Element Correction Factors Research Articles

Angular dependence of the optically stimulated luminescent dosimeter

Objective To study the angular dependence of optically stimulated luminescent dosimeter (OSLD) with solid phantoms under SSDL radiation level 60Co radiation field, and to discuss the possibility of OSLD in volumetric modulated arc therapy (VMAT) and other rotating irradiation dose audit. Methods OSLDs were embedded in the two phantoms with the same size and material, respectively. The phantom 1 was set to make the first OSLD perpendicular to the beam, and the phantom 2 was set to make the second OSLD parallel to the beam. The OSLDs were irradiated at 8 angles: 0°, 45°, 90°, 135°, 180°, –45°, –90, –135°. The counts of the OSLSs were read and the response of each angle which normalized to 0° were calculated. Results When the OSLDs are perpendicular to the beam, the angular response is between –6.76% 〜+1.5%, with the maximum angular dependence at 90° and –90°. When the OSLDs are parallel to the beam, the angular response is between –1.74%〜+1.67%, below 2%. Conclusion It is better to correct the sensitivity of dosimeters by Element Correction Factors (ECF) for dose audit. Under the condition of rotating irradiation, OSLD shoud be set parallel to the beam, which can better reduce the influence of angular dependence and facilitate further application research of VMAT dose audit. 摘要： 目的 结合定制固体模体研宄光致发光剂量计(OSLD)在放疗水平 60Co标准辐射场中的角响应性能, 探讨OSLD在容积旋转调强放射治疗(VMAT)等旋转照射剂量核查中应用的可行性。 方法 将OSLD嵌入2个外观尺寸以 及材料均相同的定制固体模体, 模体1使OSLD与射线垂直, 模体2使OSLD与射线平行, 照射8个角度：0°、45°、 90°、135°、180°、–45°、–90°和–135°, 读取每个角度的OSLD计数, 计算每个角度相对于0°的角响应值。 结果 当OSLD 垂直于射线照射时, 角响应在–6.76%〜+1.5%, 90°和–90°角响应最大;当OSLD平行于射线照射时, 角响应在–1.74%〜 +1.67%, 均在± 2%以内。 结论 在实际工作中, 应先计算元件校正因子(ECF), 校正各剂量计对射线的灵敏度, 旋转 照射条件下, 将OSLD平行于射线入射方向放置, 能更好地降低角响应影响, 便于进一步开展VMAT剂量核查应用研宄。

Dose-Response of TLD-100 in the Dose Range Useful for Hypofractionated Radiotherapy.

Purpose:The aim of the study was to exploit the feasibility of thermoluminescent dosimeters (TLDs) in radiation therapy techniques in which high dose per fraction is involved.Methods:Dose–response of TLD-100 (LiF: Mg, Ti) was investigated in both 6-MV photon and 6-MeV electron beams. The element correction factor (ECF) generation method was applied to check the variability of the TLDs response. Two batches of 50 TLDs were divided into groups and exposed in the dose range 0 to 30 Gy. Regression analysis was performed with both linear and quadratic models. For each irradiation beam, the calibration curves were obtained in 3 dose range 0 to 8 Gy, 0 to 10 Gy, and 0 to 30 Gy. The best-fitting model was assessed by the Akaike Information Criterion test.Results:The ECF process resulted a useful tool to reduce the coefficients of variation from original values higher than 5% to about 3.5%, for all the batches exposed. The results confirm the linearity of dose–response curve below the dose level of 10 Gy for photon and electron beam and the supralinear trend above.Conclusion:The TLDs are suitable dosimeters for dose monitoring and verification in radiation treatment involving dose up to 30 Gy in a single fraction.

