The IROC Houston has adopted Optically-Stimulated Luminescence (OSL) technology for periodic audits of treatment unit output
OSL offers numerous advantages over the TLD system that
has been in use at the IROC Houston since 1968
OSL technology provides a number of benefits that will help improve the quality and efficiency of the IROC Houston’s audits. The IROC Houston has acquired MicroStar “InLight” readout devices and a number of “nanoDot” dosimeters manufactured by Landauer Corp. of Glenwood, Illinois.* Evaluations were conducted during 2007-2009, and the technology was introduced into routine audits in July 2009.
The purpose of this website is to explain the IROC Houston’s decision to replace the TLD system with OSL, and to describe some of the characteristics of OSL dosimetry as used in the IROC Houston’s remote audit system. Note that the decision to adopt the Landauer MicroStar OSL system was based on the IROC Houston’s evaluation of the system from the perspective of our remote audit program. It does not imply any endorsement of Landauer Corporation or its products by either the IROC Houston or the IROC Houston’s sponsor. The IROC Houston is grateful to Landauer Corporation for their support in the preparation of this brochure.
More information is available at Landauer’s web site, in a publication in preparation as of this writing, and in two presentations made by the IROC Houston recently (1,2). 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 following lists the reasons for the IROC Houston’ decision to change to the OSL technology, 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. The readout period is only seven seconds, rather than roughly 45 seconds with TLD
d. The 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 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: the 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. The IROC Houston anticipates being able to read OSLs within a few days after irradiation, and sooner than the 10-14 days we have allowed for the fading of TLDs 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.
OSL 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 nanoDot dosimeter.
Further details about Landauer Corporation, the design and use of Landauer nanoDots, and the microStar reader are available at http://www.landauerinc.com and at http://www.osldosimetry.com.
* 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) Ibbott G, Alvarez P, Aguirre JF, Followill D. Implementation of an Optically Stimulated Luminescence (OSL) System for Remote Dosimetry Audits. Radiotherapy and Oncology 92(1):S51, 8/2009.