- Email: [email protected]
50 oral Comparison of calculated and measured data for small MLCgenerated fields defined on Varian and Elekta accelerators
$24
Monday, 17 September 2001
als (18%), institution of unit manager's positions to improve scheduling and reduce workplace stress (18%), the ability to earn more time off and improvement of pensions and wellness benefits (18%). Recommendations for improvement of the work environment and remuneration were derived from the results of the survey. Surveys of this type will be repeated in the future to assess improvements in working conditions, retention and recruitment of Radiation Therapists.
Symposia/Proffered papers
51
oral
Dynamic edge focussing: A new MLC-design to deliver IMRT with a double focussing high precision Multi-Leaf-Collimator O. Pastyr 1, G. Echner 1, G. Hartmann 1, J. Richter 2, W. Schleael 1 1DKFZ, Medical Physics, Heidelberg, Germany, 2University of Wuerzburg, Radiotherapy, Wuerzburg, Germany
IMRT - V E R I F I C A T I O N A N D DELIVERY II
Tomotherapy is a therapeutic technique that combines photon intensity modulated radiotherapy (IMRT) and computed tomography (CT) capabilities into a single piece of equipment. Tomotherapy provides a closed loop for planning, delivery, verification and adaptation of radiotherapy. Such a device allows for processes that are very difficult or even impossible with other radiotherapy modalities. Tomotherapy is an ideal tool to perform Image Guided Adaptive Radiotherapy (IGAR). The first helical tomotherapy prototype in the world had been installed at the University of WisconsinMadison. The tomotherapy capabilities will be analyzed by describing its processes and the connection between them. Processes and techniques such as planning optimization, conformal and conformal avoidance treatments, megavoltage CT, registration in projection space, image fusion, helical delivery, delivery verification, dose reconstruction, deformable contour registration, deformable dose registration, adaptive radiotherapy, commissioning and QA will be discussed. Future lines of work will also be described.
Standard Linac integrated MLCs still have severe shortcomings for delivering IMRT. The most severe ones are lacking leaf resolution and leaf positioning accuracy, a penumbra which is dependent from the leaf position and the fact that in many clinical cases the overtravel of the leaves is not sufficient. A new mechanical concept for the construction of a high precision double focussing multi-leaf-collimator has been developed, which is designed to overcome the above mentioned shortcomings. The most outstanding features of the new disign are: - physical leaf thickness below 3 mm allowing a leaf resolution of below 5 mm at isocenter distance; - linearly guided double focussing leaves. Due to a new mechanical concept with movable edges, there is no longer a need for guiding the leaves on a circular path. The edges are dynamically controlled by a simple mechanical forced drive which ensures that the edges are always directed towards the focus. Thus a very small position independent penumbra is reached - simple linear leaf guidance system. The guidance system allows to use very long leaves and thus enables a large overtravel at highest leaf precision; - high precision measurement of leaf position. The position measurement of a leaf is performed by a pair of lead integrated high precision potentiometers. According to the new mechanical design, a prototype MLC with a maximum field size of 20x20 cm 2 tuned for IMRT applications has been constructed which can be attached to the accessory holder of a Linac. Mechanical, physical and electronical details as well as results of dosimetric measurements will be presented.
50
52
49
invited
Tomotherapy: a tool for image guided adaptive radiotherapy G.H. Olivera 1,2, T.R. Mackie 1,2, K.J. Ruchala 2, J.M. Kapatoes 2, P.J. Reckwerdt2, R. Jeraj 1, J.P. Balog2, W. Lu 1,2, J. Smilowitz 1, H. Keller 1 1University of Wisconsin, Medical Physics, Madison, USA, 2Tomotherapy Inc., Middleton, USA
oral
oral
Comparison of calculated and measured data for small MLC generated fields defined on Varian and Elekta accelerators
Verification of the dose to the isocentre in stereotactic plans
L. Rock, B. McClean, P. McCavana, S. Buckney St. Luke's, Physics, Dublin, Ireland
C. McKerracher, D.I. Thwaites Western general Hospital, Oncology Physics, Edinburgh Cancer Centre, Edinburgh, United Kingdom
This work is concentrated on the use of small MLC-defined fields and low numbers of monitor units per field used in the delivery of multiple-segmented field IMRT. Prior to the delivery of multiple-segmented field IMRT, as part of a quality assurance programme, it is important that these small fields be characterised and compared to the small field data generated by the treatment planning system. During the progress of this work, small field data was measured for both a Varian 2300C/D and an Elekta SL18 linac, and compared with a Helax TMS (vers 5.1) treatment planning system (TPS). The data measured includes percentage depth dose in water, X and Y-profiles, output factors, leaf transmission and linearity of dose with monitor units for small fields centred on the central axis. Initial analysis of these results show good agreement between the TPS and the depth dose data for fields as small as l x l c m for the Varian linac and 2x2cm for the Elekta linac. Comparison of the TPS generated profiles with the measured profiles show significant differences, in both directions, for both machines at 2x2cm and l x l c m for the Elekta linac. Comparison of TPS calculated output factors with measured output factors show differences of approximately 20% for the Elekta linac at 1xl cm and 8% for the Varian linac at lxlcm, with the measured/calculated difference decreasing rapidly as the field size increases. Preliminary investigations indicate that there may also be problems with the use of both off-axis and rectangular fields. The results of this work indicates that caution should be employed with the use of small square fields and rectangular fields on the central axis and off-axis for the delivery of multiple-segmented field IMRT.
