Prog Med Phys.  2020 Dec;31(4):135-144. 10.14316/pmp.2020.31.4.135.

Linear Energy Transfer Dependence Correction of Spread-Out Bragg Peak Measured by EBT3 Film for Dynamically Scanned Proton Beams

• Lee M
• Ahn S
• Cheon W
• Han Y

• KMID: 2510474
• DOI: http://doi.org/10.14316/pmp.2020.31.4.135

Abstract

Purpose
Gafchromic films for proton dosimetry are dependent on linear energy transfers (LETs), resulting in dose underestimation for high LETs. Despite efforts to resolve this problem for singleenergy beams, there remains a need to do so for multi-energy beams. Here, a bimolecular reaction model was applied to correct the under-response of spread-out Bragg peaks (SOBPs).
Methods
For depth-dose measurements, a Gafchromic EBT3 film was positioned in water perpendicular to the ground. The gantry was rotated at 15° to avoid disturbances in the beam path.A set of films was exposed to a uniformly scanned 112-MeV pristine proton beam with six different dose intensities, ranging from 0.373 to 4.865 Gy, at a 2-cm depth. Another set of films was irradiated with SOBPs with maximum energies of 110, 150, and 190 MeV having modulation widths of 5.39, 4.27, and 5.34 cm, respectively. The correction function was obtained using 150.8-MeV SOBP data. The LET of the SOBP was then analytically calculated. Finally, the model was validated for a uniform cubic dose distribution and compared with multilayered ionization chamber data.
Results
The dose error in the plateau region was within 4% when normalized with the maximum dose. The discrepancy of the range was <1 mm for all measured energies. The highest errors occurred at 70 MeV owing to the steep gradient with the narrowest Bragg peak.
Conclusions
With bimolecular model-based correction, an EBT3 film can be used to accurately verify the depth dose of scanned proton beams and could potentially be used to evaluate the depth-dose distribution for patient plans.

Keyword

Proton beam; Spread-out Bragg peak; Depth dose; Linear energy transfer; EBT3 film

Figure

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  Fig. 1 (a) Experimental setup of the film fixed by an in-house-designed film holder in a water tank, (b) Gantry tilted as 3 degree, (c) 1 cm of the gap distance between water surface and the edge of the film.

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  Fig. 2 Scanned film irradiated by 112-MeV beams, showing pristine Bragg peaks with six different dose intensities.

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  Fig. 3 Analytically calculated linear energy transfer (LET) of spread-out Bragg peaks (SOBPs) with maximum energies of 110, 150, and 190 MeV.

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  Fig. 4 Dose distribution of cubic beam irradiated obliquely with a gantry angle of 15° in treatment planning system.

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  Fig. 5 Characteristic curve of 112-MeV pristine Bragg peak for six different dose intensities at a 2-cm depth.

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  Fig. 6 (a) D1/2 calculated using spread-out Bragg peak (SOBP) data with a maximum energy of 150 MeV represented along the water depth and the fitted curve based on a fifth-degree Gaussian curve. (b) D1/2 converted to a function along the linear energy transfer (LET).

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  Fig. 7 Pristine Bragg peaks (70, 92, 112, 150, 170, and 190 MeV from left to right) measured using the EBT3 film (corrected; solid line) and those measured using the MLIC (Zebra; inverted triangles).

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  Fig. 8 Depth profile of spread-out Bragg peak (SOBP) measured using the EBT3 film (uncorrected shown as dotted blue line and corrected as solid green line) compared with the multilayer ionization chamber (MLIC) data (purple ‘x' symbol) for maximum energies of 110, 150, and 190 MeV.

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  Fig. 9 Depth profile of the cubic treatment planning system (TPS) plan measured with film compared with the TPS data.

Reference

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