Long-Term Reliability of Optically Stimulated Luminescence Dosimeters
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Optically stimulated luminescence dosimeters (OSLDs) can be used as accurate and re-usable dosimeters for radiotherapy applications. OSLDs have been observed to decline in sensitivity with repeated use and it is important to determine whether this decline in sensitivity is associated with a decline in reliability. This study used three batches of OSLDs (purchased in 2012, 2014 and 2016) that had been repeatedly re-used in a mature in vivo dosimetry programme over a period of up to five years and evaluated the consistency of their response over repeated irradiation-readout-bleaching cycles. Each irradiation delivered 105 cGy to all OSLDs, using a 12 meV electron beam from a Varian iX linear accelerator. The five- and three-year-old OSLDs respectively displayed 86% and 89% of the sensitivity of the one year old OSLDs, but when a correction factor for each OSLD was derived based on the first measurement result and applied to each subsequent reading, all OSLDs were able to measure the 105 cGy test dose accurately, within standard deviations of 2.0% for the OSLDs from 2012 and 1.3% for the OSLDs from 2014 and 2016. If a mean calibration value was applied to the readings from each batch of OSLDs, instead of applying a measurement-derived sensitivity correction factor to each individual OSLD reading, the standard deviations increased to an unacceptable 6.1, 5.6 and 2.9%. Well-used three- and five-year-old OSLDs were shown to be capable of providing measurements with similar accuracy to a more recently-purchased batch of OSLDs, when measurement-derived sensitivity correction factors were applied to each result. If this extra step is included in the OSLD measurement process, then the same OSLDs may be reliably used for years without needing to be retired and replaced.
KeywordsRadiation therapy Solid state dosimetry Semiconductors
Optically stimulated luminescence dosimeters (OSLDs) are an attractive solution for radiotherapy dosimetry due to their small size, ease of use, sensitivity and reusability [1, 2]. OSLDs have been investigated and used for a variety of radiotherapy applications, including quality assurance and end-to-end testing [3, 4], dose measurements in non-reference and out-of-field conditions [5, 6, 7], in vivo dosimetry [8, 9, 10] and dosimetry auditing [2, 11].
OSLDs function similarly to thermoluminescent dosimeters (TLDs); incident radiation frees electrons which are then trapped between the valence and conduction bands of a doped semiconductor, until released by the application of additional energy and allowed to fall back to the valence band by the emitting a quantifiable light signal. Whereas the trapped electrons in TLDs are read out and cleared (annealed) using controlled heating, the energy needed to read out and clear (bleach) trapped electrons from OSLDs is applied using optical light.
Like TLDs and other solid state dosimeters, OSLDs have been found to exhibit a sensitivity variation with total accumulated dose [1, 12]. Different rates of decline in OSLD response have been observed after OSLD chips have been exposed to total doses above 10–20 Gy [1, 4, 12]. For example, Opp et al. measured a decrease in response of almost 7% after OSLDs were repeatedly irradiated and bleached until they were exposed to a total accumulated dose of 16 Gy , and Jursinic observed that “above 20 Gy, the OSLD sensitivity begins to drop by about 4% per 10 Gy of additional accumulated dose” .
As OSLD systems are increasingly implemented for use in routine and specialised radiotherapy dosimetry services, it is important to understand the long-term stability of OSLD response. This study therefore aimed to re-evaluate the sensitivity of thoroughly used batches of OSLDs, to provide an indication of whether decreases in OSLD sensitivity are associated with decreases in OSLD reliability and thereby provide guidance on the frequency with which batches of OSLD chips should be retired and replaced.
This study used three batches of Landauer nanoDot OSLDs (Landauer, Glenwood, USA), which had been purchased in 2012, 2014 and 2016. The OSLDs were reported by the manufacturer (and found at inital commissioning, immediately after purchase) to produce a consistent response within ±2%. The OSLDs were then repeatedly re-used in a mature in vivo dosimetry programme over a period of up to five years. The sensitivity and consistency of the response of all available OSLDs was evaluated in 2017, over three repetitions of the bleaching-irradiation-readout cycle.
A Varian iX linac (Varian Medical Systems, Palo Alto, USA) was used to deliver 105 cGy to all OSLDs at each irradiation, using a 25 × 25 cm2 12 meV electron beam, which produced a large region of relatively flat dose, sufficient for irradiating up to 100 OSLDs at once, in a purpose-fabricated array (see Fig. 1b) at the depth of maximum dose in a water-equivalent plastic phantom (see Fig. 1c).
All OSLDs were read out after each irradiation, using a Landauer Microstar OSLD reader containing an array of 38 light emitting diodes (LED) which provided a 532 nm (green) light source for stimulating the luminescence . Individual OSLDs were identified and tracked using numbers and barcodes on stickers that were attached to the OSLD housings by the manufacturer. Four OSLDs were removed from the sample due to loss (detachment) or fading of their identifying numbers and barcodes (see Fig. 1d).
3 Results and Discussion
Examination of the number of raw counts per cGy from each measurement (shown in Fig. 2a) indicates that the OSLDs from 2012 and 2014 respectively displayed 86% and 89% of the sensitivity of the newer OSLDs. This result provides a useful long-term verification of the decrease in OSLD sensitivity with total accumulated dose that was predicted via short-term OSLD response studies [1, 4, 12].
When OSLD response was corrected using a batch calibration factor (a correction calculated from the mean over/under response of all OSLDs in each batch), the OSLD measurements appeared unacceptably variable, especially for the older batches (see Fig. 2b), due to the response variations of the individual OSLD chips . The mean standard deviations calculated over the three repetitions of the bleaching-irradiation-readout cycles were 6.1, 5.6 and 2.9% for the batches of OSLDs from 2012, 2014 and 2016, respectively.
After a locally-derived correction factor for each OSLD was determined based on the first reading of the first measurement (without reference to the vendor-supplied sensitivity factor) and applied to each subsequent reading, all OSLDs were able to measure the 105 cGy test dose accurately, within standard deviations of 2.0% for the OSLDs from 2012 and 1.3% for the other two batches of OSLDs (see Fig. 2c). Using this method, approximately 90% of the OSLDs from 2014 and 2016 were able to provide measurements within ±2% of the known delivered dose, while 75% of the OSLDs from 2012 were able to provide the same level of accuracy.
Data shown in Fig. 2c indicates that several outliers exist, where measured doses differ from the known delivered doses by 5% or more. This result suggests that even when local sensitivity correction factors are measured, used and frequently updated, it may also be advisable to use more than one OSLD to perform each dose measurement, to minimise the likelihood of results being confounded by anomalous OSLD over/under-response.
Three batches of OSLDs that had been repeatedly re-used in a mature in vivo dosimetry programme have been found to be capable of providing radiotherapy dose measurements with similar levels of accuracy, despite the batches’ varying ages and levels of previous accumulated dose. To achieve this result, it was necessary to measure and apply a local sensitivity correction factor to each OSLD reading. If this extra step is included in the OSLD measurement process, then it may be possible to use the same OSLDs repeatedly and reliably for years, without needing them to be retired and replaced.
Compliance with Ethical Standards
The authors declare that they have no conflict of interest. This article does not contain any studies with human participants or animals performed by any of the authors.
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