Verification of the dose to the isocentre in stereotactic treatments should be an essential part of the quality control process. It is difficult to find a suitable detector for use in a phantom because of the non-coplanar nature of stereotactic treatments. The detector must be small, tissue equivalent and, directionally independent. This study investigated the use of a 0.125cc ion chamber (PTW), a diamond PTW) and three diodes; shielded (PFD), unshielded (EFD) and unshielded stereotactic (SFD) (Scanditronix). All were "treated" in a water equivalent phantom according to a typical stereotactic plan. In the following table of results, the symmetry is the % variation around the cross section of the detector, DD is the % directional dependence and the % errors in the measured dose to the isocentre for the largest (40mm) and smallest (12.5mm) collimators (Radionics) are shown in the final two columns: detector symmetry DD 40mm 12.5mm PFD 3-4 15-20 < 1.5 -4 EFD 1 10-15 <1.5 -4 SFD 7 or 0 7-10 <0.5 -2 PTW 0 <1 <1 diamond 0 <0.5 <1 -2 The asymmetry around the edge of the diodes is the result of variations in the manufacturing process. Scanditronix have remedied the problem in the case of the SFD. Tables of correction factors (CFs) were calculated for the diodes and the PTW, for a range of couch and gantry combinations, to correct for DD. The DD of the diamond was insignificant within the experimental uncertainty. All detectors measured the dose to the isocentre within 2% in the 40ram collimator. The PTW was within 2% for all collimators down to a 22.5mm, but was physically too large for smaller collimators. In the 12.5mm collimator, the error for a single beam with the EFD and the PFD was between 2 and 8%. For the SFD the error for a single beam was between 1.5 and 2.5%
Monday, 17 September 2001
als (18%), institution of unit manager's positions to improve scheduling and reduce workplace stress (18%), the ability to earn more time off and improvement of pensions and wellness benefits (18%). Recommendations for improvement of the work environment and remuneration were derived from the results of the survey. Surveys of this type will be repeated in the future to assess improvements in working conditions, retention and recruitment of Radiation Therapists.
Symposia/Proffered papers
51
oral
Dynamic edge focussing: A new MLC-design to deliver IMRT with a double focussing high precision Multi-Leaf-Collimator O. Pastyr 1, G. Echner 1, G. Hartmann 1, J. Richter 2, W. Schleael 1 1DKFZ, Medical Physics, Heidelberg, Germany, 2University of Wuerzburg, Radiotherapy, Wuerzburg, Germany
IMRT - V E R I F I C A T I O N A N D DELIVERY II
Tomotherapy is a therapeutic technique that combines photon intensity modulated radiotherapy (IMRT) and computed tomography (CT) capabilities into a single piece of equipment. Tomotherapy provides a closed loop for planning, delivery, verification and adaptation of radiotherapy. Such a device allows for processes that are very difficult or even impossible with other radiotherapy modalities. Tomotherapy is an ideal tool to perform Image Guided Adaptive Radiotherapy (IGAR). The first helical tomotherapy prototype in the world had been installed at the University of WisconsinMadison. The tomotherapy capabilities will be analyzed by describing its processes and the connection between them. Processes and techniques such as planning optimization, conformal and conformal avoidance treatments, megavoltage CT, registration in projection space, image fusion, helical delivery, delivery verification, dose reconstruction, deformable contour registration, deformable dose registration, adaptive radiotherapy, commissioning and QA will be discussed. Future lines of work will also be described.
