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Year : 2017  |  Volume : 13  |  Issue : 2  |  Page : 304-312

Dosimetric characterization of optically stimulated luminescence dosimeter with therapeutic photon beams for use in clinical radiotherapy measurements

1 Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Bengaluru, Karnataka; Department of Radiotherapy, Christian Medical College, Vellore, Tamil Nadu, India
2 Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Bengaluru, Karnataka, India

Date of Web Publication23-Jun-2017

Correspondence Address:
Ravikumar Manickam
Department of Radiation Physics, Kidwai Memorial Institute of Oncology, Dr. M.H. Marigowda Road, Bengaluru - 560 029, Karnataka
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0973-1482.199432

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 > Abstract 

Aim: The modern radiotherapy techniques impose new challenges for dosimetry systems with high precision and accuracy in in vivo and in phantom dosimetric measurements. The knowledge of the basic characterization of a dosimetric system before patient dose verification is crucial. This incites the investigation of the potential use of nanoDot optically stimulated luminescence dosimeter (OSLD) for application in radiotherapy with therapeutic photon beams.
Materials and Methods: Measurements were carried out with nanoDot OSLDs to evaluate the dosimetric characteristics such as dose linearity, dependency on field size, dose rate, energy and source-to-surface distance (SSD), reproducibility, fading effect, reader stability, and signal depletion per read out with cobalt-60 (60 Co) beam, 6 and 18 MV therapeutic photon beams. The data acquired with OSLDs were validated with ionization chamber data where applicable.
Results: Good dose linearity was observed for doses up to 300 cGy and above which supralinear behavior. The standard uncertainty with field size observed was 1.10% ± 0.4%, 1.09% ± 0.34%, and 1.2% ± 0.26% for 6 MV, 18 MV, and 60 Co beam, respectively. The maximum difference with dose rate was 1.3% ± 0.4% for 6 MV and 1.4% ± 0.4% for 18 MV photon beams. The largest variation in SSD was 1.5% ± 1.2% for 60 Co, 1.5% ± 0.9% for 6 MV, and 1.5% ± 1.3% for 18 MV photon beams. The energy dependence of OSL response at 18 MV and 60 Co with 6 MV beam was 1.5% ± 0.7% and 1.7% ± 0.6%, respectively. In addition, good reproducibility, stability after the decay of transient signal, and predictable fading were observed.
Conclusion: The results obtained in this study indicate the efficacy and suitability of nanoDot OSLD for dosimetric measurements in clinical radiotherapy.

Keywords: Ion chamber, optical bleaching, optically stimulated luminescence dosimeter, radiotherapy

How to cite this article:
Ponmalar R, Manickam R, Ganesh K M, Saminathan S, Raman A, Godson HF. Dosimetric characterization of optically stimulated luminescence dosimeter with therapeutic photon beams for use in clinical radiotherapy measurements. J Can Res Ther 2017;13:304-12

How to cite this URL:
Ponmalar R, Manickam R, Ganesh K M, Saminathan S, Raman A, Godson HF. Dosimetric characterization of optically stimulated luminescence dosimeter with therapeutic photon beams for use in clinical radiotherapy measurements. J Can Res Ther [serial online] 2017 [cited 2022 Nov 26];13:304-12. Available from: https://www.cancerjournal.net/text.asp?2017/13/2/304/199432

 > Introduction Top

The complexity of modern radiotherapy techniques such as intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery requires dose delivery evaluation to ensure the effectiveness of radiotherapy treatments. This imposes a new challenge for dosimetric systems with high sensitivity in high- and low-dose (LD) regions. With this aim, several types of dosimeters are being developed and the most commonly used dosimeters are ionization chambers, thermoluminescence dosimeters (TLD), diodes, and metal oxide semiconductor field effect transistors (MOSFETs).[1],[2] Ionization chambers are not used for in vivo dosimetry though they are considered as the gold standard and backbone for dosimetric measurements in radiotherapy owing to its stability, accuracy, and practicality.[3] The technology of TLD is proven and has been used extensively;[4],[5] nevertheless, re-estimation of absorbed dose is not possible due to destructive readout technique. Despite the fact that diode and MOSFET give real-time readout, diode requires corrective action due to its dependency on number of parameters,[6] reduced lifetime of MOSFET limits its utility.[2] The aforementioned drawbacks instigate the necessity to explore a new dosimetry system for dosimetric verification.

