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Thermoluminescence and electronic dosimetry for 511 kev photons
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Abstracts / Physica Medica 30 (2014) e75ee121
mSv/year for nuclear medicine physicians as well as all workers of the radiology department. Conclusions: 1. Local dose constraints can be estimated on the basis of workers’ dose records for the last 3-4 years using simple statistical analysis. 2. The level of dose constraints should be adjusted to the local conditions of radiation protection of workplaces and tasks performed by various professional groups. THERMOLUMINESCENCE AND ELECTRONIC DOSIMETRY FOR 511 KEV PHOTONS
effect on the output factor. A nominal field of 1x1 cm2 has an output factor 5.7% lower than the 1.1x1.1cm2 and 10.5% than the 1.2X1.2cm2 radiological field. We conclude that special care must be taken for small fields. An adequate algorithm modelling the beam source should be used. The source parameters should be fine-tuned for accurate dose calculations. Furthermore, careful quality control of the Linac’s mechanical components should be carried out. Keywords: Small fields, Dosimetry, External radiotherapy
K. Dalianis, K. Gogos, R. Efthimiadou, I. Andreou, V. Prassopoulos. Hygeia S.A. PET/CT Department, Athens, Greece
INVESTIGATING COLUMNAR SCINTILLATORS THROUGH ANALYTICAL MODELING. A SEMIEMPIRICAL APPROACH
Aim: The most commonly used radio nuclide in PET/CT departments is 18F-FDG. The 18 F-FDG PET/CT applications have been continuously increasing for diagnostic procedures. Although such an increase is a positive trend for the benefit of patients, the associated risk of radiation exposure of staff needs to be properly evaluated. Our aim is to compare the estimated doses from TLDs dosimeters with the electronic dosimeters regarding 511 kev annihilation photons following the ALARA principle. Method: In order to estimate the doses from external exposure, all 7 members of the staff (2 nurses, 2 medical physicists, 2 technologists, 1 secretary) had TLD badges worn at the upper pocket of their overall and digital dosimeters at their upper side pocket. Results: The dose estimated from the TLDs is evaluated monthly and is compared with the dose assessed by the electronic dosimeter. The results reveal a difference between the electronic dosimeters and the TLDs from 0,2% to 1,3%, with the dose being overestimated when assessed by TLDs. The main reason for the differences is that the dosimeters have been used for different time periods. When both TLD and electronic dosimeters have been correctly measured then the TLD is considered to show the correct value of the absorbed dose and the dose is overestimated only by a small percentage of 0.37% when measured with TLDs. Conclusion: TLDs are considered to be trustworthy in assessing the absorbed dose. However the use of electronic dosimeters cannot be ignored in the daily practice as they measure radiation exposure on a real time basis and provide immediate dose rate readings.
N. Kalyvas, C. Michail, G. Fountos, I. Valais, I. Kandarakis, D. Cavouras. Department of Biomedical Engineering, Technological Educational Institute of Athens, Greece
ADJUSTING SOURCE MODELLING PARAMETERS FOR ACCURATE DOSE CALCULATIONS WITH THE ONCENTRA MASTERPLAN TREATMENT PLANNING SYSTEM FOR SMALL FIELDS WITH DIMENSIONS OF LESS THAN 5X5CM2 Malatar Georgia a, S. Papatheodorou a, V. Anastasis b. a University Hospital Patras, Greece; b University of Patras, Greece The dosimetry of small fields in photon radiotherapy is a challenging issue. The goal of this work is to establish a reliable method to measure the absolute dose in small fields and to verify the TPS calculations, such as output factors, PDDs and profiles for small fields less than 5X5cm2. The output factors for collimator settings from 5x5cm2 down to 1X1cm2 were measured in a water tank at the reference depth of 10cm, at SSD¼100cm, with the PinPoint chamber and a calibrated MOSFET dosimeter. Output factors and dose profiles have been measured on an Elekta Synergy Linac with a 6MV photon beam. They have also been calculated using the Oncentra Masterplan version 4.3. We have found out that it is not easy to determine source-size parameters, for the TPS, which yield acceptable results for the entire range of field sizes (from 40x40 cm2 to 2x2cm2). Therefore, we have created a new “virtual” Linac in the TPS beam library dedicated to small field clinical applications. For this Linac the source-size parameters have been fine-tuned such that the calculated profiles match the measured data for small fields only. With this approach the discrepancies between measured and calculated doses have been diminished considerably. The comparisons between measured and calculated PDDs and dose profiles have been evaluated using the methodology of the g-composite criterion. For fields less than 2X2cm2 the output factor drops rapidly with field size. Small errors in the filed size due to jaw misplacements have a dominant
X-ray digital imaging systems often employ scintillator materials as part of the radiation detector (indirect detection). Recently CsI:Tl columnar phosphor is used extensively as an X-ray converter due to its higher spatial resolution compared to granular phosphors like Gd2O2S:Tb. The efficiency and the imaging performance of CsI:Tl has been already studied either experimentally or by Monte Carlo simulation. The aim of this work is to provide a semi empirical analytical model for determining CsI:Tl optical photon propagation related parameters like photon attenuation per elementary thickness and spread to the output. Hence the calculation of Detective Quantum Efficiency (DQE) of CsI:Tl for different column thicknesses (50mm up to 300mm) was achieved. The developed model considers the absorption of X-rays in the scintillator, the production of optical photons, the propagation and the reflections of the optical photons to the input and output interfaces. Finally it acknowledges the spread of the optical photons to the output. The optical photon propagation is studied by considering that the fraction of the optical photons passing through an elementary screen layer is constant. The optical spread to the output is assumed to result in a Gaussian shaped LSF. Published Monte Carlo data for 85 mm, 100 mm and 300 mm CsI:Tl columns were used to fit the analytical model equations. The optical photon fraction transmitted through each layer was found approximately 98.6%. The LSF shape was found to be depended by column thickness. Higher DQE at 0 lp/mm exhibited the 300mm column thickness CsI:Tl, while at 10lp/mm the 50mm column thickness CsI:Tl. It was interesting to notice that at approximately 4lp/mm the DQE curves of all thicknesses were comparable. Keywords: Columnar phosphors, MTF, DQE Acknowledgement: This research has been co-funded by the European Union (European Social Fund) and Greek national resources under the framework of the “ARISTEIA” project MISCIRLU code 1476 of the “Education & Lifelong Learning” Operational Programme.