Standard Linac integrated MLCs still have severe shortcomings for delivering IMRT. The most severe ones are lacking leaf resolution and leaf positioning accuracy, a penumbra which is dependent from the leaf position and the fact that in many clinical cases the overtravel of the leaves is not sufficient. A new mechanical concept for the construction of a high precision double focussing multi-leaf-collimator has been developed, which is designed to overcome the above mentioned shortcomings. The most outstanding features of the new disign are: - physical leaf thickness below 3 mm allowing a leaf resolution of below 5 mm at isocenter distance; - linearly guided double focussing leaves. Due to a new mechanical concept with movable edges, there is no longer a need for guiding the leaves on a circular path. The edges are dynamically controlled by a simple mechanical forced drive which ensures that the edges are always directed towards the focus. Thus a very small position independent penumbra is reached - simple linear leaf guidance system. The guidance system allows to use very long leaves and thus enables a large overtravel at highest leaf precision; - high precision measurement of leaf position. The position measurement of a leaf is performed by a pair of lead integrated high precision potentiometers. According to the new mechanical design, a prototype MLC with a maximum field size of 20x20 cm 2 tuned for IMRT applications has been constructed which can be attached to the accessory holder of a Linac. Mechanical, physical and electronical details as well as results of dosimetric measurements will be presented.
50
52
49
invited
Tomotherapy: a tool for image guided adaptive radiotherapy G.H. Olivera 1,2, T.R. Mackie 1,2, K.J. Ruchala 2, J.M. Kapatoes 2, P.J. Reckwerdt2, R. Jeraj 1, J.P. Balog2, W. Lu 1,2, J. Smilowitz 1, H. Keller 1 1University of Wisconsin, Medical Physics, Madison, USA, 2Tomotherapy Inc., Middleton, USA
oral
oral
Comparison of calculated and measured data for small MLC generated fields defined on Varian and Elekta accelerators
Verification of the dose to the isocentre in stereotactic plans
L. Rock, B. McClean, P. McCavana, S. Buckney St. Luke's, Physics, Dublin, Ireland
C. McKerracher, D.I. Thwaites Western general Hospital, Oncology Physics, Edinburgh Cancer Centre, Edinburgh, United Kingdom
This work is concentrated on the use of small MLC-defined fields and low numbers of monitor units per field used in the delivery of multiple-segmented field IMRT. Prior to the delivery of multiple-segmented field IMRT, as part of a quality assurance programme, it is important that these small fields be characterised and compared to the small field data generated by the treatment planning system. During the progress of this work, small field data was measured for both a Varian 2300C/D and an Elekta SL18 linac, and compared with a Helax TMS (vers 5.1) treatment planning system (TPS). The data measured includes percentage depth dose in water, X and Y-profiles, output factors, leaf transmission and linearity of dose with monitor units for small fields centred on the central axis. Initial analysis of these results show good agreement between the TPS and the depth dose data for fields as small as l x l c m for the Varian linac and 2x2cm for the Elekta linac. Comparison of the TPS generated profiles with the measured profiles show significant differences, in both directions, for both machines at 2x2cm and l x l c m for the Elekta linac. Comparison of TPS calculated output factors with measured output factors show differences of approximately 20% for the Elekta linac at 1xl cm and 8% for the Varian linac at lxlcm, with the measured/calculated difference decreasing rapidly as the field size increases. Preliminary investigations indicate that there may also be problems with the use of both off-axis and rectangular fields. The results of this work indicates that caution should be employed with the use of small square fields and rectangular fields on the central axis and off-axis for the delivery of multiple-segmented field IMRT.
Verification of the dose to the isocentre in stereotactic treatments should be an essential part of the quality control process. It is difficult to find a suitable detector for use in a phantom because of the non-coplanar nature of stereotactic treatments. The detector must be small, tissue equivalent and, directionally independent. This study investigated the use of a 0.125cc ion chamber (PTW), a diamond PTW) and three diodes; shielded (PFD), unshielded (EFD) and unshielded stereotactic (SFD) (Scanditronix). All were "treated" in a water equivalent phantom according to a typical stereotactic plan. In the following table of results, the symmetry is the % variation around the cross section of the detector, DD is the % directional dependence and the % errors in the measured dose to the isocentre for the largest (40mm) and smallest (12.5mm) collimators (Radionics) are shown in the final two columns: detector symmetry DD 40mm 12.5mm PFD 3-4 15-20 < 1.5 -4 EFD 1 10-15 <1.5 -4 SFD 7 or 0 7-10 <0.5 -2 PTW 0 <1 <1 diamond 0 <0.5 <1 -2 The asymmetry around the edge of the diodes is the result of variations in the manufacturing process. Scanditronix have remedied the problem in the case of the SFD. Tables of correction factors (CFs) were calculated for the diodes and the PTW, for a range of couch and gantry combinations, to correct for DD. The DD of the diamond was insignificant within the experimental uncertainty. All detectors measured the dose to the isocentre within 2% in the 40ram collimator. The PTW was within 2% for all collimators down to a 22.5mm, but was physically too large for smaller collimators. In the 12.5mm collimator, the error for a single beam with the EFD and the PFD was between 2 and 8%. For the SFD the error for a single beam was between 1.5 and 2.5%