ISO/TR 24422:2022
(Main)Development of a water equivalent phantom to measure the physical characteristics of specific radiosurgery treatment devices
Development of a water equivalent phantom to measure the physical characteristics of specific radiosurgery treatment devices
A water phantom is used to ensure the accurate measurement of absorbed dose delivered by a radiation therapy machine as well as standardizing the dose distribution produced by the radiation therapy device. This document describes a detailed procedure for the construction and calibration of a polystyrene phantom and the results of its use in measuring the absorbed dose profile around the mechanical centre of a radiosurgery medical device, the full width at half maximum (FWHM) of the field and the physical penumbra at the mechanical centre, as well as the associated uncertainties. According to IAEA TRS-483 document, the most common design recommended in Gamma Knife® system is a hemisphere atop a water filled or compact polystyrene cylinder, and when using a polystyrene phantom, the measurement depth of the absorbed dose to water is reported to be the centre of the hemisphere with the radius of 8 cm. This document mainly describes the procedure for measuring the absorbed dose distribution around the mechanical centre of Gamma Knife® and obtaining the FWHM and penumbra from it. The developed phantom is made of polystyrene and has a hemispherical shape in accordance with the design suggested in IAEA TRS-483. This type of phantom is specific and adapted only for the Gamma Knife® radiosurgery facilities (PerfexionTM and IconTM models) and does not apply to general dosimetry protocols in radiotherapy facilities that use a small radiation field to treat a disease such as LINAC or Cyberknife. Considering that the type of medical device corresponds to treatment using external beam radiotherapy following small static fields, this technical report follows the recommendations published in the IAEA TRS‑483.
Développement d’un fantôme équivalent eau pour le mesurage des caractéristiques physiques de modèles spécifiques de dispositif de traitement de radio-chirurgie
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 24422
First edition
2022-12
Development of a water equivalent
phantom to measure the physical
characteristics of specific
radiosurgery treatment devices
Développement d’un fantôme équivalent eau pour le mesurage des
caractéristiques physiques de modèles spécifiques de dispositif de
traitement de radio-chirurgie
Reference number
ISO/TR 24422:2022(E)
© ISO 2022
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ISO/TR 24422:2022(E)
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ISO/TR 24422:2022(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Phantom manufacturing .3
4.1 Phantom description . 3
4.2 Positioning of the inner inserts with respect to the mechanical centre . 5
4.3 Electron density calculation and determination of polystyrene phantom radius. 6
5 Measurement methods and results . 7
5.1 Beam quality correction factor . . 7
5.2 Measurement of the beam profile . 7
5.3 Measurement of FWHM – physical penumbra and sharpness . 8
5.4 Uncertainty analysis . 10
Bibliography .11
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ISO/TR 24422:2022(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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electrotechnical standardization.
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described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
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This document was prepared by Technical Committee ISO/TC 85, Nuclear energy, nuclear technologies,
and radiological protection, Subcommittee SC 2, Radiological protection.
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TECHNICAL REPORT ISO/TR 24422:2022(E)
Development of a water equivalent phantom to measure
the physical characteristics of specific radiosurgery
treatment devices
1 Scope
A water phantom is used to ensure the accurate measurement of absorbed dose delivered by a radiation
therapy machine as well as standardizing the dose distribution produced by the radiation therapy
device.
This document describes a detailed procedure for the construction and calibration of a polystyrene
phantom and the results of its use in measuring the absorbed dose profile around the mechanical centre
of a radiosurgery medical device, the full width at half maximum (FWHM) of the field and the physical
penumbra at the mechanical centre, as well as the associated uncertainties.
®
According to IAEA TRS-483 document, the most common design recommended in Gamma Knife
system is a hemisphere atop a water filled or compact polystyrene cylinder, and when using a
polystyrene phantom, the measurement depth of the absorbed dose to water is reported to be the
centre of the hemisphere with the radius of 8 cm.
This document mainly describes the procedure for measuring the absorbed dose distribution around
®
the mechanical centre of Gamma Knife and obtaining the FWHM and penumbra from it. The developed
phantom is made of polystyrene and has a hemispherical shape in accordance with the design suggested
in IAEA TRS-483.
®
This type of phantom is specific and adapted only for the Gamma Knife radiosurgery facilities
TM TM
(Perfexion and Icon models) and does not apply to general dosimetry protocols in radiotherapy
facilities that use a small radiation field to treat a disease such as LINAC or Cyberknife.
Considering that the type of medical device corresponds to treatment using external beam radiotherapy
following small static fields, this technical report follows the recommendations published in the
IAEA TRS-483.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
IAEA TRS-483, Dosimetry of Small Static Fields Used in External Beam Radiotherapy: An International
Code of Practice for Reference and Relative Dose Determination
AAPM TG-178, Recommendations on the Practice of Calibration, Dosimetry, and Quality Assurance for
Gamma Stereotactic Radiosurgery
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IAEA TRS-483 and AAPM TG-178
apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
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ISO/TR 24422:2022(E)
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
radiosurgery
medical procedure using a high single dose of radiation, directed to a stereotactically defined
intracranial volume allowing minimally invasive treatment of benign and malignant tumors and other
pathologies
Note 1 to entry: The term “radiosurgery” is also used for treatments comprising up to several fractions.