Optically stimulated luminescence (OSL) technology has been introduced into radiation dosimetry and the application of OSL dosimeter (OSLD) for dose verification in clinical radiotherapy is quickly gaining popularity.[7],[8],[9],[10],[11] Yet, OSLDs have been widely used in personal dosimetry and space dosimetry over a decade.[12],[13],[14],[15],[16],[17] The luminescent material, aluminum oxide (atomic number - 11.28) doped with carbon (Al2O3:C), is the most commonly available OSL material that has high sensitivity as a TL material, over 40–60 times that of LiF:Mg, Ti.[18],[19],[20] OSLD exhibits high accuracy and precision in dose determination, reusability, multiple readout, and readability after long time of irradiation.[21] Despite the capability to measure small and large doses, the drawback is that the phosphor material (Al2O3:C) is sensitive to light owing to the nature of OSL phenomenon.[11] However, this drawback is easily overcome by a water equivalent light-tight plastic encapsulation. The different forms of OSLDs available are OSL films, dot and nanoDot dosimeters. The OSL film strips are obtained by coating aluminum oxide powder on a roll of polystyrene film of approximately 0.3 mm thickness and 7 mm diameter and can be cut into desired size and shape. The dot dosimeters are plastic discs of 7 mm diameter and 0.2 mm thickness, infused with Al2O3:C and encased in a light-tight plastic holder of dimension 24 mm × 12 mm × 2 mm. The nanoDot OSLD has the same detector material as other OSLDs but differs in a form factor 50% of the standard dot dosimeters to better accommodate the needs of medical community. This advantage increases the feasibility to place the nanoDot OSLDs in more restricted spaces, for instance eyelid.

It is imperative to know the dosimetric characteristics of any dosimeter before using in clinical measurements. Hence, the present work is aimed to investigate the dosimetric properties and the accuracy of OSL system with cobalt-60 (60 Co), 6 and 18 MV therapeutic photon beams as a prelude to patient dose measurements.

 > Materials and Methods Top

nanoDot optically stimulated luminescence dosimeter

The nanoDot OSLDs used in this study were procured from Landauer, Inc., (Glenwood, USA) and are shown in [Figure 1]. These dosimeters consist of plastic discs of Al2O3:C of 5 mm diameter and 0.2 mm thickness. It is encased in a 1 cm × 1 cm × 0.2 cm light-tight black plastic case with the mass density of 1.03 g/cm 3, to prevent the signal depletion due to light. The sensitive element in the disc can slide out of the plastic case during read out process and optical bleaching. The bar code enables to identify each nanoDot, to track the history and to record with ease. In this study, 200 nanoDot OSLDs were used for the measurements including the repetition of all experiments. For each measurement data, minimum of four OSLDs were used for all irradiation setup. OSLDs were read three times and each data point used in the measurement was the average response of OSLDs.
Figure 1: nanoDot optically stimulated luminescence dosimeters with an adapter

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microStar optically stimulated luminescence dosimeter reader

The InLight microStar reader (Landauer, Inc., Glenwood, USA) shown in [Figure 2] was used with an external personal computer (PC) that includes a special dosimetry software to acquire and export data using Microsoft Excel. During readout, OSLD was stimulated with a light of wavelength 540 nm and the luminescence released is of 420 nm wavelength. The OSLD loader has to be operated gently and slowly or else the partial pull from the adapter or half closed position of the housing lead to inconsistent readout and subsequently poor reproducibility. Stability check of the reader was performed after the warm up time of 10 min. The readout of OSLD was done after 30 min postirradiation in order to allow the shallow trap electrons to stabilize. The preirradiation signals of OSLDs were obtained by reading the dosimeters prior to irradiation once the optical bleaching had been done. The difference between the pre- and post-irradiation signals of photomultiplier counts has been reported as the resultant OSL response.
Figure 2: microStar reader with an external PC

