Dose rate constants for125I,103Pd,192Ir and169Yb brachytherapy sources: an EGS4 Monte Carlo study
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An exhaustive revision of dosimetry data for 192Ir, 125I, 103Pd and 169Yb brachytherapy sources has been performed by means of the EGS4 simulation system. The DLC-136/PHOTX cross section library, water molecular form factors, bound Compton scattering and Doppler broadening of the Compton-scattered photon energy were considered in the calculations. The absorbed dose rate per unit contained activity in a medium at 1 cm in water and air-kerma strength per unit contained activity for each seed model were calculated, allowing the dose rate constant (DRC) A to be estimated. The influence of the calibration procedure on source strength for low-energy brachytherapy seeds is discussed. Conversion factors for 125I and 103Pd seeds to obtain the dose rate in liquid water from the dose rate measured in a solid water phantom with a detector calibrated for dose to water were calculated. A theoretical estimate of the DRC for a 103Pd model 200 seed equal to 0.669 +/- 0.002 cGy h(-1) U(-1) is obtained. Comparison of obtained DRCs with measured and calculated published results shows agreement within 1.5% for 192Ir, 169Yb and 125I sources.Keywords:
Kerma
Aim of this work is to calibrate the high dose rate (HDR) brachytherapy source 60Co. The radioactive source calibration is a very important part of the quality assurance program for dosimetry of brachytherapy source. The goal of this project is the calibration of HDR Brachytherapy source in radiation therapy is the measurement of the air kerma rate which required actual dose to deliver. The source calibration is an essential part of the quality assurance program for dosimetry of brachytherapy source. This research will help the patient who is involving brachytherapy treatment. HDR brachytherapy source 60Co is inserted directly or in close to the tumor. Most commonly using method for calibration of HDR brachytherapy source 60CO is well type ionization chamber. Calibration of the radioactive source 60Co brachytherapy source is very important for the treatment of cancer patient. We have got the variation between RAKR from TPS and measured Air Kerma Rate of 60Co brachytherapy source are 3.2% and 3.04% and which give very good agreement with the Air Kerma Rate (RAKR) is 5% (from BEBIG protocol, Germany). So, our results were satisfied for brachytherapy treatment. From these results, it must be concluded that, 60Co brachytherapy source is suitable for brachytherapy cancer treatment. It is very difficult to calculate treatment deliver dose without calibrating AKR of HDR brachytherapy source. It is very important to verify the calculated Air Kerma Rate by TPS Air Kerma Rate.
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In recent years, a change has been proposed from air kerma based reference dosimetry to absorbed dose based reference dosimetry for all radiotherapy beams of ionizing radiation. In this paper, a dosimetry study is presented in which absorbed dose based dosimetry using recently developed formalisms was compared with air kerma based dosimetry using older formalisms. Three ionization chambers of each of three different types were calibrated in terms of absorbed dose to water and air kerma and sent to five hospitals. There, reference dosimetry with all the chambers was performed in a total of eight high-energy clinical photon beams. The selected chamber types were the NE2571, the PTW-30004 and the Wellhöfer-FC65G (previously Wellhöfer-IC70). Having a graphite wall, they exhibit a stable volume and the presence of an aluminium electrode ensures the robustness of these chambers. The data were analysed with the most important recommendations for clinical dosimetry: IAEA TRS-398, AAPM TG-51, IAEA TRS-277, NCS report-2 (presently recommended in Belgium) and AAPM TG-21. The necessary conversion factors were taken from those protocols, or calculated using the data in the different protocols if data for a chamber type are lacking. Polarity corrections were within 0.1% for all chambers in all beams. Recombination corrections were consistent with theoretical predictions, did not vary within a chamber type and only slightly between different chamber types. The maximum chamber-to-chamber variations of the dose obtained with the different formalisms within the same chamber type were between 0.2% and 0.6% for the NE2571, between 0.2% and 0.6% for the PTW-30004 and 0.1% and 0.3% for the Wellhöfer-FC65G for the different beams. The absorbed dose results for the NE2571 and Wellhöfer-FC65G chambers were in good agreement for all beams and all formalisms. The PTW-30004 chambers gave a small but systematically higher result compared to the result for the NE2571 chambers (on the average 0.1% for IAEA TRS-277, 0.3% for NCS report-2 and AAPM TG-21 and 0.4% for IAEA TRS-398 and AAPM TG-51). Within the air kerma based protocols, the results obtained with the TG-21 protocol were 0.4-0.8% higher mainly due to the differences in the data used. Both absorbed dose to water based formalisms resulted in consistent values within 0.3%. The change from old to new formalisms is discussed together with the traceability of calibration factors obtained at the primary absorbed dose and air kerma standards in the reference beams (60Co). For the particular situation in Belgium (calibrations at the Laboratory for Standard Dosimetry of Ghent) the change amounts to 0.1-0.6%. This is similar to the magnitude of the change determined in other countries.