3.2
®
Gamma Knife
device used to treat benign and malignant tumors and other pathologies by delivering high-intensity
60 1)
Co external beam radiation therapy in a manner that focuses the radiation over a small volume
®
Note 1 to entry: Gamma Knife therapy, like all radiosurgery, uses doses of radiation sufficient to kill cancer cells
or shrink tumors, delivered precisely to avoid damaging healthy brain tissue. The device aims gamma radiation
®
to a target point in the patient's brain. Gamma Knife radiosurgery is able to accurately and precisely focus many
®
beams of gamma radiation to converge on one or more tumors. The Gamma Knife type C and its predecessors
60
consist of a heavily shielded assembly in the internal collimator containing 201 Co sources of approximately
1,1 TBq, each placed in a hemispherical array and interchangeable collimator located in hemispherical external
® TM TM
helmet. On the other hands, Gamma Knife Perfexion and Icon (later model) has no such external helmet
60
but 192 Co sources are arranged internally with 3 different collimator sizes on 8 independent sectors of
24 elements. Each individual beam is of relatively low intensity, so the radiation has little effect on intervening
brain tissue and is concentrated only at the lesion itself. An ablative dose of radiation is thereby delivered to the
tumor in one treatment session, while surrounding brain tissues are relatively spared.
3.3
calibration
operation that, under specified conditions, in a first step, establishes a relation between the quantity
values with measurement uncertainties provided by measurement standards and corresponding
indication with associated measurement uncertainties and, in a second step, uses this information to
establish a relation for obtaining a measurement result from an indication
3.4
machine specific reference field
field used when the absolute value of the absorbed dose to water is measured in a small field external
beam radiotherapy facility
TM TM
Note 1 to entry: According to the IAEA TRS-483, 16 mm collimator field for Perfexion and Icon is a machine
specific reference field.
3.5
beam quality correction factor
ratio of the calibration coefficient, which is used to convert the measured electric charge to the absorbed
dose to water in the machine specific reference field to the absorbed dose to water calibration factor
obtained in the reference calibration condition
ff,
msrref
Note 1 to entry: This factor corresponds to the beam quality correction factor, k , of IAEA TRS-483.
QQ,
msr 0
3.6
ionization chamber
ionization detector consisting of a chamber filled with a suitable gas, typical air, in which an electric
field applied between two electrodes, that is insufficient to induce gas multiplication, is provided for
the collection at the electrodes of charges associated with the ions and the electrons produced in the
sensitive volume of the detector by the ionizing radiation
® TM TM ®
1) Gamma Knife type C, Perfexion and Icon are the three different models of Gamma Knife radiosurgery
facilities supplied by Elekta Ltd., Sweden. This information is given for the convenience of users of this document
and does not constitute an endorsement by ISO of the product named.
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ISO/TR 24422:2022(E)
3.7
radiochromic film
colourless transparent films that give permanent coloured images on exposure to ionizing radiation
3.8
mechanical centre
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point on which multiple Co gamma radiation beams converge, supposed to coincide with a radiological
centre which was determined by the measured dose distribution
3.9
physical penumbra
spatial region surrounding the centre point of the lateral dose profile, characterized by the lateral
[1]
distance between two specified isodose curves (80 % and 20 %) at a specified depth
Note 1 to entry: This definition generally applies to a broad beam, but it is also useful for the case of the small
beam whose value of the physical penumbra is 3 mm to 10 mm, determined by the dose distribution obtained
® TM TM
using the 16 mm collimator of Gamma Knife Perfexion and Icon .
Note 2 to entry: Isodose curve is a line that joins data points of the same dose value in a dose plot such as a 2D
contour plot.
Note 3 to entry: An 80 % isodose curve indicates the data points where the dose values dropped to 80 % of the
maximum dose value at the centre of the radiation field.
4 Phantom manufacturing
4.1 Phantom description
A water equivalent plastic phantom was build according to IAEA TRS-483 recommendations.
The phantom is constructed from polystyrene water equivalent material and is customized for two
® TM TM
models of Gamma Knife radiosurgery devices (Perfexion and Icon ) for the measurement of
absorbed dose to water and several associated physical characteristics.
As described in the scope, the polystyrene phantom is designed and constructed according to the
®
reference conditions for determining absorbed dose to water of Gamma Knife , so this phantom has a
®
limitation that it can only be applied to the Gamma Knife .
The phantom has a hemi-spherical shape. The whole phantom consists of an outer and an inner part.
While the outer part is firmly attached to the Leksell Gamma frame (Elekta AB, Stockholm, Sweden),
the inner part can accommodate three different cylindrical inserts, one for hosting an ionization
chamber and the two others for film-hosting. These three inserts are easily interchangeable depending
on the type of measurement. The inner part is well fitted to the outer part so that the whole phantom
is approximated to be homogenous. One of the film-hosting inner insert was designed to set a film in
the xy-plane (axial plane) of the Leksell stereotaxic coordinate system, and the other can set a film in
the xz-plane (sagittal plane). The thickness of the outer part is 5 mm, and the equivalent water depth
(EWD) corresponding to this value is 5,08 mm. The cylindrical insert is constructed to have a EWD of
8 cm from the surface of the outer part to the centre of the insert, including the EWD of the outer part.
The schematic drawings of the polystyrene phantom with the ionization chamber insert and the film
inserts for the measurement of absorbed dose and dose profile are shown in Figure 1. In Figure 2 are
given photographs of the inner inserts.
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ISO/TR 24422:2022(E)
a) Film XY, YZ, ZX – Direction b) PTW 31010 c) Exradin A16
d) Phantom dimension e) Hexagonal plate for attaching the outer part of the phantom
to Leksell Gamma Knife frame
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ISO/TR 24422:2022(E)
Key
1 ionometric measurement of absorbed dose to water
2 radiochromic film measurement of dose distribution
3 radiochromic film measurement of dose distribution
a
M5 Volt.
b
5 pi Screw.
Figure 1 — Schematic diagram of the polystyrene phantom with different dosimeter inserts
and its technical drawings (the unit of the length is mm)
a) Photographs of th
...
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