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Optical Annealer

Optical Annealer was used to remove the residual signals of the dosimeters after readout. This optical bleaching was done with a light exposure to OSLDs after the readout to reset the OSLD for reuse. [Figure 3] shows the OSLDs placed inside the Optical Annealer during optical bleach. The OSLDs were kept for optical bleaching to get the signal almost equal to the background signal, and the annealing time of 6 h was found to be sufficient enough for this process.
Figure 3: Optically stimulated luminescence dosimeters in the Optical Annealer for bleaching

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Ionization chamber

The FC65-G Farmer type ionization chamber (Scanditronix Wellhofer AB, Sweden) with a sensitive volume of 0.65 cc, suitable for photon beam dosimetry, was used to validate the acquired data of OSLDs in a similar setup wherever applicable. The inner electrode of ionization chamber is made up of aluminum (ρ =2.7 g/cm 3) whereas the outer electrode is made up of graphite (ρ =1.82 g/cm 3). The dose 1 electrometer (Scanditronix Wellhofer AB, Sweden) used is of high precision, reference class electrometer. The calibration of chamber and electrometer has been done according to TRS-398 protocol in a 60 Co beam.

Treatment unit and phantom

Irradiations were done with photon beams from Clinac DHX linear accelerator (Varian Medical Systems, Palo Alto, CA, USA) and Theratron 780E telecobalt unit. The nominal energies of 6 MV and 18 MV X-ray beams with linac were used and calibrated according to TRS-398 protocol to deliver 1 cGy/MU. Measurements were performed in plastic water phantoms (white polystyrene material) that are plates of 30 cm × 30 cm surface size of various thicknesses (1, 2, 5, and 10 mm) with a density of 1.04 g/cm 3. The slabs of 10 cm thickness were used for backscatter for all irradiation setup. A perspex slab of dimension 30 cm × 30 cm × 0.6 cm was machined with slots to accommodate OSLDs snugly with minimized air gap during irradiation.

Measurement details: Element correction factor

In production, OSL crystals are mixed together to make huge batches of dosimeters with uniform sensitivity across the batch. However, sensitivity may vary within the same batch of dosimeters due to the inherent heterogeneity of traps in the crystal. In order to account this, an element correction factor (ECF) was determined as the ratio of the response of each OSLD to the average response of 200 OSLDs as a multiplicative factor.60 Co beam has been chosen for irradiation due to its uniform dose profile without horn effect. Two batches of 100 OSLDs were irradiated simultaneously to a known dose of 200 cGy with a field size of 20 cm × 20 cm at 5 cm depth to find the ECF of each OSLD. The corrected OSLD response (Rcorr) was calculated using the following equation:

Rcorr = ECF × S(1)

Where ECF is the ECF of the nanoDot, S = (SaSb), where Sb and Sa are the signals of the OSL before and after irradiation. The ECF obtained was applied to raw readings in the subsequent uses of each dosimeter in all measurements.