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New codes of practice for reference dosimetry in clinical high-energy photon and electron beams have been published recently, to replace the air kerma based codes of practice that have determined the dosimetry of these beams for the past twenty years. In the present work, we compared dosimetry based on the two most widespread absorbed dose based recommendations (AAPM TG-51 and IAEA TRS-398) with two air kerma based recommendations (NCS report-5 and IAEA TRS-381). Measurements were performed in three clinical electron beam energies using two NE2571-type cylindrical chambers, two Markus-type plane-parallel chambers and two NACP-02-type plane-parallel chambers. Dosimetry based on direct calibrations of all chambers in 60Co was investigated, as well as dosimetry based on cross-calibrations of plane-parallel chambers against a cylindrical chamber in a high-energy electron beam. Furthermore, 60Co perturbation factors for plane-parallel chambers were derived. It is shown that the use of 60Co calibration factors could result in deviations of more than 2% for plane-parallel chambers between the old and new codes of practice, whereas the use of cross-calibration factors, which is the first recommendation in the new codes, reduces the differences to less than 0.8% for all situations investigated here. The results thus show that neither the chamber-to-chamber variations, nor the obtained absolute dose values are significantly altered by changing from air kerma based dosimetry to absorbed dose based dosimetry when using calibration factors obtained from the Laboratory for Standard Dosimetry, Ghent, Belgium. The values of the 60Co perturbation factor for plane-parallel chambers (katt · km for the air kerma based and pwall for the absorbed dose based codes of practice) that are obtained from comparing the results based on 60Co calibrations and cross-calibrations are within the experimental uncertainties in agreement with the results from other investigators.
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Purpose: We conducted a multicenter study to investigate the current status of difference between the actual values at the patient entrance reference point (PERP) and display air kerma. Methods: We exposure dose and fluoroscopy dose were measured by 32 apparatuses at 32 member institutions of the Japanese Society of Circulation Imaging Technology (CITEC) under unified conditions, and the actual measured values and display air kerma were compared. We entrance doses during fluoroscopy and imaging were measured at the PERP, with focus detector distance (FDD) 110 cm, a copper plate of 3.5 mm in thickness adhered to the front face of flat panel detector (FPD) as absorber, field-of-view (FOV) 18 cm, and the frame rate of 15 f/s, excluding the bed. Display air kerma were recorded at the same time. JIS (Z 4751-2-43: 2012) specify “The reference air kerma rate and the cumulative reference air kerma shall not deviate from their respective display air kerma by more than ±35% over the range of 6 mGy/min and 100 mGy to the maximum value.” The number of apparatuses display air kerma deviated from this condition and its percentage were obtained. Results: The mean difference percentage between actual measured values and display air kerma in 32 apparatuses was approximately 15.6%, with some apparatuses showing substantially different display air kerma. Conclusion: In order to estimate patients’ skin exposure dose from display air kerma more accurately, it is necessary to perform calibration of the apparatus by regular dose measurement or convert values.
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A Monte Carlo computational model of CT has been developed and used to investigate the effect of various physical factors on the surface air kerma length product, the peak surface air kerma, the air kerma length product within a phantom and the energy imparted. The factors investigated were the bow-tie filter and the size, shape and position of a phantom which simulates the patient. The calculations show that the surface air kerma length product and the maximum surface air kerma are mainly dependent on phantom position and decrease along the vertical axis of the CT plane as the phantom surface moves away from the isocentre along this axis. As a result, measurements using standard body dosimetry phantoms may underestimate the skin dose for real patients. This result is specially important for CT fluoroscopic procedures: for an adult patient the peak skin dose can be 37% higher than that estimated with a standard measurement on the body AAPM (American Association of Physicists in Medicine) phantom. The results also show that the energy imparted to a phantom is mainly influenced by phantom size and is nearly independent of phantom position (within 3%) and shape (up to 5% variation). However, variations of up to 30% were found for the air kerma to regions within the AAPM body phantom when it is moved vertically. This highlights the importance of calculating doses to organs taking into account their size and position within the gantry.
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The International Organization for Standardization (ISO) has issued a standard series on photon reference radiation qualities (ISO 4037). In this series, no conversion coefficients are contained for the quantity personal dose equivalent at a 3 mm depth, Hp(3). In the past, for this quantity, a slab phantom was recommended as a calibration phantom; however, a cylinder phantom much better approximates the shape of a human head than a slab phantom. Therefore, in this work, the conversion coefficients from air kerma to Hp(3) for the cylinder phantom are supplied for X- and gamma radiation qualities defined in ISO 4037.
