Thermoluminescent dosimetry in clinical kilovoltage beams

Aim: This study aimed at calibrating a new set of GR-200A thermoluminescent dosimeters (TLDs) in low and medium kilovoltage energy photon therapy beams and in a diagnostic beam of known beam quality, in order to determine their response and to establish if the same set of TLDs could be used across both environments for in-vivo dosimetry purposes. Methods and Materials: A set of 20 TLDs was used for this study. An Oven type PCL3 was used to anneal the TLDs. The response of the TLDs was read using the Reader type LTM manufactured by Harshaw Bicron, United State of America. Vacuum tweezers were used to transfer the TLDs at the time of measurements and calibration. TLDs were kept in a subdued ultra-violet environment between the annealing and irradiation process. TLDs were placed on a 30 x 30 x 17.6 cm³ Polymethylmethacrylate (PMMA) phantom during irradiation. A calibrated Orthovoltage machine was used to deliver a known absorbed dose to the TLDs. A cylindrical ionization chamber (PTW 30001) and an electrometer (PTW 10008) were used to confirm the absorbed dose delivery of the orthovoltage machine at the time of measurement. Likewise, a calibrated LX40 radiotherapy Simulator was used to deliver a known diagnostic absorbed dose to the TLDs. A TM77334 ionization chamber was used similarly to confirm the absorbed dose. The TLDs were also irradiated on the PMMA phantom. The accepted variation in raw response of the individual TLDs from the average of the batch was compared and a deviation of less than ± 20 % was considered within tolerance. A 10 % tolerance was subsequently considered suitable for the measurement of absorbed dose. Results: Of the 20 TLDs calibrated in the 95 kVp therapy beam (3 mm Al half-value layer), 17 were within the accepted response level (i.e. ± 20 % deviation), 17 in the 180 kVp therapy beam (1 mm Cu half-value layer), 16 in the 300 kVp therapy beam (3 mm Cu half-value layer) and 15 in the diagnostic beam of 80 kVp (2.97 mm Al half-value layer). 16 of the 17 TLDs were within ± 10 % dose tolerance at 95 kVp whereas all the TLDs that were within the accepted response level at the 180 kVp and 300 kVp, were within the ± 10 % dose tolerance. 12 of the 15 TLDs at the diagnostic beam energy were within the ± 10 % dose tolerance. Three of the TLDs were therefore rejected at all energies. Conclusion: The study concludes that the same set of GR-200A TLDs could be used across both kilovoltage therapy and diagnostic fluoroscopy environments for in-vivo dosimetry purposes if an accuracy of ± 10 % is considered acceptable, however a separate calibration needs to done at each beam quality. Individual dosimeters from a batch should be carefully identified and sorted during the calibration process prior to clinical use

Determination of Calibration Factors and Uncertainties Associated with the Irradiation of MTS-N (LiF: Mg, Ti) Chips with Cesium-137 and X-ray Sources Under Low Doses for Personal Dosimetry in Diagnostic Radiology

Purpose: The purpose of this study was to compare calibration factors for deep dose equivalent Hp (10) and shallow dose equivalent Hp (0.07) between Cesium (Cs)-137 and X-ray sources when they are exposed to same dose and to determine uncertainties with MTS-N (LiF: Mg, Ti) chips when they are exposed to low dose ≤ 2mGy. Material and Methods: Thermoluminescent (TL) chips were annealed at 400oC for one hour and allowed to cool and were subjected to a temperature of 100oC for another two hours using a TLD Furnace Type LAB-01/400. They were then taken to a Secondary Standard Dosimetry Laboratory (SSDL) for irradiation using a Cs-137 source at known doses (0.2-2mGy). A RadPro Cube 400 manual TLD Reader was used to determine corresponding TL signal. The above process was replicated but with a calibrated X-ray unit as the source for calibration. Results: The calibration factors (CF) from the line graph of dose (mGy) against TL signal (count) for Cs-137 source with Hp (10) and Hp (0.07) were 3.72 x 10-6 and 5.97x10-6 mGy/count respectively. Those with X-ray source for Hp (10) and Hp (0.07) were 3.44x10-6 and 4.05x10-6 mGy/count respectively with an overall coefficient of determination (R2) = 0.99. The adjusted maximum percentage deviation between the actual and calculated dose for both sources was -2.74%. The percent (%) deviation of the mean with both sources for Hp (10) and Hp (0.07) was 3.9% and 19% respectively. Conclusion: Adjusted percent deviation from both sources were within the recommended dose limit of ±30% by the Radiological Protection Institute of Ireland (RPII) and within the International Commission on Radiological Protection (ICRP) limit respectively. Better accuracy was seen for Hp (10) with both sources compared to Hp (0.07). Calibration of the MTS-N chips using both sources was successful and can be used for personal dosimetry