Dosimetric characteristics of optically stimulated luminescence dosimeter

The dosimetric characteristics of nanoDot OSLDs were performed with 60 Co, 6 MV, and 18 MV photon beams. The irradiations were done in linac with 100 cm source-to-surface distance (SSD) at 10 cm depth with a nominal dose rate of 400 MU/min, in 60 Co beam with 80 cm SSD at 5 cm depth and a known dose of 200 cGy was delivered with a field size of 10 cm × 10 cm. These experimental setups remain same for all dosimetric measurements unless otherwise mentioned. To investigate the response of OSLDs as a function of dose, dosimeters were exposed to doses ranging from 50 to 1000 cGy insteps of 50 up to 300 cGy and insteps of 100 up to 1000 cGy. The responses of the dosimeters were normalized at 200 cGy and the corresponding linearity curves were plotted with the response of OSLDs as a function of dose. The radiation output of the linear accelerator (6 and 18 MV) and 60 Co beam as a function of field size was investigated for fields ranging from 5 cm × 5 cm to 30 cm × 30 cm in 60 Co beam and from 3 cm × 3 cm to 30 cm × 30 cm in linac. The responses of the dosimeters were normalized to reference field (10 cm × 10 cm) at the depth of dose maximum (Dmax). The effect of dose rate dependency with OSLD was evaluated by varying MU/min setting of linac in the range from 100 to 600 MU/min in steps of 100 MU/min. The responses of the dosimeters were normalized to a nominal dose rate of 400 MU/min. The variation in energy response of nanoDot OSLDs with three different energies of photon beams such as 6 MV, 18 MV, and 60 Co was investigated by maintaining similar setup during irradiation. The OSLD response for different distances from the source was determined with different SSDs of 70, 80, 90, 95, 100, 105, 110, 120, 130, and 135 cm in 6 MV, 18 MV beams and 65, 70, 75, 80, 85, 90, 95, and 100 cm in 60 Co beams.


The reproducibility of OSLD was investigated by exposing OSLDs to identical doses with repeated irradiations. It consisted of irradiation of three groups of nanoDots with and without accumulation of dose. In the first group, OSLDs were irradiated to a total dose of 8 Gy insteps of 2 Gy. These dosimeters were read with 10 min wait period after an exposure of 2 Gy. Optical bleaching was done after each subsequent irradiation of OSLDs. The second group was allowed to accumulate the dose to a total dose of 8 Gy with an increment of 2 Gy without bleaching the OSLDs between irradiations and the readout cycle was done with a wait period of 10 min. Third, a total dose of 8 Gy was delivered to OSLDs in a single session. These three different irradiations were carried out at the same experimental setup with 6 MV and 18 MV photon beams. The dosimeters were placed at 10 cm depth (100 cm SSD) in phantom with a field size of 10 cm × 10 cm.

Fading of optically stimulated luminescence signal

The fading of nanoDot OSLDs was determined by finding the decrease in optical signal of the dosimeter at room temperature with time. A study has been carried out to assess the decay of OSL signal as a function of time after irradiation by exposing the nanoDot OSLDs to LD of 2 Gy and high dose (HD) of 10 Gy with 6 MV and 18 MV photon beams at 10 cm depth with a field size of 10 cm × 10 cm with 100 cm SSD. The response of dosimeters has been checked for periods of (a) seconds/minutes/hours (short-term), (b) few days (mid-term) and (c) several weeks and months (long-term). Measurements were performed with dosimeters after irradiation by taking readings, started as early as 40 s immediately postirradiation, then at 3, 7, 10, 15, and 20 min, subsequently every 10 min for 1 h, later every 30 min for 6 h and then every half an hour for 6 h. After this period, nanoDots were readout at the same time (morning, evening) every day for 5 days. OSLD readings were taken once a week for about 5 weeks. To evaluate long-term fading, less frequent readings were taken, monthly once for about 8 months.

Reader stability

The stability of the reader is more important over the length of time during fading process. It is essential to ensure that the change in reader stability over the read out time is insignificant and the variations are within limits as recommended by manufacturer. Hence, the variation in the stability of the reader was assessed with intrinsic system check by measuring background signal as dark (DRK) current of the reader from photomultiplier tube (PMT), counts using radioactive material (CAL)14 C to indicate the consistency of PMT and counts from PMT with light emitting diode (LED) to indicate the stability of beam intensity. The procedure was repeated five times and the standard deviation (SD) was determined. The DRK, CAL, and LED readings were taken prior to each reading session of OSLD, after the reader warm up time of 10 min. During large group readout of OSLD, readings were taken in between the reading session and also at the end.

