Prog Med Phys.  2021 Dec;32(4):122-129. 10.14316/pmp.2021.32.4.122.

Design of Multipurpose Phantom for External Audit on Radiotherapy

• Lim S

• KMID: 2524158
• DOI: http://doi.org/10.14316/pmp.2021.32.4.122

Abstract

Purpose
This study aimed to design a multipurpose dose verification phantom for external audits to secure safe and optimal radiation therapy.
Methods
In this study, we used International Atomic Energy Agency (IAEA) LiF powder thermoluminescence dosimeter (TLD), which is generally used in the therapeutic radiation dose assurance project. The newly designed multipurpose phantom (MPP) consists of a container filled with water, a TLD holder, and two water-pressing covers. The size of the phantom was designed to be sufficient (30×30×30 cm³ ). The water container was filled with water and pressed with the cover for normal incidence to be fixed. The surface of the MPP was devised to maintain the same distance from the source at all times, even in the case of oblique incidence regardless of the water level. The MPP was irradiated with 6, 10, and 15 MV photon beams from Varian Linear Accelerator and measured by a 1.25 cm³ ionization chamber to get the correction factors. Monte Carlo (MC) simulation was also used to compare the measurements.
Results
The result obtained by MC had a relatively high uncertainty of 1% at the dosimetry point, but it showed a correction factor value of 1.3% at the 5 cm point. The energy dependence was large at 6 MV and small at 15 MV. Various dosimetric parameters for external audits can be performed within an hour.
Conclusions
The results allow an objective comparison of the quality assurance (QA) of individual hospitals. Therefore, this can be employed for external audits or QA systems in radiation therapy institutions.

Keyword

External audit; Radiation therapy; Phantom; Photon; Thermoluminescence dosimeter

Figure

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  Fig. 1 IAEA capsulated 155 mg of LiF powder TLD (a) the capsule measures 20 mm inner length, 3 mm inner diameter, and 1 mm wall thickness and (b) TLD in TLD holder. IAEA, International Atomic Energy Agency; TLD, thermoluminescence dosimeter.

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  Fig. 2 Component of the multipurpose phantom. (a) Water container. (b) Thermoluminescence dosimeter holder. (c) Cover for normal incident. (d) Cover for oblique incidence.

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  Fig. 3 Water-pressing cover can be selected for (a) normal incidence setup and (b) oblique incidence setup. Regardless of the water level, the source to surface distance is always constant because of the water-pressing cover. Since the two covers are designed to have the same volume when submerged in water, the water level is the same.

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  Fig. 4 The multipurpose phantom was designed to allow computed tomography (CT) scans. (a) Sagittal CT image without pressing cover: moving couch-induced slopping of water during helical CT scan causes beat phenomenon in CT image. (b) Sagittal CT image water-pressing cover: water-pressing cover prevents the water from slopping during helical CT scan.

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  Fig. 5 Location of TLDs in multipurpose phantom. The numbers from ① to ⑨ indicate the location of the TLD. Location of ① is at 5 cm depth, location of ②–⑥ is at 10 cm depth, and location of ⑦–⑨ is at 20 cm depth, and location of ⑧ and ⑨ is used for inhomogeneity. TLD, thermoluminescence dosimeter.

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  Fig. 6 Modeling of multipurpose phantom using MCNPX. (a) Front view. (b) Lateral view.

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  Fig. 7 Relative dose of each point measured by ionization chamber. (a) Relative dose for normal incident. (b) Relative dose for oblique incident. (c) Relative dose for oblique incident with a 30° wedge. MPP, multipurpose phantom.

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  Fig. 8 Correction factors plotted for each machine and energy.

Reference

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