Quantification of Various Factors Influencing Repeatability and Reproducibility of TLD-600 Detector

Various factors affecting the random and systematic error in the repeatability and reproducibility of thermoluminescent 6LiF:Mg,Ti dosimeters were investigated. These characteristics were examined by irradiating them with different doses in the range 3.8 µG to 433.94 mGy from a Cs-137 gamma source. Values of the element correction factors (ECFs) were obtained by exposing the chips to a dose of 433.93 mGy five times. This investigation denotes that more correction will be obtained by calculating the error due to non-subtracting zero dose reading from the delivered dose, which decreases the coefficient of variation as a percentage of the mean from range of 2.4%-18.6% to only 0.7%-16.9%. Using the individual sensitivity factor and repeating five calibrations decrease the coefficient of variation as a percentage of the mean to only 0.54% and the values of the Variability Index for System (SVI), Reader (RVI) and Detector (DVI) are 1.46 %, 1.40% and 0.34% respectively. Also, it is clear from the results that the best repeatability and reproducibility are going to be obtained by using the element correction factor associated with each dosimeter rather than carefully selecting the individual dosimeters. Finally, from the results we can note that at low doses the value of relative coefficient of variation becomes very large while at high doses it becomes basically constant and of the order of a few percent.

General radiographic attributes of optically stimulated luminescence dosimeters: A basic insight

We report the ubiquitous radiographic characteristics of optically stimulated luminescence dosimeters (OSLD) so called nanoDot OSLDs (Landauer Inc., Glendwood, IL). The X-ray irradiations were performed in free air ambiance to inspect the repeatability, the reproducibility, the signal depletion, the element correction factors (ECFs), the dose response and the energy dependence. Repeatability of multiple readouts after single irradiation to 10 mGy revealed a coefficient of variation below 3%, while the reproducibility in repeated irradiation-readout-annealing cycles was above 2%. The OSL signal depletion for three nanoDots with simultaneous irradiation to 20 mGy and sequential readouts of 25 times displayed a consistent signal reduction ≈0.5% per readout with R2 values over 0.98. ECFs for individual OSLDs were varied from 0.97 to 1.03. In the entire dose range under 80 kV, a good linearity with an R2 exceeding 0.99 was achieved. Besides, the percentage difference between OSLD and ion-chamber dose was less than 5%, which was superior to TLD. The X-ray photon irradiated energy response factors (between 0.76 and 1.12) in the range of 40–150 kV (26.1–61.2 keV) exhibited significant energy dependence. Indeed, the nanoDot OSLDs disclosed good repeatability, reproducibility and linearity. The OSLDs measured doses were closer to ion-chamber doses than that of TLD. It can be further improved up to ≈3% by applying the individual dosimeter ECF. On top, the energy dependent uncertainties can be minimized using the energy correction factors. It is established that the studied nanoDot OSLDs are prospective for measuring entrance dose in general radiographic practices.

A Preliminary Study for Dose Calculation using optically Stimulated Luminescence Dosimeters in High-Energy Radiation

Background/Objectives: To determine the reproducibility, homogeneity, and sensitivity correction factor of optically stimulated luminescence dosimeters (OSLDs) for accurate measurement in Co-60. Methods/Statistical analysis: In this study, ananoDotOSLD (Landauer, Inc., Glenwood, USA), a Micro Star read out device (Landauer, Inc., Glenwood, USA), and a Co-60 Theratron 780 irradiator (AECL, Kanata, Canada) were used for the experiments. OSLDs were divided into three groups by radiation sensitivity and each consisted of dosimeters with radiation sensitivity variations within ± 1.5%. The dosimetric characteristics of OSLD were evaluatedin 60-Co. Findings: The division into three groups was performed to place an error within ± 1.5% on the basis of the normalized relative sensitivity equaling1.00. The first group(A) contained 30 devices within the range of 1.015 to 1.045, the second group (B, 26EA) within 0.985 to 1.015, and the third group (C, 19EA) within 0.955 to 0.985. The reproducibility evaluation for OSLD is expressed with a CoV and compared. All of the reproducibility was within 2.0%: The A group was within the .66-1.94%range, the B group within.84-1.99%, and the C group within 0.54%-1.88%. The evaluation of batch homogeneity is expressed as the CoV in the whole group and the A, B, and C groups’ values were 0.38%, 1.48%, and 1.47%, respectively. Improvements/Applications: This study intended to improve the accuracy of dose evaluations with Co-60 (r-ray) by using sampling and grouping and by applying an Element Correction Factor (ECF) in the process of using OSLDs.