Signal depletion per readout

The successive readout of OSLD depletes the trapped charges partially and reduces the OSL signal when multiple readout of the same dosimeter is performed. The analysis of signal loss per readout helps in incorporating the required correction to OSL. The depletion fraction of trapped charges that are emptied during each of the stimulation was estimated by exposing OSLDs to 2 Gy (LD), 10 Gy (HD) with 10 cm × 10 cm field at 10 cm depth in 6 and 18 MV photon beams (100 cm SSD). The OSLDs were read after half an hour following irradiation in order to allow the shallow trap electrons to stabilize. The dosimeters were repeatedly read 200 times sequentially using microStar reader. The depletion in optical signal of OSLDs per readout and also the drop in signal over 200 successive readouts were determined.

 > Results Top

Element correction factor

The ECF for each nanoDot was determined from a batch irradiation of 200 OSLDs. The histogram presented in [Figure 4] shows the spread in ECF values between 0.91 and 1.07. The SD and coefficient of variation of OSLD readings were evaluated. The mean SD of 0.75% and the coefficient of variation <1.5% were observed. Ninety percent of OSLDs fall within 5% of the mean value and the remaining 10% dosimeters fall within 8.6% of the mean.
Figure 4: Histogram of the distribution of element correction factors for 200 optically stimulated luminescence dosimeters

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Dose linearity

[Figure 5] presents the dose-response behavior of nanoDot OSLDs with 60 Co, 6 MV, and 18 MV photon beams and the error bar indicates the standard uncertainty in OSLD readings. The OSLD response was linear for doses from 50 to 300 cGy. The linear model fit for the relationship between OSL response and dose with values of R2 are 0.9962, 0.9967, and 0.9960 for 60 Co, 6 MV, and 18 MV photon beams, respectively. The supralinearity behavior was observed for doses >300 up to 1000 cGy, maximum dose delivered in the study. The supralinear behavior was carefully analyzed for the doses ranging from 400 to 1000 cGy and the supralinearity factor was determined from the ratio of the supralinear response of OSLD to the linear response. The calculated values of supralinear factors are summarized in [Table 1].
Figure 5: Dose response of nanoDot optically stimulated luminescence dosimeters with photon beams

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Table 1: Supralinearity factor for photon beams with doses from 400 to 1000 cGy

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Field size dependency

The relative output factors as a function of field size at Dmax are represented in [Figure 6]. Error bars represent the standard uncertainty in OSLD readings. The response of OSLDs with field size was compared with ionization chamber response. The maximum experimental uncertainty of 1.10% ± 0.4%, 1.09% ± 0.34%, and 1.20% ± 0.26% was observed in 6 MV, 18 MV, and 60 Co beam, respectively.
Figure 6: Field size dependency of optically stimulated luminescence dosimeter in comparison with ion chamber

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Dose rate dependency

[Figure 7] represents the OSLD response with dose rates of 6 and 18 MV photon beams and the error bar indicates the SD between OSLD readings. The largest variation of 1.3% ± 0.4%, 1.4% ± 0.4% was observed for 6 and 18 MV, respectively, when the values were normalized to a nominal dose rate of 400 MU/min. These results show the dose rate independency of OSLD from 100 to 600 MU/min.
Figure 7: Dose rate dependence of the response of optically stimulated luminescence dosimeter

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Energy dependency

The results obtained regarding the beam energy dependence of OSLD with photon beams were normalized to the relative response of 6 MV photon beam. The dependency with energy was tested and observed a deviation of 1.5% ± 0.7% and 1.7% ± 0.6% at 18 MV and 60 Co with 6 MV beam. It is evident that the nanoDot OSLDs have no energy dependence in the energy range from 6 to 18 MV photon beam and with 60 Co beam. The response of OSLDs with photon beams relative to 6 MV beam is shown in [Figure 8]. The error bar indicates the SD with appropriate uncertainty propagation of OSLD data.
Figure 8: Energy dependence of optically stimulated luminescence dosimeters for photon beams

