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Clinical applications of a high speed matrix ionization chamber portal imaging system
S45
169
170 TRANSIT DOSIMETRY
Vibeke N.Hansen, Phil M. Evans and William Swindell Institute of Cancer Research and Royal Marsden NHS Trust, Sutton, Surrey, London, SMI 3LB. UK
CUNICAL APPLICATIONS OF A HIGH SPEED MATRIX IONIZATION CHAMBER PORTAL IMAGING SYSTEM M. van Herk, W. Fencl', K. Gilhuijs, A. van Dalen and P. van de Yen. The Netherlands Cancer Institute / Antoni van Leeuwenhoek Huis, Amsterdam, the Netherlands; 'Varian International, Baden, Switserland.
An Electronic Portal Imaging Device (EPID) measures the exit fluence of the radiation beam used for radiotherapy. This exit f1uence contains information of the position of the radiation field relativeto anatomical land marks. This was first developed for and is mainly used for patient positioning measurements. However, in addition to replacingconventional films the EPID intensity can be used to extract dosimetric information on the actual radiation dose delivered. This may be done todifferent levels of complexity. When the patienthas been CT scannedin the planningposition, the primary f1uence as extracted from the EPID intensity may be used for backprojection. The EPID can be backprojected all the way throughthe patientas a Back ProjectedBeamProfile(BPBP). The BPBP will be very homogeneous, if the dose delivered was as planned, using the CT scan. The EPID can also be backprojected to generatea complete3D primaryf1uence distribution, that in tum can be convolved with point spread functions to generate a dose distribution of the dose delivered. This is a transit dosimetry generateddose distribution. The transitdosimetry dose distribution may be evaluated just as the planning dose distribution by dose volume histogram and other dose evaluation tools. Anotherdosimetriccheck using the EPID. that does not require CT scans of the patient is to calculate the radiological thickness that the radiation has transversed, This may be compared with the actual outline thicknessof the patient that is often the basic input parameter to the actual dose calculation. The simplest dosimetric check is to have the EPID calibrated to exit dose (this may be compared with exit dose calculatedby a planning computer).
A main disadvantage of the present matrix ionization chamber system for electronic portal imaging is its relatively slow image acquisition of 6 s at full resolution. We have solved this problem by modifying the read-out electronics in two ways: First, faster high voltage switches are applied which work with a higher voltage; Second, faster read-out amplifiers are applied which have reduced cross-talk . With these improvements circuit noise is no longer dominant at typical radiotherapy dose rates. Because the quantum noise level in the matrix ionization chamber system is purely determined by signal integration in the liquid medium, the image scan can now be reduced to as short as 055 s with little loss of image quality. However, there is some loss of resolution at readout speed faster than 1.5 s due to speed limitations of the read-out amplifiers. One of the applications of the new device is double exposures for larynx fields. At a reduced dose rate of 125 MU/min, only about 5 MUs are required for a single frame on a 4 MV ABB Dynaray accelerator. Other applications which benefit from the reduced image scan time are time lapse movies. Typically 15 frames per field are made during one fraction. The movies offer both information on patient motion and improved image quality by averaging the frames. Finally, on-line analysis of the images can be performed more easily and has been included in the software package. In can be concluded that the higher speed of the new matrix ionization chamber system is an important improvement for several clinical applications.
171
172
Treatment verification using a digital photostimulable phosphor system.
A STRATEGY FOR FIELD SHAPEEVALUATION IN DIGITAL PORTAL IMAGING
ChingE.J., Hutchings R.E. and Thomson E.S. Norfolk & Norwich HealthCare NHS Trust, Norwich, United Kingdom.
P.HVos', M. Quist', J. weistra', A.M,Vossepoer , Dr. Bernard Verbeeten Institute, P.O.Box 50120, 5000LA nburg, The Netherlands; , Delft University of Technology, Delft, The Nether1anda
Goodquality megavoltage portal images can be gainedusinga digital photostimulable phosphor imaging system. One such computed radiography (CR) system, designed for use in diagnostic radiology departments as a replacement for film-based systems, has beenused at the Norfolk and Norwich Hospital, UK to produce localisation portal images. The CR system provides a fast imaging medium, which is useful for single or double exposure techniques. It has certain advantages overfast localisation film inthatthereare no optimum film dose and latitude considerations to take into accountas the system adjusts itsreadout parameters to compensate for a rangeof exposures and required latitudes. With respect to electronic portal imaging devices (EPIDs), CR portalimaging hasthe advantage that it is not machine specific, however it cannotbe usedfor on-line assessment of patient position or on-line adjustment. The practical problems associatedwith this method of portalimaging are discussed. Thereis also the possibility of passing CR images in digitalformatdirectly to a portal image assessment terminal. thus eliminating the needfor hard copiesandwithout the loss inquality which wouldbe associated with the Iransfer of film images into the system usinga digitiser.
