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286 poster A QA tool for absolute dosimetry: an electronic archive and on-line access to the IAEA protocols
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286
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Calculations of electron fluence correction factors using the Monte Carlo code Penelope
A QA tool for absolute dosimetry: an electronic archive and on-line access to the IAEA protocols
J.M. Fernandez Varea2, A. Sieobahn 1, B. Nilsson 1, P. Andreo 1 I stockholm University and Karofinska Institutet, Medical radiation physics, Stockholm, Sweden, 2Universitat de Barcelona, Facultat de Fisica (ECM), Barcelona, Spain,
H. Valen Haukeland University Hospital, Department of Radiation Physics, Bergen, Norway
Purpose: in electron beam dosimetry plastic phantom materials may be used instead of water. A correction factor is then needed for converting the electron fluence in plastic phantoms to the fluence at the equivalent depth in water. The recommended values for this factor given by AAPM and IAEA differ, in particular at large depths. Thus there is an interest to further investigate these factors and in particular their variation with depth Methods: Simulations of the electron fluence have been done, in semi-infinite phantoms of water and in different common plastic phantom materials, using the Monte Carlo code PENELOPE. The calculations have been carried out for incident electron energies between 5 and 20 MeV. Fluence correction factors were calculated at different depths as the ratio of fluence in water to fluence in plastic at equivalent depths. Comparisons are made with the recommended values from the protocols of IAEA and AAPM. Results: At depth of dose maximum generally a higher value of about one percent was found compared with the values in the protocols. Between dose maximum and the surface the calculated values of the correction factor are higher than those given by AAPM and IAEA, in particular for lower incident energies. Beyond the dose maximum the values suggested by AAPM, obtained as primary electron fluence ratios tend to overestimate the fluence correction factor. On the other hand the values given by IAEA, based on measured ionization ratios, seem to underestimate slightly the fluence correction factor in this region. In the calculations done to obtain the correction factor in the AAPM protocol, only primary electrons were considered. In the present study it was found that including the secondary electrons do not change the fluence factor significantly. Conclusions: The variation of fluence correction factor with depth in phantom obtained using PENELOPE differ somewhat from the recommended values in AAPM and IAEA but agree in shape with factors obtained by Ding et al using EGS4. The results also show that determining the fluence correction factor toward the end of the electron range is uncertain due to increasing relative errors in this region caused by systematic errors in the depth-scaling procedure. Due to this uncertainty we recommend to use a constant fluence factor from depths 0.5 to 1 cm in front of R100 down to Rp. A linear interpolation from a surface value of 1.0 to the constant value may also be an acceptable approximation.
Introduction: The recommendations given by dosimetry protocols such as the IAEA TRS277 and 381, are a common method of calibrating ionisation chambers and measuring radiotherapy beams. In 1990 the TRS277 protocol became part of our clinic's QA programme. A computer based archiving system for dosimetric parameters of the installed equipment and the observed output parameters from the therapeutic beams were established three years later. Recently, another module was added to enable a computer based interface to the IAEA protocols. Methods: A computer program has been designed in order to facilitate access to a database containing physical parameters for the beams, dosimetry factors for the equipment and the records from routine measurement of beam output. Networked PCs are used for data input during the measurement procedure. Initially, prior to data acquisition for the archive, parameters describing dosimetric properties both for beams and measurement equipment have to be entered. The interface to the database contains a sheet for input of parameters acquired during the measurement procedure. The various factors might be monitored versus time by means of a supplied chart tool. Names and definition of factors and quantities within the programme comply with those of the IAEA protocols. TRS277, 381 and 398 might be selected as the preferred protocol for absolute dosimetry. The program might look up dosimetric factors in tables taken from the selected protocol, thus reducing time for manual interpolation. Subsequently the QC of new factors relating to new equipment will be easier, and performing dosimetry may start sooner. Alarm levels for both energy checks and beam output might be entered in advance to help the physicist to decide whether or not to adjust. Conclusion: The software tool described has been part of the department's QA programme since 1994. Establishment of the electronic archive in 1994 simplified input to as well as output from the QC records. With aid of the integrated database query tool we have been able to monitor shifts in physical parameters of the therapeutic beams. Possible variations of those constants might then be discovered at an earlier stage. As demands on QA programmes increases, a medium sized clinic as ours, with a moderate number of beams and a variety of dosimetric equipment it has proven vital to keep database records of all relevant constants and parameters and a software tool for convenient access to those data. 287
285
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High energy photon beams dosimetry with films digitized by a professional digital camera O. Caselles, P. Francois Claudius Regaud, physics, Toulouse, France The use of film as absolute dosimeter seems to be limited because of the non-linear relation between dose and optical density, and the variation of film processing parameters. But it presents many advantages, which have been often described. For example, to obtain 2D relative measured dose distributions, film dosimetry gives high spatial resolution within short measurement times. Their use for electron beam dosimetry has been shown to be accurate and reliable. Many digitizing devices have appeared on the market. They allow to quickly get numerical data which can be analyzed easily using commercially available software. For high energy photon beams the problem seems to be more complicated due to the energy, depth, field size and dose dependence of the film response. The main problem is to linearize the film dose response after digitization, and to get a quick and robust optical density to dose conversion method. We have developed a conversion method which separates and takes into account every parameter influencing the film dosimetry process. The film is digitized using a professional camera and the resulting gray levels are converted to dose. In this paper, the method is shown to be applicable for photon beam energies of 6 and 18MV.