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Source-to-surface distance dependency

The SSD dependence of OSLD with 6 MV, 18 MV, and 60 Co beams are depicted in [Figure 9]a and [Figure 9]b. The response of the dosimeters with 6 MV and 18 MV beams were normalized to 100 cm SSD value whereas 60 Co beam values were normalized to 80 cm SSD value. The maximum variation observed with ion chamber was 1.5% ± 1.2% for 60 Co, 1.5% ± 0.9% for 6 MV, and 1.5% ± 1.3% for 18 MV photon beams.
Figure 9: Source-to-surface distance dependence of optically stimulated luminescence dosimeters for (a) 6 MV and 18 MV photons (b) 60Co gamma rays

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The OSLDs were exposed to identical doses three times as described earlier to find the reproducibility of OSLDs and the response is depicted in [Figure 10]. The data obtained are normalized to OSLD response values that are bleached after each subsequent irradiation. A second order polynomial was fitted to the measured data and the standard uncertainty was determined. The SD estimated was 1.9% and the error bar represents the SD of the mean. The maximum percentage difference observed between the bleached and accumulated response was 7.8% in 6 MV and 9.9% in 18 MV photon beams. The effect due to accumulation of dose is small, but still noticeable (<2%) when OSLDs were exposed to 8 Gy in a single session. The inter-OSL response variation was found to be <1.03% (1 SD) that indicated a good reproducibility of OSLD during multiple irradiations.
Figure 10: Reproducibility of optically stimulated luminescence dosimeters in repeated irradiations

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Fading characteristics

The rapid drop in optical signal of OSLD from 40 s to 10 min was found to be 8.8%. This is due to the transient signal originated from the spontaneous emission of unstable nondosimetric electron trap. The data obtained revealed that the time taken to stabilize this low energy trap is approximately 8–10 min postirradiation with 6 and 18 MV photon beams. Thus, a minimum wait time of 10 min was given throughout the measurement before the signal readout. The rapid drop and the stability in OSL signal when the signal is normalized to 60 min postirradiation is shown in [Figure 11]. Moreover, the fading effect of OSL signal extended over a period of 8 months was observed. It was noticed that every 1-month period, the fading decay rate stayed constant with a percentage reduction in signal <1%. Over a period of 8 months, the percentage reduction in signal when the response was normalized to 1 day reading was 4.8%, 5.7% with 6 and 18 MV for LD whereas 5.7%, 7.1% with 6 and 18 MV for HD, respectively.
Figure 11: The transient signal decay and the stability of optically stimulated luminescence signal

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Reader stability

The control limits suggested by the manufacturer for reader stability are DRK should be <30, CAL should be within ± 10%, and LED should be within ±10%. In this study, the dark current values obtained were always well below the manufacturer's recommended value, a maximum DRK count of 11 was observed. The maximum variation between the results of CAL and LED was around 3% which is well below the tolerance level that indicated the acceptable stability of the reader.

Signal depletion per read out

[Figure 12]a and [Figure 12]b shows the magnitude of signal depletion of OSLD per readout over 200 sequential readouts when exposed to low and HDs with 6 and 18 MV photon beams. The statistical analysis of signal depletion per readout of OSLD was determined and compared with published results that are tabulated in [Table 2]. The readout process of nanoDot OSLD was nondestructive unlike TLD, with only a small portion of the OSL signal was being removed per reading. In this work, over 200 readings, nanoDot lost 0.05% signal per readout for 2 Gy and 0.06% signal loss per readout was observed for 10 Gy. With 200 sequential read out, the loss in OSL signal at 50th, 100th, 150th, and 200th from original signal is plotted in [Figure 13]. The percentage signal loss in 50th readout was found to be 2.9% for 6 MV and 3.2% for 18 MV with the irradiation of LD, whereas 3.2% and 3.7% with HD for 6 and 18 MV. Similarly on 200th readout, the percentage signal loss in 6 and 18 MV was found to be 9.4% and 9.8% with LD whereas 11.5% and 11.7% with HD.
Figure 12: The magnitude of signal depletion per readout (a) 6 MV (b) 18 MV photon beams