Digital portal imagers allow accurate ITlIllIIUrement of the field ahlIpe in radiotherapy. A strategy is introduced to detennine origin and magnitude ofdiscrepancies ~ the prescribed and rnealIInd field ouUlne. After measurement ofthe actual detector posiIlon relaliYe to \he be8m·a conversion is made from pixels in \he image matrix to mm in \he plane of the isocenter, without using information from \he ilTl8ged field. Using a distance lrall8form a quick checlI is performed: \he ouUine is accepted if all outline points deviate leal then • predefined minimum (usuatly 5 mm). Subsequent evaluation ltarts if IIOl'MWhenl in the ouUine this minimum is exceeded. The collimator defined parts in the field ouUine are disctiminated from \he lIhieiding bloclcs using an enc:IoaIng rectangle of \he portal ouUine. This rectangle is found by minimization of \he area as a function of rotation. Ifmore thanone solution il available, minimization of the entropy of the field outline projeeliona determines which rectangle cooesponds best to the field ouUine. A checlI for the validity of the determined conimator parts il performed with a Iep8rate Wnear fit through these parts. An oulline part is lICCllPIed as a coIWITllItor outline part if ~ is longer than a predefined length . Using thisprocedure the position foreach of the collimator jlIwlI can be individually ITlIllIIUred and compared with ~s prescription, thus allowing discrimination between symmetric and asymmetric collimator aet-llp5. Using the distance iranIform again. for eachof the detected (aecondaJy) shielding bloclcs the largest discrepancy or the area giving underdouge or overdosage can be computed to evaluate their shape and poIiIion. Parametel(s) and criteria thatsllould be used to evaluate the field I18t-llP lIIll apecified in clinical protocols. For standard shielding blocks ulUlllly only a maximum tolerated difference is apecified, wl1enlas for manlle fields also maximum allowed over- and underdose areas are apecified. The procedure is performed completely automatically, within a few seconds. Appropriate messages are displayed on \he screento inform the operator about the 18SU~ of the field shapeevaluation. 'Qn-Ino _ 01... _ _ posIIon in digital portal imaging, P.HV.., ..... procoeding• .
169
170 TRANSIT DOSIMETRY
Vibeke N.Hansen, Phil M. Evans and William Swindell Institute of Cancer Research and Royal Marsden NHS Trust, Sutton, Surrey, London, SMI 3LB. UK
CUNICAL APPLICATIONS OF A HIGH SPEED MATRIX IONIZATION CHAMBER PORTAL IMAGING SYSTEM M. van Herk, W. Fencl', K. Gilhuijs, A. van Dalen and P. van de Yen. The Netherlands Cancer Institute / Antoni van Leeuwenhoek Huis, Amsterdam, the Netherlands; 'Varian International, Baden, Switserland.
An Electronic Portal Imaging Device (EPID) measures the exit fluence of the radiation beam used for radiotherapy. This exit f1uence contains information of the position of the radiation field relativeto anatomical land marks. This was first developed for and is mainly used for patient positioning measurements. However, in addition to replacingconventional films the EPID intensity can be used to extract dosimetric information on the actual radiation dose delivered. This may be done todifferent levels of complexity. When the patienthas been CT scannedin the planningposition, the primary f1uence as extracted from the EPID intensity may be used for backprojection. The EPID can be backprojected all the way throughthe patientas a Back ProjectedBeamProfile(BPBP). The BPBP will be very homogeneous, if the dose delivered was as planned, using the CT scan. The EPID can also be backprojected to generatea complete3D primaryf1uence distribution, that in tum can be convolved with point spread functions to generate a dose distribution of the dose delivered. This is a transit dosimetry generateddose distribution. The transitdosimetry dose distribution may be evaluated just as the planning dose distribution by dose volume histogram and other dose evaluation tools. Anotherdosimetriccheck using the EPID. that does not require CT scans of the patient is to calculate the radiological thickness that the radiation has transversed, This may be compared with the actual outline thicknessof the patient that is often the basic input parameter to the actual dose calculation. The simplest dosimetric check is to have the EPID calibrated to exit dose (this may be compared with exit dose calculatedby a planning computer).
