IROC Houston has adopted Optically-Stimulated Luminescence (OSL) technology for periodic audits of treatment unit output
OSL offers advantages over the TLD system that
has been in use at the IROC Houston since 1968
The purpose of this website is to explain the IROC Houston’s decision to expand the remote dosimetry system to OSL, and to describe some of the characteristics of OSL dosimetry as used in the IROC Houston’s remote audit system.
OSL technology provides a number of benefits that will help improve the quality and efficiency of the IROC Houston’s audits.
The following are characteristics of OSL technology that are relevant to our mailed dosimetry program, and some of the benefits that we believe will result from this change:
1. Simpler readout procedures
a. Optical technology means that no heating is required.
b. Dosimeters are illuminated by a laser to stimulate emission of light that is proportional to absorbed dose.
c. We have chosen a readout period of only seven seconds, rather than roughly 45 seconds with TLD.
d. IROC Houston acquires several readings from each dosimeter, and uses two dosimeters at each measurement location.
e. Acquisition of the signal from the dosimeters at each location thus requires approximately 30 seconds, rather than the 6 minutes needed for TLD.
2. Dosimeters are environmentally stable
a. IROC Houston evaluations show that the dosimeter readings are unaffected by normal variations of temperature and humidity.
b. Dosimeters must be protected from light; as long as the dosimeter cassette is not opened, exposure to room light and sunlight does not affect the reading.
3. Readout is nondestructive
a. Each measurement depletes the dosimeter signal by approximately 0.2% (using the read cycle chosen by the IROC Houston).
b. Multiple readings of each dosimeter are possible.
c. Repeat readings can be made, even weeks or months later.
d. Dosimeters can be archived for repeat analysis (Note: IROC Houston presently has no plans to archive dosimeters for this purpose.
e. Cumulative exposures are possible.
f. Dosimeters can be used to assure reader stability.
4. Minimal fading of signal
a. The correction for fading is smaller than with TLD, is at least as reproducible and occurs over the first few days after irradiation.
b. IROC Houston anticipates being able to read OSL dosimeters within a few days after irradiation, and sooner than the 10-14 days we have allowed for the fading of TLD’s to stabilize.
5. Minimal energy dependence
a. Measurements in full phantom demonstrate essentially no energy dependence.
b. Measurements in the IROC Houston’s mini phantoms show a small dependence on the combination of energy and backscatter that is similar to the dependence of TLD and is well behaved.
The Science and Technology of OSL
The OSL dosimetry technology originated from archeological dating. The method was also used for retrospective dosimetry following the Chernobyl nuclear reactor accident.
OSLD and TLD are quite similar in that both dosimeters respond to the absorption of energy from ionizing radiation by trapping electrons that are excited to the conduction band by the interactions. Unlike TLD, in which electrons are released from the traps by the application of heat, the trapped electrons in an OSL dosimeter are excited back to the conduction band by exposure to green light from a laser or LED light source. The OSL reader contains filters to select the blue light emitted from the dosimeter and capture it by a photomultiplier tube. The quantum nature of the process results in only a small fraction (0.2% under the conditions used by the IROC Houston) of the stored energy to be released through the readout process, so multiple readings can be obtained to improve the uncertainty of the measurement.
OSL dosimeter material is made through a proprietary manufacturing process. High-purity Al2O3 is melted at high temperatures. It is recrystallized to introduce dopants and oxygen vacancies, which form traps in the material. This creates a unique structure that is able to trap electrons that are excited by exposure to ionizing radiation. The crystal can be used itself, or it can be ground into powder and coated onto a base material, as is the case in the dosimeters used by the IROC Houston.
Commissioning of the IROC Houston’s OSL dosimetry system
Over the past two years, the IROC Houston has been in the process of acceptance testing, commissioning and designing of an OSL system for the mailed audit activities that has included acceptance and commissioning of readers, dosimeter characteristics, design of procedures for data taking, dose calculation, data processing, and quality assurance.
The following properties have been determined as part of the characterization of the system:
• Reading cycle
• Depletion rate
• Dependence of depletion rate on reader parameters
• Cumulative dose limit
• Number of readings per dosimeter
• Relative dose response or element correction factor (ECF)
• Variability of ECF with reader
• Variability of ECF with dose
• Dose linearity correction
• Signal fading correction
• Energy/block correction
• Optimal annealing time
• Stability of ECF with annealing
Prior to the introduction of the system to the participant community, it was tested within the M.D. Anderson Cancer Center. A round of tests was then conducted to compare OSLD and TLD irradiations at 11 institutions.
IROC Houston has acquired MicroStar “InLight” readout devices and a number of “nanoDot” dosimeters manufactured by Landauer Corp. of Glenwood, Illinois* after evaluations conducted during 2007-2010. The target date for introduction of this technology into routine audits is January 2010.
More information is available in presentations made by the IROC Houston recently (1-4). These publications describe the IROC Houston’s testing and commissioning of the OSL system, specifically for use under the conditions imposed by the IROC Houston’s auditing procedures. The OSL dosimeters were commissioned for photon and electron beams. Future plans call for commissioning of the OSL system for use in the IROC Houston’s phantoms, and for audits of proton beams.
*The selection of the Landauer “microStar” reader and “nanoDot” dosimeters does not imply an endorsement of Landauer Corporation by either the IROC Houston or its sponsor.
(1) J Homnick, G Ibbott, A Springer, and J Aguirre. Optically Stimulated Luminescence (OSL) Dosimeters Can Be Used for Remote Dosimetry Services, Med. Phys. 35 2994 (2008).
(2) Homnick J, Ibbott G, Springer A, Aguirre F. OSL Dosimeters can be used for Remote Dosimetry. Int J of Radiat Oncol Biol Phys 72(1):S672-3, 2008.
(3) J. Aguirre, P. Alvarez, D. Followill, G. Ibbott, C. Amador, A. Tailor. SU-FF-T-306: Optically Stimulated Light Dosimetry: Commissioning of An Optically Stimulated Luminescence (OSL) System for Remote Dosimetry Audits, the Radiological Physics Center Experience. Medical Physics 36(6):2009.