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Table 2: Statistical analysis of low and high doses with 6 and 18 MV photon beams

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Figure 13: Percentage loss in optically stimulated luminescence signals at 50th, 100th, 150th, and 200th readout

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 > Discussion Top

Element correction factor

From batch irradiation of 200 nanoDot OSLDs, ECF was obtained and three times readout of OSL readings were found to be satisfactory in reducing uncertainty. Among the group, 90% of the dosimeters found to have a deviation <5% of mean that indicates the stability and uniformity of OSL production. The individual correction factor applied to raw readings of OSL is better than a single correction factor for the entire batch from the mean response as the variance obtained for the entire batch is greater than the variance of an individual dosimeter.[22] Hence, the correction factor for each dosimeter was applied to the raw readings of OSLD during each measurement.

Dose linearity

The analysis on the linear and the supralinear response of OSLD shows that the supralinearity behavior starts anywhere in the dose ranges from 200 to 400 cGy [2], 21, [23],[24],[25],[26] with 6 MV photon beam; nevertheless, supralinear response of nanoDot OSLDs with 18 MV and 60 Co beam was not reported in literature. The supralinear response of nanoDot dosimeter is due to the extra luminescence emitted from deeper electron traps in the dosimeter during irradiation at higher doses [27] and variation in the supralinearity dose value may be due to the dissimilarity in the OSLD reader and the readout methods.[11] It was observed that the supralinear factor increases with increase in dose. The supralinear factor has to be accounted in dose calculation for higher doses when OSLDs are used in patient measurements.

Field size dependency

The experimental uncertainty found for field size variation with 6 MV, 18 MV, and 60 Co beam indicate the independency of nanoDot OSLD with field size. Viamonte et al.[2] have observed a difference of 1% in dot dosimeters with 60 Co beam for all field sizes considered. However, Schembri and Heijmen reported a maximum discrepancy of ±2.5% in 6 MV photon beams with OSL films.[24] As OSLDs cover a small area, these dosimeters can be used for the measurements of small field sizes, for instance those used in radiosurgery.[2] The relative output factor measurement can be carried out using OSL instead of ion chamber because of no change in trend in the effect of OSL response compared to ion chamber.

Dose rate dependency

The results of the dosimeter response show the dose rate independency of nanoDot OSLD from 100 to 600 MU/min with 6 and 18 MV beams and consistent with OSL film, dot dosimeters in 6 MV beam.[2],[21],[24],[28] No dose rate dependence was reported for dose rates from 100 to 600 MU/min for 18 MV beams. As the OSLD does not have any change in sensitivity with dose per pulse within the experimental uncertainty, it can provide accurate measurement for a large range of dose per pulse values. The accurate measurement of dose for a wide range of dose per pulse could occur at extended distances for total skin electron or whole body irradiation, under wedges, under blocks, or under multileaf collimator leaves in IMRT beams.[21]

Energy dependency

The results of energy response of nanoDot OSLD show no energy dependence in the energy range from 6 to 18 MV photon beam and with 60 Co beam and are consistent with the results of Dot dosimeter.[8],[29] The variation noticed with OSL film [24] could be due to the various phantom materials used such as water, solid water phantom, and polystyrene during measurements.[26]

Source-to-surface distance dependency

The SSD dependency of nanoDot OSLD measurements showed good agreement with the published results of dot dosimeter in 60 Co beam;[2] nevertheless, the dependency of nanoDot OSLDs with SSD has not been reported in 60 Co beam, 6 MV, and 18 MV beams.


