Dosimetry Principle

DOSIMETRY PRINCIPLES IN CALCULATING PATIENT DOSES

RADIATION SCIENCES

LEARNING OBJECTIVES

• At the end of the session, students should be able to explain:
• Dosimetry and dosimeter
• Properties of dosimeters
• Ionization chamber dosimetry systems
• Film dosimetry
• Luminescence dosimetry
• Semiconductor dosimetry

Function of dosimeters

Dosimetry

Dosimetry is the measurement, calculation and assessment of the ionizing radiation absorbed by an object, usually the human body

DOSIMETRY

Deals with the measurement of the absorbed dose or dose rate resulting from the interaction of ionizing radiation with matter.

• -It also refers to the determination of radiologically relevant quantities such as:
• Exposure
• Kerma – kinetic energy released in matter
• Fluence

etc

DOSIMETER

Dosimeter can be defined generally as any device that is capable of providing a reading ‘r’ that is a measure of the absorbed dose ‘D’, deposited in its sensitive volume V by ionizing radiation

Dosimterer

• Dosimeter is a device that measures directly or indirectly
• Exposure
• Kerma
• Absorbed dose
• Equivalent dose
• Or other related quantities.

The dosimeter along with its reader is referred to as a Dosimetry System.

Properties of dosimeters

• A useful dosimeter exhibits the following properties:
• High accuracy and precision
• Linearity of signal with dose over a wide range
• Dose and dose rate dependence
• Energy response
• Small directional dependence
• High spatial resolution

Large dynamic range

Accuracy and Precision

Accuracy specifies the proximity of the mean value of a measurement to the true value (How a close measurement is to the correct value).

Precision specifies the degree of reproducibility of a measurement.

The accuracy and precision associated with a measurement is often expressed in terms of its uncertainty.

Uncertainty shows the range within which the true value is likely to be, considering errors in the measurement process

The standard deviation of the mean value is used to express the uncertainty for the best estimate:

Characteristics of Dosimeters

• Linearity
• The dosimeter reading should be linearly proportional to the dosimetric quantity.
• Beyond a certain range, usually there is non linearity.

This effect depends on the type of dosimeter.

Ideally, the dosimeter reading M should be linearly proportional to the dosimetric quantity Q.

However, beyond a certain dose range a non-linearity sets in. The linearity range and the non-linearity behavior depend on the type of dosimeter and its physical characteristics

In Fig: Curve A first exhibits linearity with dose, then a supralinear behavior, and finally saturation.

Curve B first exhibits linearity and then saturation at high doses In general, a non-linear behavior should be corrected for.

A dosimeter and its reader may both exhibit non-linear characteristics, but their combined effect could produce linearity over a wider range

Or dose dependence

Dosimetry Principle

Ideally, the response of a dosimetry system M/Q at two different dose rates ((dQ/dt)1 and (dQ/dt)2) should remain constant.

In reality, the dose rate may influence the dosimeter readings and appropriate corrections are necessary, for example: recombination corrections for ionization chambers in pulsed beams

Energy response/dependency

The response of a dosimetry system M/Q is generally a function of radiation beam quality (energy).

Since the dosimetry systems are calibrated at a specified radiation beam quality (or qualities) and used over a much wider energy range, the variation of the response of a dosimetry system with radiation quality (called energy dependence) requires correction.

DIRECTIONAL DEPENDENCE

The variation in response of a dosimeter with the angle of incidence of radiation is known as the directional, or angular, dependence of the dosimeter.

Dosimeters usually exhibit directional dependence, due to their constructional details, physical size and the energy of the incident radiation.

Dosimetry Principle

Directional dependence is important in certain applications, for example in in vivo dosimetry while using semiconductor dosimeters.

Therapy dosimeters are generally used in the same geometry as that in which they are calibrated.

SPATIAL RESOLUTION

• The quantity absorbed dose is a point of quantity

Ideal measurement requires a point-like detector Measurement result can be attributed to a point within the volume referred to as the effective point of measurement

COMMON DOSIMETERS

• Ionization chamber
• Film dosimeter
• Luminescence dosimeter

Semiconductor

IONIZATION CHAMBER

Are-gas filled spaces between two electrodes, typically parallel plates or a hollow cylinder and a thin wire They operate at a saturation voltage, and the current passing through them can be recorded by a voltmeter

Types of ionization chamber

• Free air ionization chamber
• Thimble ionization chamber
• Plane-parallel ionization chamber
• Well-type chamber
• Pocket ionization chamber
• Extrapolation ionization chamber
• Geiger-Muller counter

Proportional counter

Ionization Chamber

• Cylindrical (thimble) ionization chamber
• Most popular design
• Independent of radial beam direction
• Typical volume between 0.05 -1.00 cm3
• Typical radius ~2-7 mm
• Length~ 4-25 mm
• Thin walls: ~0.1 g/cm2

Used for: electron, photon, proton, or ion beams.