Effective attenuation coefficients for a cement based compensator material determined by measurements in different phantoms M.S. Thomsen Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark In radiation therapy compensators have been used for decades to obtain a more homogeneous dose distribution in the target volume. Today, compensators are also used for intensity-modulated radiation therapy. However, to obtain a correct dose in the patient, the attenuation properties of the compensator material have to be well established. A comparison of the broad beam effective attenuation coefficients for a cement based material determined from measurements in different phantoms is presented. The measurements were carried out with 6 and 18 MV x rays and with the ionization chamber in: a large water phantom, a water equivalent plastic phantom, and a cylindrical mini-phantom. Furthermore, measurements 'in air' by using a brass build-up cap were performed. At the low energy, the mini-phantom measurements showed an up to 2-3% higher transmission for the compensators compared to the transmission found from the measurements in the other phantoms. At the high energy, the difference in the transmission determined from measurements in the water phantom and the mini-phantom was much smaller. It is shown that the difference may be divided into a contribution due to a change in phantom scatter because of beam hardening in the attenuator and a contribution which is proportional to the thickness of the absorber. Thus, the application of a miniphantom for determination of effective attenuation coefficients may at low photon energies lead to a small underestimation of the attenuation of the
284
Posers
poster
286
poster
Calculations of electron fluence correction factors using the Monte Carlo code Penelope
A QA tool for absolute dosimetry: an electronic archive and on-line access to the IAEA protocols
J.M. Fernandez Varea2, A. Sieobahn 1, B. Nilsson 1, P. Andreo 1 I stockholm University and Karofinska Institutet, Medical radiation physics, Stockholm, Sweden, 2Universitat de Barcelona, Facultat de Fisica (ECM), Barcelona, Spain,
H. Valen Haukeland University Hospital, Department of Radiation Physics, Bergen, Norway
Purpose: in electron beam dosimetry plastic phantom materials may be used instead of water. A correction factor is then needed for converting the electron fluence in plastic phantoms to the fluence at the equivalent depth in water. The recommended values for this factor given by AAPM and IAEA differ, in particular at large depths. Thus there is an interest to further investigate these factors and in particular their variation with depth Methods: Simulations of the electron fluence have been done, in semi-infinite phantoms of water and in different common plastic phantom materials, using the Monte Carlo code PENELOPE. The calculations have been carried out for incident electron energies between 5 and 20 MeV. Fluence correction factors were calculated at different depths as the ratio of fluence in water to fluence in plastic at equivalent depths. Comparisons are made with the recommended values from the protocols of IAEA and AAPM. Results: At depth of dose maximum generally a higher value of about one percent was found compared with the values in the protocols. Between dose maximum and the surface the calculated values of the correction factor are higher than those given by AAPM and IAEA, in particular for lower incident energies. Beyond the dose maximum the values suggested by AAPM, obtained as primary electron fluence ratios tend to overestimate the fluence correction factor. On the other hand the values given by IAEA, based on measured ionization ratios, seem to underestimate slightly the fluence correction factor in this region. In the calculations done to obtain the correction factor in the AAPM protocol, only primary electrons were considered. In the present study it was found that including the secondary electrons do not change the fluence factor significantly. Conclusions: The variation of fluence correction factor with depth in phantom obtained using PENELOPE differ somewhat from the recommended values in AAPM and IAEA but agree in shape with factors obtained by Ding et al using EGS4. The results also show that determining the fluence correction factor toward the end of the electron range is uncertain due to increasing relative errors in this region caused by systematic errors in the depth-scaling procedure. Due to this uncertainty we recommend to use a constant fluence factor from depths 0.5 to 1 cm in front of R100 down to Rp. A linear interpolation from a surface value of 1.0 to the constant value may also be an acceptable approximation.
