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A quality management system for radiotherapy physics
S48 182
181 CALCtJLATIO.
0..
A
STAJlDARD TABLE
0..
PBAIfTClM
SCATTBR .. ACTOR
CALCULATION OF THE PRIMARY DOSE IN THE ABSENCE OF ELECTRONIC EQUILIBRIUM
p.St:orchi, Dr. DaDiel deD Botld CADcer CADter, Rotter:d. .
J.VaD Gast:ereD, Radiotherapy IDatitute Arab . ., Arub. . In radiotherapy planning, the phantom scatter factor sp is neceBBary for the calculation of irregular fields, but ueually Sp is not included in the standard do&!matry of a treatment unit and is not .... uured. Therefore, the pouibility of a standard table, giving the sp-curve of an arbitrary treatment unit u a function of its quality index, has been considered. A aet of spcurves at reference depth of 10 em have been derived from total output factors Se.p and head scatter factors Se' which have been previoualy measured for a number of square fields of 27 treatment units in the Netherlands. The Se have been meuured according to a standard procedure, uaing a beam co-axial narrow cylinder phantom of polystyrene. Also for each treatment unit, the quality index QI has been recorded. A computer program hu now been written, which converts thia set of data into a complete table giving Sp as a function of the field size of the square field and of the quality index QI. In order to make this table, the sp-curves of each treatment unit are firstly interpolated for the field sizes ranging from 4 up to 40 em, in steps of 1 em, from the meaaured values. Secondly, for each field aize, the S -curvee are computed by fitting a function of QI to the available valuea. The Sp-values are computed for QI ranging from 0.600 up to 0.800, in ateps of 0.010. A second program has been written which makea an evaluation of the differences between the .... asured input sp'a and the calculated table. From this evaluation we draw the conclusions that there is a atrong relation between the quality index QI of a treatment unit and the phantom scatter factor, and that the calculated table gives Sp as a function of the field aize and QI with an error of less than 1 percent.
PETROV D.E., Moscow Engineering Physics Institute, Moscow, Russia In a mulilayered media at points close to the boundary of any two layers of different electronic densities, the absorbed dose due to primary radiation, can be estimated by means of secondary electrons. Assuming monoenergetic megavoltage photons incident on a two layered media, the Compton effect is a predominant mode of interaction, the primary dose is due to the electrons set into motion from the first interaction. The dose is divided into components from the upper and lower layers. The beam width must be large enough so that all electrons scattered at some angle could pass through detector point. General expression for the dose is a product of the initial photon flux at some depth, the K1ein-Nisha cross section, and the ionizational mass stopping power integrated over azimuthal angle. The factor taking into account real geometry is out of the sign of integral. The latter depends only on the distance between the interface and detector for the given consequence of layers. At the interface there is a peak or drop of the dose determined by the ratio of the stopping powers under the sign of the integral. At points beyond the interface the dose decreases or increases up to electronic equilibrium region where the absorbed dose and the kerma are parallel to each other. The peak width at the base is determined by the free range of the electron with the maximum possible energy after the Compton scattering.
184
183 RADIOTIIERAPY PHYSICS
MonteCarlo calculation of spectral lines for use in the Italian Protocol for dosimetry in Brachytherapy
K.A.Ball, D.S.Biggs,R.E.Hutehings,E.S.Thomson, Norfolk and Norwich Hospital, England.
M.Galelli, E.Moretti Medical Physics Department • General Hospital - Brescia (Italy)
A
QUALITY
MANAGEMENT
SYSTEM
FOR
The importance of quality in Radiotherapy Physics cannot be
overemphasized and the need for having controlled, written procedures for all tasks is increasing in importance, There are a number of quality standards currently in use such as ISO 9000. In the UK, the Quality Assurance in Radiotherapy (QART) Standard has been produced based on IS09000 and recommended for implementation in all Radiotherapy Departments. A Quality Management System has been implemented in the RadiotherapyPhysics Section at the Norfolk and Norwich Hospital, England, to fulfil the QART and IS09000 recommendations. This Poster will illustrate the system used at the Norfolk and Norwich Hospital. The system offers a standardised document format and allows the issue and re-issue of all written procedures and proforma. It has enhanced the quality and consistency of tasks, when the task is performed by varying members of staff and thereby improving the quality of the service offered from the safety and organisational perspectives. It is believed that the Quality Management System used in the Radiotherapy Physics Section at the Norfolk and Norwich Hospital would be simple to implement in the majority of Radiotherapy Physics Departments.
The Italian Committee for Dosimetry in Radiotherapy is about to produce a protocol for the dosimetry of brachytherapy sources that defines methods to measure the quantity 'air kerma rate in free air in a reference point' using ionisation chambers. Several parameters and quantities necessary to apply the protocol have to be calculated. In this presentation we show the methods used to calculate two of them: Pair, that account for the attenuation and scattering of photons in air; Nk(source), the calibration factor for each dosimeter and source type. Both quantities have been calculated by means of MonteCarlo simulations. '.To calculate Pair we score the photon fluence in the detector area, separately for 'primary photons', i.e. photons coming directly from the source without interacting in air; 'scattered photons', i.e. photons that are diffused from the air towards the scoring region; 'attenuated photons', i.e. primary photons directed towards the scoring region that are subtracted from the primary fluence by interactions in air. Pair is calculated as a combination of those fluences. Nk(source) is calculated starting from the air kerma rates due to spectral lines emitted by the source and from the corresponding calibrationfactors. The MonteCarlo code EGS4 is used, in a version modified in order to take into account the characteristicsX-ray production. Results are shown for some of the sources most used in Italy.