NEW CHALLENGES IN OPTIMIZATION OF RADIATION-DOSE IN INFANTS HIP-EXAMINATIONS X-RAY PROCEDURES € stur Finbogasson MD, PhD, Britt-Marie Shahla Mobini MSc, Thro € OstlundRadiologic Nurse, Theo Chouliaras MSc, Angela Karambatsakidou € m MSc licentiate. Department of Medical Physics, MSc, Jan Lindstro Karolinska University Hospital, S-171 76 Stockholm Sweden; Department of Pediatric Radiology Astrid Lindgrens at Karolinska University Hospital, S171 76 Stockholm, Sweden The risk of radiation effect is age dependent according to International Commission on Radiological Protection ICRP (2007). Since children have a longer life expectancy than adults, it follows a higher probability of late effects. The variation in sensitivity of different organs and tissue also differ from that of adults (BEIR V 1990). Optimization of radiation dose for pediatric patients is therefore of high priority. A dose survey was performed with the aim to establish the Local Dose Reference level (DRL) for various examination types. The result showed that there is room for further optimization in hipexaminations and the corresponding anatomic specific programs. A
Abstracts / Physica Medica 30 (2014) e75ee121
mSv/year for nuclear medicine physicians as well as all workers of the radiology department. Conclusions: 1. Local dose constraints can be estimated on the basis of workers’ dose records for the last 3-4 years using simple statistical analysis. 2. The level of dose constraints should be adjusted to the local conditions of radiation protection of workplaces and tasks performed by various professional groups. THERMOLUMINESCENCE AND ELECTRONIC DOSIMETRY FOR 511 KEV PHOTONS
effect on the output factor. A nominal field of 1x1 cm2 has an output factor 5.7% lower than the 1.1x1.1cm2 and 10.5% than the 1.2X1.2cm2 radiological field. We conclude that special care must be taken for small fields. An adequate algorithm modelling the beam source should be used. The source parameters should be fine-tuned for accurate dose calculations. Furthermore, careful quality control of the Linac’s mechanical components should be carried out. Keywords: Small fields, Dosimetry, External radiotherapy
K. Dalianis, K. Gogos, R. Efthimiadou, I. Andreou, V. Prassopoulos. Hygeia S.A. PET/CT Department, Athens, Greece
INVESTIGATING COLUMNAR SCINTILLATORS THROUGH ANALYTICAL MODELING. A SEMIEMPIRICAL APPROACH
Aim: The most commonly used radio nuclide in PET/CT departments is 18F-FDG. The 18 F-FDG PET/CT applications have been continuously increasing for diagnostic procedures. Although such an increase is a positive trend for the benefit of patients, the associated risk of radiation exposure of staff needs to be properly evaluated. Our aim is to compare the estimated doses from TLDs dosimeters with the electronic dosimeters regarding 511 kev annihilation photons following the ALARA principle. Method: In order to estimate the doses from external exposure, all 7 members of the staff (2 nurses, 2 medical physicists, 2 technologists, 1 secretary) had TLD badges worn at the upper pocket of their overall and digital dosimeters at their upper side pocket. Results: The dose estimated from the TLDs is evaluated monthly and is compared with the dose assessed by the electronic dosimeter. The results reveal a difference between the electronic dosimeters and the TLDs from 0,2% to 1,3%, with the dose being overestimated when assessed by TLDs. The main reason for the differences is that the dosimeters have been used for different time periods. When both TLD and electronic dosimeters have been correctly measured then the TLD is considered to show the correct value of the absorbed dose and the dose is overestimated only by a small percentage of 0.37% when measured with TLDs. Conclusion: TLDs are considered to be trustworthy in assessing the absorbed dose. However the use of electronic dosimeters cannot be ignored in the daily practice as they measure radiation exposure on a real time basis and provide immediate dose rate readings.
