Qing Liang, PhD Medical Physicist Mercy Health System, Janesville, WI Radiation Dosimeters Qing Liang, PhD Medical Physicist Mercy Health System, Janesville, WI

Learning Objectives Overview of different types of dosimeters Understand the radiation-induced process and the meaning of measured signal Learn about the clinical applications and advantages/issues with each type of dosimeter

Ionization Chamber

Ionization Chambers Work with electrometer (high V, charge collection) Measure radiation dose from charge/current Requires calibration

Ionization Chambers Thimble ionization chamber Farmer typed chamber

Ionization Chambers Parallel plate chamber (electron dosimtery)

Ionization Chambers Spherical chamber: Gamma Knife

Ionization Chambers Well chamber: brachytherapy

Ionization Chambers Well chamber: brachytherapy Sweet spot (maximum response)

Ionization Chambers Pencil chamber: CT dosimetry for CTDI (CT dose index) measurements 10 cm nominal length

Chamber Reading Corrections Pion: recombination correction PTP: air density correction Ppol: polarity correction Pelec: electrometer calibration

Ionization Chambers: Other Issues Stem effect: response from stem not thimble Depends on chamber design (guarding) Well guarded chamber: <0.1% Guarded chamber: 0.1-0.3% Unguarded chamber: 0.3-0.6% Function of energy and beam size

Ionization Chambers: Other Issues Leakage: Source: chamber, cable, electrometer Should measure leakage as a system before taking measurements Leakage in the chamber is generally 1 to 10fA for a good chamber

Ionization Chambers: Other Issues Energy response: It is desired to have a uniform energy response for all energies Farmer chamber

TLD

Thermoluminescent Dosimeter (TLD) Advantages Small size Wider linear dose response range Reusable

Luminescence Process Physics

Thermoluminescent Dosimeter (TLD) Luminescence induced by heat Impurities play important roles. TLD readout process

TLD Glow Curve Time-temperature profile Glow curve composed with difference peaks

Thermoluminescent Dosimeter (TLD) Supralinearity Materials: LiF:Mg,Ti LiF:Mn,Cu,P CaF2:Mn Li2B4O7:Mn Dose response linearity: function of matieral Dose response linearity

Energy Dependance of TLD

How to use TLD? Annealing: 400-450 oC for 1hour + 80 oC for 24 hours to reset the trap structure and eliminate any electrons in residual traps Irradiation: wait 24 hours to let low temperature traps fade Readout: to get the signal from high temperature traps Annealing Irradiation Readout

TLD Applications Radiation therapy (external beam and brachy therapy): Dose verification In-vivo dosimetry: patient skin Dose rate constant determination for BT

TLD Applications Diagnostic physics Health physics Personnel exposure monitoring Neutron measurements

OSLD

Optically Stimulated Luminescence Dosimeter (OSLD) Instead of using heat, luminescence is stimulated with optical light Material: Al2O3:C Used in space by NACS Used for personnel exposure monitoring Now popular in radiation therapy and diagnostic physics

OSLD Readout system

OSLD Dose response linearity: supralinearity

OSLD Dose rate dependence:

OSLD Energy dependence f(Q): f(Q) Al2O3 LiF Al2O3/LiF

OSLD: signal fading

Characteristics of OSLD

Applications of OSLD External beam and brachytherapy Output verification In-vivo dosimetry IROC uses OSLD to perform output audit

DIODE

Diode Dosimeter Designs Cylindrical Flat

Diode Dosimeter Theory Semiconductor: narrow energy band width n type: Doping “donor” impurity to produce additional electrons P type: Doping “acceptor” impurity to produce additional holes

Diode Dosimeter Theory Diode: p-n junction made by doping the semiconductor with donors and acceptors at adjacent junctions Diode: no external voltage applied

Diode Dosimeter Theory n-type diode: high doping level of n-type semiconductors, and low doping level of p-type conductors p-type diode: high doping level of p-type semiconductors, and low doping level of n-type conductors

Dose Rate Dependence p-type is less dependent on dose rate then n-type

Dose Rate Dependence

SSD Dependence Related to dose rate

Accumulated Dose Dependence Sensitivity of diode depends on accumulated dose. Theory: crystal lattice gets changed with dose

Temperature Dependence

Energy Dependence MV beams

Angular Dependence

Application of Diodes In-vivo dosimetry Small field dosimetry TBI (Total Body Irradiation) Dose verification Small field dosimetry

MOSFET

MOSFET Metal Oxide Semiconductor Field Effect Transistor Capable of dose measurements immediately after irradiation or can be sampled in predefined time intervals (on-line dosimetry)

Types of MOSFET

Dual-MOSFET

MOSFET Products Mobile system Wireless system

MOSFET Products Implanted MOSFET detector: Dose Verified System

MOSFET: Temperature Dependence TN dual-MOSFET-dual-bias detector: temperature independent Other types of MOSFET: Temperature dependent DVS MOSFET 3.3% more sensitive for 37 oC Calibration should be performed at 37 oC (body temp)

MOSFET: Energy Dependence

MOSFET: Dose Linearity Dual-bias MOSFET

MOSFET: Accumulated Dose

MOSFET: Angular Dependence Depends on design Can be different for different energies

MOSFET: Angular Dependence To determine angular dependence

Characteristics of MOSFET

Clinical Applications MOSFET

Clinical Applications MOSFET In-vivo dosimetry

Radiographic Film

Radiographic Film

Radiographic Film

Radiographic Film Film development process

Radiographic Film: Temp. Dependence

Radiographic Film: Temp. Dependence

Radiographic Film: Optical Density OD can be measured with film scanner/digitize or densitometer.

Radiographic Film: Dose Rate Dependence

Radiographic Film: Energy Dependence

Radiographic Film: Effect of Depth and Field Size

Radiographic Film: Clinical Application

Radiographic Film: Summery

Radiochromic Film

Radiochromic Film Radiochromic film consists of a single or double layer of radiation-sensitive organic microcrystal monomers, on a thin polyester base with a transparent coating

Radiochromic Film Color of the radiochromic films turns to a shade of blue upon irradiation. Darkness of the film increases with increasing absorbed dose. No processing is required to develop or fix the image.

Radiochromic Film Flat bed film scanner

Radiochromic Film: Dose Linearity Depends on scanning instruments

Radiochromic Film: Applications

Radiochromic Film: Summery Advantage: high resolution, no processing necessary

Chemical Dosimeters

Chemical Dosimeters Fricke

Chemical Dosimeters Alanine

Chemical Dosimeters Polymer Gel

Others Diamond detector Scintillator/plastic dosimeter Optical fiber dosimeter ….

Summery

Thank you!