Determination of Calibration Factors and Uncertainties Associated with the Irradiation of MTS-N (LiF: Mg, Ti) Chips with Cesium-137 and X-ray Sources Under Low Doses for Personal Dosimetry in Diagnostic Radiology

Purpose: The purpose of this study was to compare calibration factors for deep dose equivalent Hp (10) and shallow dose equivalent Hp (0.07) between Cesium (Cs)-137 and X-ray sources when they are exposed to same dose and to determine uncertainties with MTS-N (LiF: Mg, Ti) chips when they are exposed to low dose ≤ 2mGy. Material and Methods: Thermoluminescent (TL) chips were annealed at 400oC for one hour and allowed to cool and were subjected to a temperature of 100oC for another two hours using a TLD Furnace Type LAB-01/400. They were then taken to a Secondary Standard Dosimetry Laboratory (SSDL) for irradiation using a Cs-137 source at known doses (0.2-2mGy). A RadPro Cube 400 manual TLD Reader was used to determine corresponding TL signal. The above process was replicated but with a calibrated X-ray unit as the source for calibration. Results: The calibration factors (CF) from the line graph of dose (mGy) against TL signal (count) for Cs-137 source with Hp (10) and Hp (0.07) were 3.72 x 10-6 and 5.97x10-6 mGy/count respectively. Those with X-ray source for Hp (10) and Hp (0.07) were 3.44x10-6 and 4.05x10-6 mGy/count respectively with an overall coefficient of determination (R2) = 0.99. The adjusted maximum percentage deviation between the actual and calculated dose for both sources was -2.74%. The percent (%) deviation of the mean with both sources for Hp (10) and Hp (0.07) was 3.9% and 19% respectively. Conclusion: Adjusted percent deviation from both sources were within the recommended dose limit of ±30% by the Radiological Protection Institute of Ireland (RPII) and within the International Commission on Radiological Protection (ICRP) limit respectively. Better accuracy was seen for Hp (10) with both sources compared to Hp (0.07). Calibration of the MTS-N chips using both sources was successful and can be used for personal dosimetry

Determination of reference dose levels among selected X-ray centers in Lagos State, South-West Nigeria

Background: With increasing use of diagnostic X-ray machines across Lagos, South-West Nigeria, relevant international bodies have proposed the use of reference dose levels (RDLs) to help manage radiation dose to patients without compromising image quality. Objectives: The purpose of this study was to determine the entrance surface dose (ESD) at third quartile (75 th percentile) in Lagos metropolis for normal adult radiographic examinations and to compare them with national and international established reference dose guidelines. Materials and Methods: One dedicated X-ray unit in each diagnostic center was used for this study denoted as H1-H10. A noninvasive Unfors ThinX RAD kilovoltage (kVp) meter which served as the phantom (mimicked patient) was used for measurements. The ESDs were determined by placing the Unfors ThinX RAD kVp meter on the patient′s table bucky at a source to image distance (SID) of 100 cm and at the erect bucky at a SID of 180 cm. The peak tube kVp was varied at different milliampere seconds (mAs). Results : The mean ESD for adult postero-anterior (PA) chest, antero-posterior (AP) Abdomen and (AP) lumbar spine X-ray examination were 0.603, 2.57, and 2.57 mGy, respectively. While the ESD for the third quartile for adult (PA) chest, (AP) abdomen, and (AP) lumbar spine X-ray examination among the ten X-ray centers were 0.93, 2.74, and 2.47 mGy, respectively. Conclusion: Third quartile ESD which translate to RDL for adult (PA) chest examination for this study was higher compared to other national and international RDLs but adult (AP) abdomen and lumbar spine examinations were within accepted national and international range

