Implementing a new optically stimulated luminescence dosimetry system

Conheady, C 2013, Implementing a new optically stimulated luminescence dosimetry system, Masters by Research, Applied Sciences, RMIT University.

Document type: Thesis
Collection: Theses

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Title Implementing a new optically stimulated luminescence dosimetry system
Author(s) Conheady, C
Year 2013
Abstract This project documents the attempted implementation of a new optically stimulated luminescence (OSL) system. The system comprised aluminium oxide OSLDs (without cassettes) and a reader. The primary aim was to improve upon the reader’s simple internal dose calculation. The secondary aim was to uncover practical considerations which would guide the safe and accurate use of the OSLD system in the clinic.
The reader’s internal dose calculation assumed a linear relationship between reader dose and delivered dose. The assumed gradient was found in the dose report for each read routine, the intercept was found by observing a read routine in engineering mode. The gradient and intercept were required to obtain the raw signal.
The relationship between raw signal and delivered dose for the OSL system is linear below 4 Gy and progressively more sub-linear above 4 Gy. A non-linearity correction has been estimated and can be applied to the raw signal of each OSLD.
The response of the OSL system has no statistically significant dependence on nominal dose rate or irradiation angle.
Scanning electron microscope images provided insight into the composition and structure of the OSLDs. The distribution of Al2O3:C crystals suggests that if the OSLDs were cut down to a smaller size, there may be a greater sensitivity difference between OSLDs.
A two minute exposure to typical office lighting produced a 3.3 ± 2.9 % underestimation of the calculated dose. Ideally, the OSLDs should not be handled in a room with the lights on.
Inconsistent raw signal values were observed in repeated read routines for a group of uniformly irradiated OSLDs. The dominant cause of the inconsistency was concluded to be slippage of the hub on the spindle. The inconstancy was reduced but not completely eliminated after replacement of the original hub. Further repairs or preferably spindle redesign will be required because hub slippage introduces errors that may change at any time.
By restricting irradiation to specific zones on several OSLDs it was observed that approximately 33 % of the OSL originated from within 0.5 mm of the edge of the gap through which the OSLD was stimulated. A portion of the OSL was lost if the OSLD was not positioned exactly above the stimulating laser due to slippage.
Factors to correct for a wobble in the reader’s carousel were estimated from raw signal values and are consistent with dial indicator measurements.
A method was devised which allows the calculation of an unknown dose from the values reported by the reader and incorporates a number of necessary corrections. The standard uncertainty of patient doses was estimated to be greater than 5.9 % and may potentially lead to incorrect dose monitoring. An error in an in vivo dosimetry estimate may lead to an ill-informed clinical decision.
Therefore the OSL system will not be used clinically until the issues of hub slippage and carousel wobble can be fully resolved. This recommendation and the above conclusions apply only to the specific devices used at WBRC and do not necessarily apply to other products by the same manufacturer or others.

Degree Masters by Research
Institution RMIT University
School, Department or Centre Applied Sciences
Keyword(s) Optically stimulated luminescence
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Created: Fri, 04 Jul 2014, 17:02:58 EST by Maria Lombardo
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