181 CALCtJLATIO.
0..
A
STAJlDARD TABLE
0..
PBAIfTClM
SCATTBR .. ACTOR
CALCULATION OF THE PRIMARY DOSE IN THE ABSENCE OF ELECTRONIC EQUILIBRIUM
p.St:orchi, Dr. DaDiel deD Botld CADcer CADter, Rotter:d. .
J.VaD Gast:ereD, Radiotherapy IDatitute Arab . ., Arub. . In radiotherapy planning, the phantom scatter factor sp is neceBBary for the calculation of irregular fields, but ueually Sp is not included in the standard do&!matry of a treatment unit and is not .... uured. Therefore, the pouibility of a standard table, giving the sp-curve of an arbitrary treatment unit u a function of its quality index, has been considered. A aet of spcurves at reference depth of 10 em have been derived from total output factors Se.p and head scatter factors Se' which have been previoualy measured for a number of square fields of 27 treatment units in the Netherlands. The Se have been meuured according to a standard procedure, uaing a beam co-axial narrow cylinder phantom of polystyrene. Also for each treatment unit, the quality index QI has been recorded. A computer program hu now been written, which converts thia set of data into a complete table giving Sp as a function of the field size of the square field and of the quality index QI. In order to make this table, the sp-curves of each treatment unit are firstly interpolated for the field sizes ranging from 4 up to 40 em, in steps of 1 em, from the meaaured values. Secondly, for each field aize, the S -curvee are computed by fitting a function of QI to the available valuea. The Sp-values are computed for QI ranging from 0.600 up to 0.800, in ateps of 0.010. A second program has been written which makea an evaluation of the differences between the .... asured input sp'a and the calculated table. From this evaluation we draw the conclusions that there is a atrong relation between the quality index QI of a treatment unit and the phantom scatter factor, and that the calculated table gives Sp as a function of the field aize and QI with an error of less than 1 percent.
PETROV D.E., Moscow Engineering Physics Institute, Moscow, Russia In a mulilayered media at points close to the boundary of any two layers of different electronic densities, the absorbed dose due to primary radiation, can be estimated by means of secondary electrons. Assuming monoenergetic megavoltage photons incident on a two layered media, the Compton effect is a predominant mode of interaction, the primary dose is due to the electrons set into motion from the first interaction. The dose is divided into components from the upper and lower layers. The beam width must be large enough so that all electrons scattered at some angle could pass through detector point. General expression for the dose is a product of the initial photon flux at some depth, the K1ein-Nisha cross section, and the ionizational mass stopping power integrated over azimuthal angle. The factor taking into account real geometry is out of the sign of integral. The latter depends only on the distance between the interface and detector for the given consequence of layers. At the interface there is a peak or drop of the dose determined by the ratio of the stopping powers under the sign of the integral. At points beyond the interface the dose decreases or increases up to electronic equilibrium region where the absorbed dose and the kerma are parallel to each other. The peak width at the base is determined by the free range of the electron with the maximum possible energy after the Compton scattering.
184
183 RADIOTIIERAPY PHYSICS
MonteCarlo calculation of spectral lines for use in the Italian Protocol for dosimetry in Brachytherapy
K.A.Ball, D.S.Biggs,R.E.Hutehings,E.S.Thomson, Norfolk and Norwich Hospital, England.
M.Galelli, E.Moretti Medical Physics Department • General Hospital - Brescia (Italy)
A
QUALITY
MANAGEMENT
SYSTEM
FOR
The importance of quality in Radiotherapy Physics cannot be
overemphasized and the need for having controlled, written procedures for all tasks is increasing in importance, There are a number of quality standards currently in use such as ISO 9000. In the UK, the Quality Assurance in Radiotherapy (QART) Standard has been produced based on IS09000 and recommended for implementation in all Radiotherapy Departments. A Quality Management System has been implemented in the RadiotherapyPhysics Section at the Norfolk and Norwich Hospital, England, to fulfil the QART and IS09000 recommendations. This Poster will illustrate the system used at the Norfolk and Norwich Hospital. The system offers a standardised document format and allows the issue and re-issue of all written procedures and proforma. It has enhanced the quality and consistency of tasks, when the task is performed by varying members of staff and thereby improving the quality of the service offered from the safety and organisational perspectives. It is believed that the Quality Management System used in the Radiotherapy Physics Section at the Norfolk and Norwich Hospital would be simple to implement in the majority of Radiotherapy Physics Departments.
The Italian Committee for Dosimetry in Radiotherapy is about to produce a protocol for the dosimetry of brachytherapy sources that defines methods to measure the quantity 'air kerma rate in free air in a reference point' using ionisation chambers. Several parameters and quantities necessary to apply the protocol have to be calculated. In this presentation we show the methods used to calculate two of them: Pair, that account for the attenuation and scattering of photons in air; Nk(source), the calibration factor for each dosimeter and source type. Both quantities have been calculated by means of MonteCarlo simulations. '.To calculate Pair we score the photon fluence in the detector area, separately for 'primary photons', i.e. photons coming directly from the source without interacting in air; 'scattered photons', i.e. photons that are diffused from the air towards the scoring region; 'attenuated photons', i.e. primary photons directed towards the scoring region that are subtracted from the primary fluence by interactions in air. Pair is calculated as a combination of those fluences. Nk(source) is calculated starting from the air kerma rates due to spectral lines emitted by the source and from the corresponding calibrationfactors. The MonteCarlo code EGS4 is used, in a version modified in order to take into account the characteristicsX-ray production. Results are shown for some of the sources most used in Italy.