Long term OSLD reader stability in the ACDS level one audit.

The Australian Clinical Dosimetry Service (ACDS) has demonstrated the capacity to perform a basic dosimetry audit on all radiotherapy clinics across Australia. During the ACDS’s three and a half year trial the majority of the audits were performed using optically stimulated luminescence dosimeters (OSLD) mailed to facilities for exposure to a reference dose, and then returned to the ACDS for analysis. This technical note investigates the stability of the readout process under the large workload of the national dosimetry audit. The OSLD readout uncertainty contributes to the uncertainty of several terms of the dose calculation equation and is a major source of uncertainty in the audit. The standard deviation of four OSLD readouts was initially established at 0.6 %. Measurements over 13 audit batches—each batch containing 200−400 OSLDs—showed variability (0.5−0.9 %) in the readout standard deviation. These shifts have not yet necessitated a change to the audit scoring levels. However, a standard deviation in OSLD readouts greater than 0.9 % will change the audit scoring levels. We identified mechanical wear on the OSLD readout adapter as a cause of variability in readout uncertainty, however, we cannot rule out other causes. Additionally we observed large fluctuations in the distribution of element correction factors (ECF) for OSLD batches. We conclude that the variability in the width of the ECF distribution from one batch to another is not caused by variability in readout uncertainty, but rather by variations in the OSLD stock.

SU-E-T-173: Evaluation of the MicroSTAR Reader for the OSLD System for Remote Dosimetry Audits at the IROC HQAC

Purpose: Analysis of the performance of the microSTARii reader for optically stimulated luminescence dosimeters (OSLD) used by the IROC Houston Quality Assurance Center (IROC HQAC) for external audits compare to the characteristics of the Microstar reader. Methods: The IROC HQAC uses the Microstar reader for the OSLD program for verification of output of photon, electron and proton beams. The calculation of dose from the OSLD system is based on a group of factors defined at time of the commissioning of a batch of detectors. Factors like system sensitivity (SS), depletion (KD), element correction factor (ECF), linearity (KL), energy correction (KE). The new microSTARii unit presents many hardware and software upgrades that were considered useful for this program. Based on these changes many factors, that were considered reader dependent, were revised in order to analyze the effect of the new reading process. The SS, KD, ECF and KL were evaluated and compared with data defined based on reading done on Microstar reader. Results: The SS is reader specific and specified at 100 cGy dose level. This value is define per reading session and monitored over time. The KD factor was found to be different because of reading procedure are different. Themore » ECF values changed for a group of nano dots compare to values defined based on reading done on the Microstar reader. The KL was defined for the reader. Conclusion: The new microSTARii reader presents new features that improve the efficiency of the OSLD program at the IROC HQAC. New characterization is needed before final implementation is done to match the requirements of the existing OSLD system defined for the Microstar reader. Changes in uncertainty of the results has not been analized.« less

SU‐E‐T‐31: The Evaluation of Dosimetric Characteristics of OSLDs Based On Output Correction Factor in Low Energy

Purpose: The output of General X‐ray unit (low energy) has variation against 60Co unit. So it is necessary to minimize output variation when evaluating dosimetric characteristics of optical stimulated luminance dosimeters (OSLDs). Considering output variation of X‐ray unit, we evaluated dosimetric characteristics of OSLDs in low energyMethods: The OSLDs that used were nanoDotTM Dosimeter (Landauer Inc, Glenwood, USA) and this dosimeter had never been irradiated. Through single irradiate them (test dose was 7mGy), we certified batch homogeneity and sampled dosimeters with variation of radiation sensitivity within ± 1.5% among them. Using to these dosimeters certified reproducibility considering output variation every time when irradiated OSLDs. Through this process, we certified element correction factor (ECF) and coefficient of variation (COV) about each OSLD. Based on these OSLDs, we studied linearity, energy dependence and angular dependenceResults: The batch homogeneity was 1.21% of the coefficient of variation (after sampling). The average value of COV about reproducibility of OSLDs was reduced from 1.3% to 0.96% after applying to output correction factor. The linearity was that the correlation of between dose and count was fitted by linear function (R2 =0.997). The energy dependence study showed a range of ion chamber‐to OSLD rations from 0.23 (24.7 keV) to 0.27 (34.5 keV). According to energy, the range of angular dependence was from 0.1% to 8.4% variation when each degree was normalized by zero degreeConclusion: Considering output correction factor, reduced uncertainty occurred in general x‐ray unit. We acquired information that was ECF, COV of OSLDs in low energy. Using to these OLDs, It is feasible to measure patient dose of diagnostic radiography and Cone beam computed tomography for radiotherapy

