Prog Med Phys.  2022 Dec;33(4):80-87. 10.14316/pmp.2022.33.4.80.

Measurement of Proton Beam Dose-Averaged Linear Energy Transfer Using a Radiochromic Film

• An S
• Pak Si
• Jeong S
• Min S
• Kim TJ
• Shin D
• Lim Y
• Jeong JH
• Kim H
• Lee SB

• KMID: 2537875
• DOI: http://doi.org/10.14316/pmp.2022.33.4.80

Abstract

Purpose
Proton therapy has different relative biological effectiveness (RBE) compared with X-ray treatment, which is the standard in radiation therapy, and the fixed RBE value of 1.1 is widely used. However, RBE depends on a charged particle’s linear energy transfer (LET); therefore, measuring LET is important. We have developed a LET measurement method using the inefficiency characteristic of an EBT3 film on a proton beam’s Bragg peak (BP) region.
Methods
A Gafchromic EBT3 film was used to measure the proton beam LET. It measured the dose at a 10-cm pristine BP proton beam in water to determine the quenching factor of the EBT3 film as a reference beam condition. Monte Carlo (MC) calculations of dose-averaged LET (LET _d ) were used to determine the quenching factor and validation. The dose-averaged LETs at the 12-, 16-, and 20-cm pristine BP proton beam in water were calculated with the quenching factor.
Results
Using the passive scattering proton beam nozzle of the National Cancer Center in Korea, the LET _d was measured for each beam range. The quenching factor was determined to be 26.15 with 0.3% uncertainty under the reference beam condition. The dose-averaged LETs were measured for each test beam condition.
Conclusions
We developed a method for measuring the proton beam LET using an EBT3 film. This study showed that the magnitude of the quenching effect can be estimated using only one beam range, and the quenching factor determined under the reference condition can be applied to any therapeutic proton beam range.

Keyword

Proton therapy; Linear energy transfer; Film dosimetry; EBT3

Figure

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  Fig. 1 Data and MC comparison of depth dose curves of 10-, 12-, 16-, and 20-cm water equivalent range beams measured by the Markus chamber (point plots) and MC (solid lines). MC, Monte Carlo; PDD, percentage depth dose.

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  Fig. 2 (a) Top view of a schematic diagram of the measurement system, (b) side view of the 3D printed structure, (c) side view of the measurement system.

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  Fig. 3 EBT3 film calibration curve. Data points are measured behind the 3.135-cm solid water phantom.

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  Fig. 4 Under the water equivalent range 10-cm reference beam condition, (a) comparison of PDD curves measured by the EBT3 (point plots), and the Markus chamber (solid line), (b) the response ratio of the EBT3 film to the Markus chamber about LETd (c) comparison between estimated LETd (point plot) and LETd calculated by MC (solid line). LET, linear energy transfer; MC, Monte Carlo; PDD, percentage depth dose.

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  Fig. 5 In the water equivalent 12-, 16- and 20-cm beam ranges, (a) PDD curves measured by EBT3 (points) and the Markus chamber (solid lines), (b) the response ratio of film to Markus about LETd. LET, linear energy transfer; PDD, percentage depth dose.

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  Fig. 6 Measured LETd (points) of water equivalent 12-, 16-, and 20-cm beam ranges, they were compared to LETd calculated by MC (solid lines). LET, linear energy transfer; MC, Monte Carlo.

Reference

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