison of the simulation results for the absorbed dose with actual measurements showed a slight difference at the surface above the water phantom, but the width of the SOBP was well matched with a difference of less than 1%. The kQ;Q0 factor calculated at the reference depth of 7.5 g/cm2 was 1.045, which is within the error range of the value of 1.030 provided by the TRS-398 protocol. The kQ;Q0 factors calculated at the depths of 4 g/cm2 and 13 g/cm2 were 1.041 and 1.048, respectively. While all the calculated values were within the error range of the value suggested by TRS-398, the observed increase in the kQ;Q0 factor with increasing depth suggests that a position-dependent beam quality correction factor determined through precise measurements may be required to calculate the correct dose.
Keywords: Beam quality correction factor, Monte carlo simulation, Proton therapy, TOPAS
The dosimetry protocol TRS-398 developed by the International Atomic Energy Agency (IAEA) presents a protocol to measure absorbed doses of external radiation . Absorbed doses are measured by using an ionization chamber placed inside a water phantom, with the ionization chamber being periodically calibrated to determine the correct absorbed dose. Most ionization chambers are calibrated based on 60Co gamma rays, but since patient treatments actually use radiation of various particles and energies, a correction compensating for the difference in beam quality is required. TRS-398 proposes a correction factor
Among the radiation used for patient treatment, the physical characteristic of a single-energy proton beam is an energy peak that is generated by transferring a large amount of energy at the end of the range, which is called a Bragg peak. In addition, the Spread Out Bragg Peak (SOBP), which is made by the superposition of proton beams of various energies, can irradiate radiation uniformly to the target volume. Due to these physical characteristics, the proton beam can focus radiation on the target and minimize damage to surrounding normal tissues compared to treatments using gamma and electron beams.
The determination of the beam quality correction factor through experimental measurements presents many difficulties. Monte Carlo simulations can be used for an efficient and accurate calculation of the beam quality correction factor, as shown in previous studies [2,3]. The Monte Carlo simulation toolkit used in this study is TOPAS , which is developed based on Geant4. It was shown that TOPAS is able to calculate a sufficiently accurate beam quality correction factor in comparison with previous Monte Carlo simulations .
The beam quality correction factor
In order to calculate the beam quality correction factor, the geometries of the ionization chamber and of the point volume are required to determine
The beam quality correction factor of the proton beam used in this study was calculated for three depth values. The correction factor was first calculated at the depth of 7.5 g/cm2, which is the central depth of the SOBP and also the reference depth suggested by TRS-398. Then, the correction factors were calculated at the depths of 4 g/cm2 and 13 g/cm2 to observe any position dependence.
Fig. 2 shows the proton beam nozzle of the National Cancer Center (NCC) as reconstructed in TOPAS. The proton beam is modulated with a range of 15 cm and a width of SOBP of 15 cm, and the field size of the proton beam on the water phantom surface is 10×10 cm2. The proton beam passes through the nitrogen gas between the nozzle and the water phantom and enters the water phantom which has a size of 30 × 30 × 30 cm3.
When performing the simulations with TOPAS, the physics lists of radioactive decay process g4decay, inelastic nuclear interaction process g4hphy_QGSP_BIC_HP, ion physics process g4ionbinarycascade, elastic hadron processes g4helastic_HP, stopping power process g4stopping, electromagnetic process g4emstandard_opt4 were used .
Fig. 3 presents a graph comparing the simulation results with experimental measurement results of percent depth dose (PDD) to check whether the proton beam is generated well as intended. In the case of the simulation, the absorbed dose was measured by placing a voxel of 0.5 × 0.5 × 0.5 mm3 on the water phantom. For the experimental measurement, IBA Zebra was used to measure the proton beam at NCC. IBA Zebra measured the absorbed dose using vented ionization chambers spaced 2 mm apart . The results from the simulations and the experiment show a slight difference in the absorbed dose on the surface of the water phantom, but the width of SOBP is about 1% different, with TOPAS and IBA Zebra presenting values of 14.9 g/cm2 and 15.0 g/cm2, respectively.
Fig. 4 shows the beam quality correction factor calculated by using Monte Carlo simulations at several depths to determine the position dependence. The calculated values at the depths of 4 g/cm2, 7.5 g/cm2, and 13 g/cm2 are 1.041, 1.045, and 1.048, respectively. Thus the beam quality correction factors calculated by using TOPAS are within the error range of the value presented by TRS-398. However, the results show that the beam quality correction factor gradually increase with the depth. Compared to the value at 7.5 g/cm2, the value at 4 g/cm2 was found to be about 0.4% smaller, and the value at 13 g/cm2 is about 0.3% larger. These results imply that the beam quality correction factor may not be constant depending on the position of the ionization chamber in the SOBP.
In this study, the proton beam nozzle of NCC was simulated using the Geant4-based Monte Carlo simulation toolkit TOPAS to generate a proton beam and calculate the beam quality correction factor
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government, Ministry of Science, ICT and Future Planning (MSIP) (NRF-2019R1F1A1060665), and was also supported by the Korea Research Institute of Standards and Science (KRISS), the research program of “Proton beam (double scattered) water absorbed dose precision measurement technology development”.