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PRECISE MEASUREMENT OF STEREOTACTIC LINAC RADIATION POSITION PARAMETERS USING A PHOTODIODE ARRAY
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P ROFFERED PAPER
f-up, nurse-led f-up, and f-up by the general practitioner, show promising results on QoL and patient satisfaction. Replacing clinic visits by telephone interviews is also increasingly being used as a potentially cost-effective way of f-up. Furthermore, a short educational program may be able to replace some outpatient clinic visits to the specialist. Nevertheless, we still seem to adhere to the conventional oncologist f-up, as alternative strategies are not yet widely applied in clinical practice. Additional evidence on the costs and effects of alternative strategies may speed up the implementation process. The RCT has been successfully conducted, with the 320th patient included in March 2008. With at least 12-month f-up needed, data analyses started in April 2009 and preliminary results are expected to be presented at the 10th ESTRO conference in September 2009. Conclusion: The value of frequent outpatient follow-up in the first few years after curative treatment for breast cancer is still a controversial issue. Based on what is known from the literature, we successfully conducted an RCT investigating the cost-effectiveness of both a reduced i.e. nurse-led telephone f-up and a short EGP.
W EDNESDAY, S EPTEMBER 2, 2009
Proffered paper Stereotactic body radiotherapy 308 oral PRECISE MEASUREMENT OF STEREOTACTIC LINAC RADIATION POSITION PARAMETERS USING A PHOTODIODE ARRAY D. Spencer1 , I. Nygren1 , M. Balderson1 , D. Brown1 1 2 3
TOM B AKER C ANCER C ENTRE, Medical Physics, Calgary, Canada U NIVERSITY OF C ALGARY, Oncology, Calgary, Canada U NIVERSITY OF C ALGARY, Physics and Astronomy, Calgary, Canada
Purpose: A linear accelerator used for stereotactic radiosurgery must have a very good isocentre and the lasers must point to that isocentre with high accuracy. Typically, the isocentre is determined carefully at the time of installation both mechanically and with radiation, and then checked periodically, along with the lasers, using a Winston-Lutz test or something similar. The worst-case combination of error due to laser setup and collimator, gantry and couch rotation should be less than 1 mm. Lasers drift and need to be adjusted periodically. Gantry sag (GS) and head tilt (HT) might change with bearing wear or accidents such as collisions. MLC or cone centring might be affected by preventive maintenance. It is important to have a QC program which can detect all these things. Our thinking was that if we wanted to detect a drift of 0.1 mm, our measurement accuracy must be at least 0.05 mm. Materials: We have developed a prototype system we call MagicEye to measure these properties using a photodiode array, the RadEye1om Rad-Icon Imaging with a board from National Instruments. The array is 2.5 cm by 5.0 cm with a 48 micron pitch, and 512x1024 elements. This array is sensitive to light; to increase its sensitivity to X-Rays we have added a translucent fluorescent screen which still allows us to see the light field and lasers. We have taken images of the 4 mm cone, 6mm and 12mm square MLC fields for various collimator and gantry angles. We analyzed these images by finding the 50% edge and fitting it to a circle or rectangle as appropriate. We also took images of the laser crosshairs which we fit to two intersecting lines. We then calculated GS and HT over time, the distance from the centre of our microMLC field to the collimator rotational axis, and the agreement of our patient set-up lasers with the radiation field and the isocentre. We also measured the tongue and groove effect due to the overlap of adjacent leaves. These measurements were done on a Novalischine from BrainLAB with the m3 Micro Multileaf Collimator. Results: We have followed GS and HT over a period of 15 months and contrary to our expectations we measured no deterioration, with an upper limit of 20 microns. The centre of the MLC field differs from the rotational axis of the collimator by 0.3 mm. The lasers are less than 0.2 mm from the reference point. The tongue and groove effect reduces the apparent field size by 0.9 mm. Conclusions: Our device is capable of precision measurements of linac radiation position parameters. To our surprise our GS and HT do not appear to be deteriorating. We intend to include the couch rotation in future measurements. 309 oral PRELMINARY CHARACTERIZATION OF A DEDICATED IMAGEGUIDED SBRT SYSTEM M. Speiser1 , P. Medin1 , W. Mao1 , L. Papiez1 , F. Gum2 , T. Solberg1 1 2
UT S OUTHWESTERN M EDICAL C ENTER, Dallas, USA B RAIN LAB AG, Heimstetten, Germany