The approaches that were made to compare the reproducibility of OSLD system demonstrated that these nanoDots can be conveniently reused with bleaching between irradiations provided taking care of the changes in the sensitivity of OSLD with repeated irradiations. The percentage difference observed between the bleached and accumulated response with 6 MV photon beam was comparable with the published result,[30] nonetheless no reported results with 18 MV photon beams. The significant over response of OSL signal observed from dosimeters during accumulation of doses could be due to the supralinearity effect with accumulated dose.[25]

Fading characteristics

The transient signal decay observed immediately after irradiation is due to the spontaneous emptying of electron traps without the external stimulation either by heat or light.[21] Thus, a minimum wait time of 10 min is required after irradiation and before the signal readout.[12],[31] The knowledge of postirradiation fading characteristics [28] of OSLD signal would be useful in postal audit program where the irradiated dosimeters were read after several days. The reported results in literature with 6 MV photon beam have shown 2.5% reduction over 30 days when the signal was normalized to 2 days postirradiation [22] and 4%–5% reduction over 9-month period.[15] The decreased fading effect [15],[21],[24],[28],[30] of nanoDot OSLD compared to TLD is due to the thermal stability of the dosimeter and the light-proof plastic case.[22]

Reader stability

No considerable variation in the stability of microStar reader was found during each measurement session using position dials DRK, CAL, and LED by monitoring the variations of the dark PMT counts, PMT output, and LED intensity, respectively.

Signal depletion per readout

The statistical analysis of the signal depletion per readout of OSLD was comparable with the values reported in literature.[21],[22] The readout process of nanoDot OSLD was nondestructive unlike TLD, only a small portion of the OSL signal was being removed per reading. The rate of loss of OSL signal depended on dose and energy. Higher rate of loss in OSL signal was observed when they were exposed to HD.[31],[32] The readings of nanoDot OSLD could be found out from the sequential readout by giving the appropriate correction using this value. The signal loss per readout of 6 MV beam was consistent with the results reported in literature.[12],[21] Dunn et al.[22] suggested that though the results are consistent with each other, small difference between the results could arise from the differences in reader generation and/or detector generation.

 > Conclusion Top

In this study, the dosimetric characteristics of nanoDot OSLD have been examined with 6 MV, 18 MV, and 60 Co photon beams. The dose response behavior, independency with dosimetric parameters such as dose rate, field size and energy, fading characteristics, reproducibility and stability of the system indicate the suitability of nanoDot to an enhanced utilization in various radiotherapy dosimetric measurements. Investigated OSLD system with high precision, accuracy, reusability, nondestructive readout, multiple readout, optical bleach rather than annealing, barcode facility to identify dosimeter have made nanoDot OSLD, a viable dosimeter to be used in routine clinical measurements in various radiotherapy applications with comparable accuracy.


This study was supported by a grant (AERB/CSRP/58/06/2014) from the Atomic Energy Regulatory Board, India. The authors would like to thank Mr. Vijaya Reddy, Ms. Kavya Medical Equipments for designing dosimeter accessories.

Financial support and sponsorship

AERB funded project.

Conflicts of interest

There are no conflicts of interest.

 > References Top

Dyk JV. The Modern Technology of Radiation Oncology: A Compendium for Medical Physicists and Radiation Oncologists. Madison, Wisconsin: Medical Physics Pub.; 1999.  Back to cited text no. 1
Viamonte A, da Rosa LA, Buckley LA, Cherpak A, Cygler JE. Radiotherapy dosimetry using a commercial OSL system. Med Phys 2008;35:1261-6.  Back to cited text no. 2
Alfonso R, Andreo P, Capote R, Huq MS, Kilby W, Kjäll P, et al. A new formalism for reference dosimetry of small and nonstandard fields. Med Phys 2008;35:5179-86.  Back to cited text no. 3
Attix FH. Introduction to Radiological Physics and Radiation Dosimetry. New York: John Wiley & Sons; 2008.  Back to cited text no. 4
Essers M, Mijnheer BJ.In vivo dosimetry during external photon beam radiotherapy. Int J Radiat Oncol Biol Phys 1999;43:245-59.  Back to cited text no. 5
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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13]

  [Table 1], [Table 2]

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