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. 2022 Mar 29;67(7):10.1088/1361-6560/ac5c8f.
doi: 10.1088/1361-6560/ac5c8f.

Dosimetry of a novel focused monoenergetic beam for radiotherapy

Jamshid Moradi-Kurdestany  1 , Dirk A Bartkoski  1 , Ramesh Tailor  1 , Dragan Mirkovic  1 , Ze'ev Harel  2 , Aharon Bar-David  2 , Michael Kleckner  2 , Shirly Borukhin  2 , Mohammad Salehpour  1
Affiliations

Dosimetry of a novel focused monoenergetic beam for radiotherapy

Jamshid Moradi-Kurdestany et al. Phys Med Biol. .

Abstract

Objective. A novel treatment modality is currently being developed that produces converging monoenergetic x-rays. Conventional application of dosimetric calibration as presented in protocol TG61 is not applicable. Furthermore, the dosimetry of the focal point of the converging beam is on the order of a few millimeters, requiring a high-resolution dosimeter. Here we present a procedure to calibrate radiochromic film for narrow-beam monoenergetic 60 keV photons as well as absolute dosimetry of monoenergetic focused x-rays. A study of the focal spot dose rate after passing through a bone-equivalent material was also done to quantify the effects of heterogeneous materials.Approach.This was accomplished by configuring a polyenergetic beam of equivalent energy using a clinical orthovoltage machine. Calibrated films were then used to perform absolute dosimetry of the converging beam by measuring the beam profile at various depths in water. Main Results.A method for calibrating radiochromic film has been developed and detailed that allows absolute dosimetry of a monoenergetic photon beam. Absolute dosimetry of a focused, mono-energetic beam resulted in a focal spot dose rate of ∼30 cGy min-1at a depth of 5 cm in water.Significance.This work serves to establish a dosimetry protocol for mono-energetic beam absolute dosimetry as well as the use of such a method for measurement of a novel teletherapy modality.

Keywords: energy response; gafchromic film dosimetry; orthovoltage dosimetry.

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Figures

Figure 1.
Figure 1.
Cross-sectional conceptual representation of a Bragg reflecting lens. A polyenergetic diverging beam, such as produced by an x-ray tube, is reflected from a lens structure using Bragg refraction with the central non-reflecting portion of the beam being blocked. The result is a converging monoenergetic beam with a highly localized dose distribution at the focal point. Figure taken from (Bartkoski et al 2021).
Figure 2.
Figure 2.
Sample exposed films for dose-calibration. EPSON 10000XL scanner settings where: transmission mode, positive film, 75 dpi, and 48-bit color. The scanned images were analyzed with ImageJ software using red channel. The left two films were both exposed to 150.4 cGy with the right two films being exposed to 300 cGy.
Figure 3.
Figure 3.
EBT3 calibration curve for an equivalent polyenergetic beam configured using 180 kVp, 15 mA, modified filter No. 6, and HVL 9.0 mm Al. The line represents a third order polynomial fitted to the optical density data.
Figure 4.
Figure 4.
The results show that EBT3 film response is practically independent in the presented energy range, i.e., for a given spatial resolution, negligible difference between beam energies can be seen.
Figure 5.
Figure 5.
Site pattern used for measurement of EBT3 homogeneity.
Figure 6.
Figure 6.
Combed film homogeneity and film-to-film variations for three EBT3 (Lot 10231801) films. Expanded uncertainty (k=2) is shown as a highlighted box around data.
Figure 7.
Figure 7.
Tissue-maximum ratio (TMR) at the focal spot for various depths for a beam produced with the CRnR system using 125 kVp, 28.6 mA source parameters. Polynomial fit and equation are also shown.
Figure 8.
Figure 8.
Depth dose curves for focused monoenergetic 60 kV x-rays in water taken longitudinally down the central axis passing through the focal spot where the different curves represent the spot at different depths.
Figure 9.
Figure 9.
(Left) CT image of a cup of water, polybutylene terephthalate (Teflon), and solid water. CT numbers along the indicated line are plotted. (Right) CT number across the three materials.
See this image and copyright information in PMC

References

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