Introduction: The recommendations given by dosimetry protocols such as the IAEA TRS277 and 381, are a common method of calibrating ionisation chambers and measuring radiotherapy beams. In 1990 the TRS277 protocol became part of our clinic's QA programme. A computer based archiving system for dosimetric parameters of the installed equipment and the observed output parameters from the therapeutic beams were established three years later. Recently, another module was added to enable a computer based interface to the IAEA protocols. Methods: A computer program has been designed in order to facilitate access to a database containing physical parameters for the beams, dosimetry factors for the equipment and the records from routine measurement of beam output. Networked PCs are used for data input during the measurement procedure. Initially, prior to data acquisition for the archive, parameters describing dosimetric properties both for beams and measurement equipment have to be entered. The interface to the database contains a sheet for input of parameters acquired during the measurement procedure. The various factors might be monitored versus time by means of a supplied chart tool. Names and definition of factors and quantities within the programme comply with those of the IAEA protocols. TRS277, 381 and 398 might be selected as the preferred protocol for absolute dosimetry. The program might look up dosimetric factors in tables taken from the selected protocol, thus reducing time for manual interpolation. Subsequently the QC of new factors relating to new equipment will be easier, and performing dosimetry may start sooner. Alarm levels for both energy checks and beam output might be entered in advance to help the physicist to decide whether or not to adjust. Conclusion: The software tool described has been part of the department's QA programme since 1994. Establishment of the electronic archive in 1994 simplified input to as well as output from the QC records. With aid of the integrated database query tool we have been able to monitor shifts in physical parameters of the therapeutic beams. Possible variations of those constants might then be discovered at an earlier stage. As demands on QA programmes increases, a medium sized clinic as ours, with a moderate number of beams and a variety of dosimetric equipment it has proven vital to keep database records of all relevant constants and parameters and a software tool for convenient access to those data. 287
285
poster
poster
High energy photon beams dosimetry with films digitized by a professional digital camera O. Caselles, P. Francois Claudius Regaud, physics, Toulouse, France The use of film as absolute dosimeter seems to be limited because of the non-linear relation between dose and optical density, and the variation of film processing parameters. But it presents many advantages, which have been often described. For example, to obtain 2D relative measured dose distributions, film dosimetry gives high spatial resolution within short measurement times. Their use for electron beam dosimetry has been shown to be accurate and reliable. Many digitizing devices have appeared on the market. They allow to quickly get numerical data which can be analyzed easily using commercially available software. For high energy photon beams the problem seems to be more complicated due to the energy, depth, field size and dose dependence of the film response. The main problem is to linearize the film dose response after digitization, and to get a quick and robust optical density to dose conversion method. We have developed a conversion method which separates and takes into account every parameter influencing the film dosimetry process. The film is digitized using a professional camera and the resulting gray levels are converted to dose. In this paper, the method is shown to be applicable for photon beam energies of 6 and 18MV.
Effective attenuation coefficients for a cement based compensator material determined by measurements in different phantoms M.S. Thomsen Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark In radiation therapy compensators have been used for decades to obtain a more homogeneous dose distribution in the target volume. Today, compensators are also used for intensity-modulated radiation therapy. However, to obtain a correct dose in the patient, the attenuation properties of the compensator material have to be well established. A comparison of the broad beam effective attenuation coefficients for a cement based material determined from measurements in different phantoms is presented. The measurements were carried out with 6 and 18 MV x rays and with the ionization chamber in: a large water phantom, a water equivalent plastic phantom, and a cylindrical mini-phantom. Furthermore, measurements 'in air' by using a brass build-up cap were performed. At the low energy, the mini-phantom measurements showed an up to 2-3% higher transmission for the compensators compared to the transmission found from the measurements in the other phantoms. At the high energy, the difference in the transmission determined from measurements in the water phantom and the mini-phantom was much smaller. It is shown that the difference may be divided into a contribution due to a change in phantom scatter because of beam hardening in the attenuator and a contribution which is proportional to the thickness of the absorber. Thus, the application of a miniphantom for determination of effective attenuation coefficients may at low photon energies lead to a small underestimation of the attenuation of the