Study of Using Optical Stimulated Luminescence (OSL) in Clinical 3DCRT and IMRT Planning QA

Yang Wang ^1,3, Carr Munoz-Ferrada ^1,2, Vivien Munoz-Ferrada ^1,2, Bin Hu¹, William Zealey¹

^1. University of Wollongong, Sydney Australia

². Gammasonic Research Institute, Sydney Australia

³. Radiation Oncology, St. Vincent’s Private Hospital Sydney, Australia

Optical Stimulated Luminescence (OSL) is a material with luminescent properties in a crystal structure that can absorb radiation energy and temporarily keep the mobile electrons in an ionization trap. The trapped electrons can be stimulated by a laser light then become a countable amount of luminescence. The amount of luminescence is corresponding to the radiation dose irradiated to the material. All procedure performed by a designed laser reader we call this process is OSL dosimetry.

First report of OSL was published by Roy et al (1997). The study of using OSL for radiation dose measurement in radiotherapy was proposed by Huston et al in 1998 (In Vivo Radiotherapy Dose Monitoring System). In recent years, OSL is widely used in radiation safety monitoring by stepping forward to replace TLD technique. In radiotherapy clinical study, most research reports were focused on In-Vivo real time dosimetry, as well the depth dose measurement with point by point dose measurement in solid phantom (Marianne et al 2004 Phys. Med. Biol.). A common OSL material used is aluminium oxide (Al₂O₃). This crystal structured material in nowadays is not only can made into small peaces for point dose measurement, also a shape as film, thing (0.2mm – 0.4mm) flexible, and water proof.

This clinical study in University of Wollongong is focused on radiotherapy using sheet shaped OSL. The tasks include characteristic study for how the material response to different radiation beams, measurement technology development, and clinical 3DCRT and IMRT plan checks. In characteristic study, the experiment included the beam/energy dependence, dose response range and linearity, signal reliability / reproducibility and noise level, and the impacts of art-facts. The development of measurement technology includes research on 2D X-Y scanning process, 3DCRT and IMRT phantom design for OSL measurement and software development for clinical dose distribution analysis.

The characteristic result from the analysis is showing that with different type of beam and energy, the dose response and linearity to OSL are similar. The perfect dose linear range is from 0cGy to up to 350cGy. The reasonable linear region could up to 1000cGy. The experiment compared low dose rate (I-125 seeds) and 6MV photon beam. The reading reproducibility and reliability tested by giving repeat 5 readings at 0cGy (background) and 1000cGy material exposed. The result shows maximum 2.5% standard deviation over average for no exposure background material, but maximum 0.5% and average 0.3% for the readings from the OSL with given dose 1000cGy.

From the experiment result shows that the characteristic of OSL as a new clinical dosimetry tool is capable to take the place in real time dosimetry and planning QA checks. Compare OSL with TLD technique, although the noise level is no showing significant differences, the processing on saving time can be very big. Compare OSL sheet with EDR-2 film, OSL has far better dose response range and linearity, low impact from art-fact. Compare with small volume ionization chamber measurement in an IMRT phantom, OSL could provide better flexibility on setting and with very small size of reading point. The further study of this project is towards to the processing of 2D dose scan analysis in 3DCRT and IMRT phantom. The OSL dosimetry in radiotherapy is intend to combine point dose measurement, In-Vivo and iso-dose analysis into one as new QA technology tool in radiotherapy clinical work.