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Reference dosimetry for high doserate 192 Ir brachytherapy in North America is based on an interpolated air kerma standard maintained at the U.S. ADCLs. Clinical well‐chambers are calibrated in terms of air kerma and the AAPM TG‐43 protocol is used to obtain absorbed dose to water. This first part of the presentation will review: i. The calibration chain for the dissemination of the quantity air kerma strength for 192 Ir, from the primary standards maintained by NIST, through the intermediate realization at the ADCLs, to the calibration of clinical well‐type ionization chambers. ii. How the accuracy of dose measurements may be affected in situations where the source design differs for calibration and use. iii. What uncertainty components contribute to the overall uncertainty in the measurement of air kerma strength in the clinical situation, focusing on the recently published AAPM/ESTRO TG‐138 report. There are significant developments worldwide focused on moving to an absorbed dose‐to‐water basis for HDR 192 Ir dosimetry. Various techniques have been proposed — water and graphite calorimetry, Fricke dosimetry, and ionization chambers — and all offer the potential for increased accuracy and a simpler protocol for users. The second part of the presentation will: i. Outline how absorbed dose standards provide a potential improvement in dosimetry for HDR 192 Ir. An absorbed dose standard would mean that the calibration the clinical physicist obtains for their well chamber would be much closer to what they need, thereby potentially reducing the uncertainty on the clinical reference dose. ii. Review the new approaches for direct absorbed dose to water realization in detail. Water calorimetry would seem to be the most direct option for a primary standard but other approaches under investigation may ultimately offer a lower uncertainty. The AAPM TG‐43 protocol is the standard dosimetry protocol used worldwide for brachytherapy clinical dose calculations. While there have been a number of updates refining the approach, the basic formalism based on superposition of single‐source dose distributions in a fixed water sphere has remained. The joint AAPM/ESTRO HEBD report on high‐energy (> 50 keV) photon‐emitting brachytherapy source dosimetry further refines the basic TG‐43 formalism. Specific to high‐energy radionuclide sources such as 192 Ir, the report includes the following: a) consensus datasets of brachytherapy dosimetry parameters and recommended methods for evaluating these consensus datasets, b) recommendations on dosimetry methods to characterize the source dose distribution (based on experimental procedures and Monte Carlo methods), and c) interpolation/extrapolation techniques for the 2D anisotropy function and radial dose function, specific to high‐energy sources, which differ from the low‐energy techniques in the 2004 and 2007 AAPM updates. In addition to explaining this report, the final part of the presentation will discuss other advances such as using model‐based dose calculation algorithms (AAPM/ESTRO/ABG TG‐186 report) and clinical implementation of primary HDR 192 Ir dose rate‐to‐water calibrations. Learning Objectives: 1. Understand the present basis for HDR 192 Ir dosimetry in North America 2. Understand how proposed standards of absorbed dose will benefit clinical dosimetry 3. Understand how the TG‐43 protocol is being refined through joint AAPM/ESTRO activities.
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Introduction:In the last 5 years the American Association of Physicists in Medicine Task Group 51 (AAPM TG-51) and the International Atomic Energy Agency (IAEA) published a new high-energy photon and electron dosimetry protocol.These protocols are based on the use of an ion chamber having an absorbed-dose to water calibration factor.These are different from the previous NCS report-2 and IAEA TRS-277 protocols, which require air kerma calibration factor.Aim of the Study: Is to present the dose comparison between various dosimetry protocols and the IAEA TRS-398 protocol for clinical reference dosimetry of high energy photon beams.The absorbed-dose to water measured according to the NCS Report-2, International Atomic Energy Agency technical Report Series No. 277 (IAEA TRS-277) and, TG-51 are compared to that measured using the TRS-398 protocol. Results and Discussion:This study shows that the absorbed dose which is measured with The IAEA TRS-398 formalisms is higher than that calculated with NCS Report-2 and IAEA TRS-277 formalisms within range from 0.4 to 1.3% and from 0.7 to 2.1%, respectively, for different higher energy photon beams of Co-60, 6, 8 and 18 MV.as sensed by different ionization chambers, The chambers used are PTW 30001, 30004, and NE-2571; which have calibration factors N K and N D,W traceable to the Bureau International des Poids et Mesures (BIPM).In contrast, the absorbed-dose to water measured according to TG-51 is in good agreement with TRS-398 within about 0.3% for photon beams.
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Optically stimulated luminescence
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The present study grasped the radiation exposure per examination by incident air kerma (air kerma-area product; KAP and incident air kerma; Ka, e) using an air kerma-area product meter of our division with mobile population based gastric cancer screening. Initially, we measured the air kerma rate at the patient entrance reference point using an air kerma-area product meter and calibrated dosimeter, for three devices which an air kerma-area product meter was equipped, inspected the indication error of them. The error was 4.3% at the maximum, and accuracy was confirmed. The 816 patients who underwent gastric cancer screening in our division, the median values of KAP and Ka, e of the standard gastrography method 1 were 645.7 mGy·cm2, 37.4 mGy, respectively. The radiation dose of males were significantly higher than females, and the radiation dose increased in proportion to the BMI. The median values of calculated KAP and Ka, e of the standard gastrography method 1 for standard body size were 633.8 mGy·cm2, 37.0 mGy, respectively. We suggest that the patient exposure in gastrography can be optimized using an air kerma-area product meter.
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