ISO/TR 24422:2022

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

• Status: Published
• Publication Date: 15-Dec-2022
• ICS: 13.280 - Radiation protection
• Technical Committee: ISO/TC 85/SC 2 - Radiological protection
• Drafting Committee: ISO/TC 85/SC 2 - Radiological protection
• Current Stage: 6060 - International Standard published
• Start Date: 16-Dec-2022
• Due Date: 12-Jun-2022
• Completion Date: 16-Dec-2022

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)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2022

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

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Published in Switzerland
© ISO 2022 – All rights reserved
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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

iii
© ISO 2022 – All rights reserved
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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

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

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

editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www.iso.org/patents).

Any trade name used in this document is information given for the convenience of users and does not

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URL: www.iso.org/iso/foreword.html.

This document was prepared by Technical Committee ISO/TC 85, Nuclear energy, nuclear technologies,

and radiological protection, Subcommittee SC 2, Radiological protection.
© ISO 2022 – All rights reserved
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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
© ISO 2022 – All rights reserved
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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

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

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

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
M5 Volt.
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
...