A main disadvantage of the present matrix ionization chamber system for electronic portal imaging is its relatively slow image acquisition of 6 s at full resolution. We have solved this problem by modifying the read-out electronics in two ways: First, faster high voltage switches are applied which work with a higher voltage; Second, faster read-out amplifiers are applied which have reduced cross-talk . With these improvements circuit noise is no longer dominant at typical radiotherapy dose rates. Because the quantum noise level in the matrix ionization chamber system is purely determined by signal integration in the liquid medium, the image scan can now be reduced to as short as 055 s with little loss of image quality. However, there is some loss of resolution at readout speed faster than 1.5 s due to speed limitations of the read-out amplifiers. One of the applications of the new device is double exposures for larynx fields. At a reduced dose rate of 125 MU/min, only about 5 MUs are required for a single frame on a 4 MV ABB Dynaray accelerator. Other applications which benefit from the reduced image scan time are time lapse movies. Typically 15 frames per field are made during one fraction. The movies offer both information on patient motion and improved image quality by averaging the frames. Finally, on-line analysis of the images can be performed more easily and has been included in the software package. In can be concluded that the higher speed of the new matrix ionization chamber system is an important improvement for several clinical applications.
171
172
Treatment verification using a digital photostimulable phosphor system.
A STRATEGY FOR FIELD SHAPEEVALUATION IN DIGITAL PORTAL IMAGING
ChingE.J., Hutchings R.E. and Thomson E.S. Norfolk & Norwich HealthCare NHS Trust, Norwich, United Kingdom.
P.HVos', M. Quist', J. weistra', A.M,Vossepoer , Dr. Bernard Verbeeten Institute, P.O.Box 50120, 5000LA nburg, The Netherlands; , Delft University of Technology, Delft, The Nether1anda
Goodquality megavoltage portal images can be gainedusinga digital photostimulable phosphor imaging system. One such computed radiography (CR) system, designed for use in diagnostic radiology departments as a replacement for film-based systems, has beenused at the Norfolk and Norwich Hospital, UK to produce localisation portal images. The CR system provides a fast imaging medium, which is useful for single or double exposure techniques. It has certain advantages overfast localisation film inthatthereare no optimum film dose and latitude considerations to take into accountas the system adjusts itsreadout parameters to compensate for a rangeof exposures and required latitudes. With respect to electronic portal imaging devices (EPIDs), CR portalimaging hasthe advantage that it is not machine specific, however it cannotbe usedfor on-line assessment of patient position or on-line adjustment. The practical problems associatedwith this method of portalimaging are discussed. Thereis also the possibility of passing CR images in digitalformatdirectly to a portal image assessment terminal. thus eliminating the needfor hard copiesandwithout the loss inquality which wouldbe associated with the Iransfer of film images into the system usinga digitiser.
Digital portal imagers allow accurate ITlIllIIUrement of the field ahlIpe in radiotherapy. A strategy is introduced to detennine origin and magnitude ofdiscrepancies ~ the prescribed and rnealIInd field ouUlne. After measurement ofthe actual detector posiIlon relaliYe to \he be8m·a conversion is made from pixels in \he image matrix to mm in \he plane of the isocenter, without using information from \he ilTl8ged field. Using a distance lrall8form a quick checlI is performed: \he ouUine is accepted if all outline points deviate leal then • predefined minimum (usuatly 5 mm). Subsequent evaluation ltarts if IIOl'MWhenl in the ouUine this minimum is exceeded. The collimator defined parts in the field ouUine are disctiminated from \he lIhieiding bloclcs using an enc:IoaIng rectangle of \he portal ouUine. This rectangle is found by minimization of \he area as a function of rotation. Ifmore thanone solution il available, minimization of the entropy of the field outline projeeliona determines which rectangle cooesponds best to the field ouUine. A checlI for the validity of the determined conimator parts il performed with a Iep8rate Wnear fit through these parts. An oulline part is lICCllPIed as a coIWITllItor outline part if ~ is longer than a predefined length . Using thisprocedure the position foreach of the collimator jlIwlI can be individually ITlIllIIUred and compared with ~s prescription, thus allowing discrimination between symmetric and asymmetric collimator aet-llp5. Using the distance iranIform again. for eachof the detected (aecondaJy) shielding bloclcs the largest discrepancy or the area giving underdouge or overdosage can be computed to evaluate their shape and poIiIion. Parametel(s) and criteria thatsllould be used to evaluate the field I18t-llP lIIll apecified in clinical protocols. For standard shielding blocks ulUlllly only a maximum tolerated difference is apecified, wl1enlas for manlle fields also maximum allowed over- and underdose areas are apecified. The procedure is performed completely automatically, within a few seconds. Appropriate messages are displayed on \he screento inform the operator about the 18SU~ of the field shapeevaluation. 'Qn-Ino _ 01... _ _ posIIon in digital portal imaging, P.HV.., ..... procoeding• .