N. Kalyvas, C. Michail, G. Fountos, I. Valais, I. Kandarakis, D. Cavouras. Department of Biomedical Engineering, Technological Educational Institute of Athens, Greece
ADJUSTING SOURCE MODELLING PARAMETERS FOR ACCURATE DOSE CALCULATIONS WITH THE ONCENTRA MASTERPLAN TREATMENT PLANNING SYSTEM FOR SMALL FIELDS WITH DIMENSIONS OF LESS THAN 5X5CM2 Malatar Georgia a, S. Papatheodorou a, V. Anastasis b. a University Hospital Patras, Greece; b University of Patras, Greece The dosimetry of small fields in photon radiotherapy is a challenging issue. The goal of this work is to establish a reliable method to measure the absolute dose in small fields and to verify the TPS calculations, such as output factors, PDDs and profiles for small fields less than 5X5cm2. The output factors for collimator settings from 5x5cm2 down to 1X1cm2 were measured in a water tank at the reference depth of 10cm, at SSD¼100cm, with the PinPoint chamber and a calibrated MOSFET dosimeter. Output factors and dose profiles have been measured on an Elekta Synergy Linac with a 6MV photon beam. They have also been calculated using the Oncentra Masterplan version 4.3. We have found out that it is not easy to determine source-size parameters, for the TPS, which yield acceptable results for the entire range of field sizes (from 40x40 cm2 to 2x2cm2). Therefore, we have created a new “virtual” Linac in the TPS beam library dedicated to small field clinical applications. For this Linac the source-size parameters have been fine-tuned such that the calculated profiles match the measured data for small fields only. With this approach the discrepancies between measured and calculated doses have been diminished considerably. The comparisons between measured and calculated PDDs and dose profiles have been evaluated using the methodology of the g-composite criterion. For fields less than 2X2cm2 the output factor drops rapidly with field size. Small errors in the filed size due to jaw misplacements have a dominant
X-ray digital imaging systems often employ scintillator materials as part of the radiation detector (indirect detection). Recently CsI:Tl columnar phosphor is used extensively as an X-ray converter due to its higher spatial resolution compared to granular phosphors like Gd2O2S:Tb. The efficiency and the imaging performance of CsI:Tl has been already studied either experimentally or by Monte Carlo simulation. The aim of this work is to provide a semi empirical analytical model for determining CsI:Tl optical photon propagation related parameters like photon attenuation per elementary thickness and spread to the output. Hence the calculation of Detective Quantum Efficiency (DQE) of CsI:Tl for different column thicknesses (50mm up to 300mm) was achieved. The developed model considers the absorption of X-rays in the scintillator, the production of optical photons, the propagation and the reflections of the optical photons to the input and output interfaces. Finally it acknowledges the spread of the optical photons to the output. The optical photon propagation is studied by considering that the fraction of the optical photons passing through an elementary screen layer is constant. The optical spread to the output is assumed to result in a Gaussian shaped LSF. Published Monte Carlo data for 85 mm, 100 mm and 300 mm CsI:Tl columns were used to fit the analytical model equations. The optical photon fraction transmitted through each layer was found approximately 98.6%. The LSF shape was found to be depended by column thickness. Higher DQE at 0 lp/mm exhibited the 300mm column thickness CsI:Tl, while at 10lp/mm the 50mm column thickness CsI:Tl. It was interesting to notice that at approximately 4lp/mm the DQE curves of all thicknesses were comparable. Keywords: Columnar phosphors, MTF, DQE Acknowledgement: This research has been co-funded by the European Union (European Social Fund) and Greek national resources under the framework of the “ARISTEIA” project MISCIRLU code 1476 of the “Education & Lifelong Learning” Operational Programme.
NEW CHALLENGES IN OPTIMIZATION OF RADIATION-DOSE IN INFANTS HIP-EXAMINATIONS X-RAY PROCEDURES € stur Finbogasson MD, PhD, Britt-Marie Shahla Mobini MSc, Thro € OstlundRadiologic Nurse, Theo Chouliaras MSc, Angela Karambatsakidou € m MSc licentiate. Department of Medical Physics, MSc, Jan Lindstro Karolinska University Hospital, S-171 76 Stockholm Sweden; Department of Pediatric Radiology Astrid Lindgrens at Karolinska University Hospital, S171 76 Stockholm, Sweden The risk of radiation effect is age dependent according to International Commission on Radiological Protection ICRP (2007). Since children have a longer life expectancy than adults, it follows a higher probability of late effects. The variation in sensitivity of different organs and tissue also differ from that of adults (BEIR V 1990). Optimization of radiation dose for pediatric patients is therefore of high priority. A dose survey was performed with the aim to establish the Local Dose Reference level (DRL) for various examination types. The result showed that there is room for further optimization in hipexaminations and the corresponding anatomic specific programs. A