Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2022 Jul-Sep;47(3):301-308.
doi: 10.4103/jmp.jmp_40_22. Epub 2022 Nov 8.

Determination of the Small-Field Output Factor for 6 MV Photon Beam Using EGSnrc Monte Carlo

K W Chuah  1 , M Z Abdul Aziz  2 , J Jayamani  1
Affiliations

Determination of the Small-Field Output Factor for 6 MV Photon Beam Using EGSnrc Monte Carlo

K W Chuah et al. J Med Phys. 2022 Jul-Sep.

Abstract

Accuracy of ionization chamber (IC) to measure the scatter output factor (Scp) of a linear accelerator (linac) is crucial, especially in small field (<4 cm × 4 cm). The common IC volume of 0.6 cc is not adequate for small-field measurement and not all radiotherapy centers can afford to purchase additional IC due to the additional cost. This study aimed to determine the efficiency of the EGSnrc Monte Carlo (MC) to calculate the Scp for various field sizes including small field in Elekta Synergy (Agility multileaf collimator) linac. The BEAMnrc and DOSXYZnrc user codes were used to simulate a 6 MV linac model for various field sizes and calculate the radiation dose output in water phantom. The modeled linac treatment head was validated by comparing the percentage depth dose (PDD), beam profile, and beam quality (Tissue Phantom Ratio (TPR)20,10) with the IC measurement. The validated linac model was simulated to calculate the Scp consisting of collimator scatter factor (Sc) and phantom scatter factor (Sp). The PDD and beam profile of the simulated field sizes were within a good agreement of ±2% compared with the measured data. The TPR20,10 value was 0.675 for field size 10 cm × 10 cm. The Scp, Sc, and Sp simulated values were close to the IC measurement within ±2% difference. The simulation for Sc and Sp in 3 cm × 3 cm field size was calculated to be 0.955 and 0.884, respectively. In conclusion, this study validated the efficiency of the MC simulation as a promising tool for the Scp calculation including small-field size for linac.

Keywords: Collimator scatter factor; linear accelerator; phantom scatter factor.

PubMed Disclaimer

Conflict of interest statement

There are no conflicts of interest.

Figures

Figure 1
Figure 1
The water phantom and IC positioning for the (a) Scp in water phantom at SAD 100 cm and (b) Sc in air at SAD 100 cm. SAD: Source-to-axis distance, Sc: Collimator scatter factor, Scp: Scatter output factors, IC: Ionization chamber
Figure 2
Figure 2
Modeled linac in BEAMnrc/EGSnrc: (a) XZ view, (b) YZ view. linac: Linear accelerator
Figure 3
Figure 3
(a) Spectral distribution of 6 MV photon beam at the water phantom surface. The maximum energy absorbed by the water phantom was 6.4 MeV. (b): X-Y scatter plot of 6 MV photon beam for the field size 10 cm × 10 cm at 100 cm distance from the linac's target to the surface of the water phantom. linac: Linear accelerator
Figure 4
Figure 4
The PDD versus depth for field sizes (a) 3 cm × 3 cm, (b) 5 cm × 5 cm, (c) 10 cm × 10 cm, (d) 15 cm × 15 cm, and (e) 20 cm × 20 cm between the IC measurement and MC simulation for Elekta Synergy (Agility MLC) linac. IC: Ionization chamber, MC: Monte Carlo, PDD: Percentage depth dose, linac: Linear accelerator, MLC: Multileaf collimator
Figure 5
Figure 5
The relative dose in beam profile between IC measurement and MC simulation: (a) cross-plane and (b) in-plane for field size 3 cm × 3 cm; (c) cross-plane and (d) in-plane for field size 5 cm × 5 cm; (e) cross-plane and (f) in-plane for field size 10 cm × 10 cm; (g) cross-plane and (h) in-plane for field size 15 cm × 15 cm; (i) cross-plane and (j) in-plane for field size 20 cm × 20 cm. IC: Ionization chamber, MC: Monte Carlo
Figure 6
Figure 6
(a) The Scp, (b) Sc, and (c) Sp of IC measurement and MC simulation for field size 3 cm × 3 cm, 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm, respectively. The error bar indicated ± 2% difference. Sc: Collimator scatter factor, Sp: Phantom scatter factor, Scp: Scatter output factor, IC: Ionization chamber, MC: Monte Carlo
See this image and copyright information in PMC

References

    1. Chen HH, Kuo MT. Improving radiotherapy in cancer treatment: Promises and challenges. Oncotarget. 2017;8:62742–58. - PMC - PubMed
    1. Citrin DE. Recent developments in radiotherapy. N Engl J Med. 2017;377:1065–75. - PubMed
    1. Sánchez-Nieto B, Medina-Ascanio KN, Rodríguez-Mongua JL, Doerner E, Espinoza I. Study of out-of-field dose in photon radiotherapy: A commercial treatment planning system versus measurements and Monte Carlo simulations. Med Phys. 2020;47:4616–25. - PMC - PubMed
    1. Rosenblatt E, Zubizarreta E. Radiotherapy in Cancer Care: Facing the Global Challenge. Non-serial Publications, IAEA, Vienna: International Atomic Energy Agency; 2017
    1. Hashimoto S, Fujita Y, Katayose T, Mizuno H, Saitoh H, Karasawa K. Field-size correction factors of a radiophotoluminescent glass dosimeter for small-field and intensity-modulated radiation therapy beams. Med Phys. 2018;45:382–90. - PubMed