Evaluation of kilovoltage failure in conventional X-ray machines among selected X-ray Centers in Jos North Local government area of Plateau State, Nigeria

Background: The goal of quality assurance of x-ray machine is to obtain accurate and timely diagnosis and low dose to patients.Aims and Objectives: To determine peak kilovoltage (kVp) accuracy and reproducibility of 23 individual X-ray machine and compare their values with recommended standards.Materials and Methods: Measurements were taken in the 23 diagnostic centers (XRY1 – XRY23 ). Unfors Thinx RAD detector which served as the mimicked patient was placed at Source to Image Distance (SID) of 100 cm. At a constant tube current, several kVp values were used for both the accuracy and reproducibility test. Any results above ±5% were termed “Not Acceptable”. Data analysis value was done using GraphPad Prism version 5.0 statistics software.Results: A total of 16 X-ray machine passed the kVp accuracy test (69.57%) while 7 X-ray machine failed the test (30.43%). Also, 15 X-ray machine passed the kVp reproducibility test (65.22%), while 8 X-ray machine failed the test (34.78%).There was no correlation (P = 0.916) between kVp accuracy and reproducibility test also no significant difference (P = 0.5134). Very good statistical significant difference was seen between the machine age, kVp accuracy and reproducibility respectively (P<0.05)Conclusion: This study has shown that more than one third of the total X-ray machines failed both kVp accuracy and reproducibility test. There are strong indications that machine age had significant effect on kVp accuracy and reproducibility.Keywords: Peak kilovoltage, Radiation dose, X-ray, Unfors Thinx RAD detector, Milliampere second

Dosimetric effect of the gantry rotations of a novel trunk phantom using an area integration algorithm

Background: Treatment planning systems (TPSs) have proved to be a useful tool in predetermining how a treatment outcome will be in radiotherapy. The accuracy of any TPS to calculate dose to any arbitrary point within a material is largely dependent on the mathematical algorithm used. Aims: The purpose of this study was to design a local trunk phantom and use the phantom to check the percentage dose accuracy of the Area Integration Algorithm of a Precise PLAN 2.16 TPS if it is in agreement with results obtained from manufacturer's verification by varying the gantry angle and whether it is within ± 5% International Commission on Radiation Units and Measurements (ICRU) minimal limit. Materials and Methods: The study was executed with a locally designed phantom made of Plexiglas with six insert and an ionization chamber port. The phantom was simulated using a HiSpeed NX/i computed tomography scanner and Precise PLAN 2.16 TPS for application of beam setup parameters. The mimicked organs for the inserts were: 25%–75% Glycerol-Water for liver, pure carboxyl methyl cellulose was used for lungs, 30%–70% Glycerol-Water for muscle, 40%–60% Glycerol-Water was used for adipose, pure Sodium hypochlorite was used for bone and pure sodium laureth sulfate (Texapon) for kidney. Results: The maximum percentage (%) deviation with a large field for six inhomogeneous inserts and with bone only homogeneous inserts were 3.4% and 2.9%, respectively. The maximum % deviation with a small field for six inhomogeneous inserts was 3.2%. The % deviation between the solid water phantom and the locally designed phantom was 3.5%. Conclusion: The Area Integration Algorithm has shown an overall accuracy of 4% below 5% ICRU minimal limit. There was no statistically significant difference in field sizes and in inhomogeneity/homogeneity, respectively. Variation exists in % deviation for small field size with parallel opposed field between our verification and the manufacturers