Thimble Chamber

PARALLEL PLATE CHAMBER (ELECTRON DOSIMETRY)

Consist of two plane walls, one serving as an entry window and polarizing electrode and the other as the black wall and collecting electrode, as well as the guard system ring system.

The black wall is usually a block of conducting plastic or a non-conducting material with a thin conducting layer of graphite forming the collecting electrode and the guard ring system on top.

Plane-Parallel Ion Chamber

• (1) Polarizing electrode
• (2) Measuring electrode
• (3) Guard ring
• (a) height (electrode
• separation) of the air
• cavity
• (d) diameter of the
• polarizing electrode
• (m) diameter of the
• collecting electrode

(g) width of the guard ring.

Uses of parallel-plate chamber

• Parallel-plate (Plane-parallel) chamber is recommended for:
• Dosimetry of electron beams with energies below 10 MeV.
• Depth dose measurements in photon and electron beams.
• Surface dose measurements of photon beams.
• Depth dose measurements in the build-up region of megavoltage

photon beams.

Extrapolation Chambers

Extrapolation chambers are parallel-plate chambers with a variable electrode separation.

They can be used in absolute radiation dosimetry (when embedded into a tissue equivalent phantom).

• Cavity perturbation for electrons can be eliminated by:
• Making measurements as a function of the cavity thickness

Extrapolating electrode separation to zero.

Using this chamber, the cavity perturbation for parallel plate chambers of finite thickness can be estimated.

How ionization chamber work

When ionizing radiation passes through a gas-filled chamber, it ionizes the gas atoms, creating ion pairs (positive and free electrons) A strong electric field is applied across the chamber, which accelerates the ions and electrons towards the oppositely charged electrodes

The movement of these charged particles creates a small electric current that is proportional to the amount of ionizing radiation

By measuring this current, the radiation dose can be calculated

Function of ionization chamber

• Generaly, are used;
• to measure radiation/radioactive samples

To determine dose rates from gamma radiation

FILM DOSIMETRY

A film dosimeter, also known as a film badge, is a device used to measure and record the level of ionizing radiation exposure a person receives over time.

It consists of photographic film contained in a small badge, which is worn by the person being monitored for radiation exposure.

Film Dosimetry

• Radiochromic film
• A new type of film well suited for both radio‐therapy and diagnostic dosimetry.
• This film type is self‐developing, requiring Neither a developer

Nor a fixer.

Principle: Contains a special dye that is polymerized and develops a blue color upon exposure to radiation.

Similarly to radiographic film, the radiochromic film dose response is determined with a suitable densitometer.

The most commonly used radiochromic film type is the GafChromic film.

How film dosimeter works

-The film is placed in a badge or holder and worn by the individual or placed in an area where radiation exposure is expected -After exposure, the film is developed in a darkroom using chemicals similar to those used for photographic film -The developed film is analyzed using a densitometer, which measures the degree of darkening -The more dark means individual has received more doses -The densitometer reading is then converted into a radiation dose using a calibration curve

Advantages of film dosimeter

-Simple and inexpensive

-Sensitive to a wide range of doses (can measure a wide range of doses from low levels to high levels) Disadvantages of film dosimeter

• -Time consuming

-Can be affected by environmental conditions (e.g heat, humidity, light, etc) -They have limited spatial resolution i.e they cannot measure radiation exposure in small areas

LUMINESCENCE DOSIMETRY

Is a technique used to measure the amount of radiation exposure by utilizing materials that emit light when stimulated This light emission is known as LUMINESCENCE, which is proportional to the absorbed radiation dose

If the exciting agent is light, the phenomena is called OPTICALLY STIMULATED LUMINESCENCE DOSIMETRY (OSD)

OSLD materials (often aluminium oxide doped with carbon) also trap electrons upon radiation exposure Instead of heating, they are stimulated with light to release the trpped electrons, causing them to emit light

SEMICONDUCTOR DOSIMETRY

Are type of radiation detector that utilize semiconductor materials to measure ionizing radiation

HOW SEMICONDUCTOR DIODE WORK

When ionizing radiation interacts with the semiconductor material, it creates electron-hole pairs An electric field is applied across the semiconductor, causing the egenareted charge carriers to drift towards oppositely charged electrodes

Dosimetry Principle

The movement of these charge carriers generates an electrical signal, which is proportional to the energy deposited by the radiation The electrical signal is processed and converted into a radiation dose measurement

TYPES OF SEMICONDUCTOR DOSIMETERS

• Diode dosimeters
• MOSFET dosimeters

Silicon diode detectors