Energy responses of the LiF series TL pellets to high-energy photons in the energy range from 1.25 to 21 MV

The energy responses for the KLT-300(LiF:Mg,Cu,Na,Si, Korea), GR-200(LiF:Mg,Cu,P, China) and MCP-N(LiF:Mg,Cu,P, Poland) thermoluminescence(TL) pellets were studied for a photon radiation with energies from 1.25 MeV(60Co) to 21 MV (Microtron) to verify the usefulness of the calibration for the radiotherapy beams. The International Atomic Energy Agency (IAEA) and the World Health Organization (WHO) have performed thermoluminescence dosimetry (TLD) audits to verify the calibration of the beams by TL powder, but TL pellets were used in this study because the element correction factor (ECF), defined as the factor to correct the variations that all TL dosemeters cannot be manufactured to have exactly the same TL efficiency, for each TL pellet could be accurately derived and be handled conveniently when compared with the powder. Also several works for the energy response of the TLDs were done for the low-energy photon beams up to 60Co, but they will be extended in this experiment to the high photon energies (up to 20 MV), which are widely used in the therapy level of a radiation. The PTW 30006 ionisation chamber was calibrated by the Korea primary standards to establish the air-kerma rates and the TL pellets were irradiated in a specially designed waterproof pellet holder in a water phantom (30 x 30 x 30 cm3) just like the IAEA postal audits programme. This result was compared with that of another type of phantom [10 (W) x 10 (L) x 10 (H) cm3 PMMA Perspex phantom for the 60Co and 6 MV photon, and 10 x 10 x 20 (H) cm3 for the 10 and 21 MV photon] for its convenient use and easy handling and installation in a hospital. The results show that the differences of the responses for the water phantom and PMMA Perspex phantom were negligible, which is contrary to the general conception that a big difference would be expected. For an application of these results to verify the therapy beams, an appropriate energy correction factor should be applied to the energies and phantom types in use.

Characteristics of the eigenvalue sensitivities to the change of element correction factors for beams

Some characteristics of the eigenvalue sensitivities have been found for beams in the paper. For cantilever beams and simply supported beams, the sensitivities of the eigenvalues to the stiffness correction factor of one element are equal and opposite to the sensitivities to the mass correction factor of the symmetrically positioned element. For beams with other boundary conditions, however, the relationship does not hold. The relationship has been proven analytically for simply supported beams.

Sensitivity of the eigenvalues of beams to the change of element correction factors

Sensitivity of the eigenvalues of beams to the change of element correction factors

Production of element correction factors for thermoluminescent dosimeters.

Approximately 80 processors of personal dosimetry in the United States use thermoluminescent dosimeters (TLDs). Recent demands that dosimetry processors be able to measure radiation doses to within +/- 50% of the correct value have focused attention on the reproducibility of the TL elements within each TLD. The phosphors for these TLDs are manufactured by three companies. A dosimetry processor faces three options concerning the quality of the TL elements purchased; trust the supplier's quality control program, screen new TL elements and discard those that are extremely bad, or use element correction factors (ECFs). The first option results in dosimetry processors failing the +/- 50% accuracy requirement due to excessive variability among the TL elements. The second option still permits large precision errors that come close to the +/- 50% accuracy requirement. This paper advocates the third option and presents a 10-step procedure to produce ECFs. The procedure ensures that the ECFs represent only variations among the TL elements and not variations caused by stability problems with the TLD reader. Following is an example of ECF production for 3000 TLDs.