Purpose: Preliminary commissioning measurements and stereotactic "hidden target" tests were performed to assess dosimetric and targeting characteristics as well as clinical capabilities of a new system dedicated to Stereotactic Body Radiation Therapy (SBRT). Materials: The "Vero SBRT System" (known as "TM2000" in Japan) is a joint product of MHI (Mitsubishi Heavy Industries Ltd., Tokyo, Japan) and BrainLAB (BrainLAB AG, Feldkirchen, Germany). The Vero uses a rigid, rotating ring structure to integrate a beam delivery platform with image guidance systems. The beam delivery platform consists of a 6 MV C-band LINAC with MLC is mounted on orthogonal gimbals that are in turn built into the ring. An electronic portal imaging device (EPID) and two orthogonal kV imaging devices are also integrated into the ring. Percent depth dose curves and beam profiles were measured in a water phantom and subsequently analyzed in accordance with industry standards. In addition to symmetric fields, depth dose and off axis characteristics were measured for a 5x5 cm2 beam "tilted" 1.5 cm off the central axis using the gimbals mechanism. Additional characteristics including dose and dose rate linearity, relative output with field size, and output constancy with gantry angle were also evaluated. A commercial stereotactic phantom (LucyTM, Standard Imaging, Middleton, WI) was used to assess end-to-end localization accuracy of the image guidance system (through a "hidden-target" test) and to compare dose measurements against treatment planning calculations. Results: The essential characteristics of the Vero are commensurate with modern stereotactic requirements. Dosimetric characteristics are similar to
P ROFFERED PAPER
f-up, nurse-led f-up, and f-up by the general practitioner, show promising results on QoL and patient satisfaction. Replacing clinic visits by telephone interviews is also increasingly being used as a potentially cost-effective way of f-up. Furthermore, a short educational program may be able to replace some outpatient clinic visits to the specialist. Nevertheless, we still seem to adhere to the conventional oncologist f-up, as alternative strategies are not yet widely applied in clinical practice. Additional evidence on the costs and effects of alternative strategies may speed up the implementation process. The RCT has been successfully conducted, with the 320th patient included in March 2008. With at least 12-month f-up needed, data analyses started in April 2009 and preliminary results are expected to be presented at the 10th ESTRO conference in September 2009. Conclusion: The value of frequent outpatient follow-up in the first few years after curative treatment for breast cancer is still a controversial issue. Based on what is known from the literature, we successfully conducted an RCT investigating the cost-effectiveness of both a reduced i.e. nurse-led telephone f-up and a short EGP.
W EDNESDAY, S EPTEMBER 2, 2009
Proffered paper Stereotactic body radiotherapy 308 oral PRECISE MEASUREMENT OF STEREOTACTIC LINAC RADIATION POSITION PARAMETERS USING A PHOTODIODE ARRAY D. Spencer1 , I. Nygren1 , M. Balderson1 , D. Brown1 1 2 3
TOM B AKER C ANCER C ENTRE, Medical Physics, Calgary, Canada U NIVERSITY OF C ALGARY, Oncology, Calgary, Canada U NIVERSITY OF C ALGARY, Physics and Astronomy, Calgary, Canada
Purpose: A linear accelerator used for stereotactic radiosurgery must have a very good isocentre and the lasers must point to that isocentre with high accuracy. Typically, the isocentre is determined carefully at the time of installation both mechanically and with radiation, and then checked periodically, along with the lasers, using a Winston-Lutz test or something similar. The worst-case combination of error due to laser setup and collimator, gantry and couch rotation should be less than 1 mm. Lasers drift and need to be adjusted periodically. Gantry sag (GS) and head tilt (HT) might change with bearing wear or accidents such as collisions. MLC or cone centring might be affected by preventive maintenance. It is important to have a QC program which can detect all these things. Our thinking was that if we wanted to detect a drift of 0.1 mm, our measurement accuracy must be at least 0.05 mm. Materials: We have developed a prototype system we call MagicEye to measure these properties using a photodiode array, the RadEye1om Rad-Icon Imaging with a board from National Instruments. The array is 2.5 cm by 5.0 cm with a 48 micron pitch, and 512x1024 elements. This array is sensitive to light; to increase its sensitivity to X-Rays we have added a translucent fluorescent screen which still allows us to see the light field and lasers. We have taken images of the 4 mm cone, 6mm and 12mm square MLC fields for various collimator and gantry angles. We analyzed these images by finding the 50% edge and fitting it to a circle or rectangle as appropriate. We also took images of the laser crosshairs which we fit to two intersecting lines. We then calculated GS and HT over time, the distance from the centre of our microMLC field to the collimator rotational axis, and the agreement of our patient set-up lasers with the radiation field and the isocentre. We also measured the tongue and groove effect due to the overlap of adjacent leaves. These measurements were done on a Novalischine from BrainLAB with the m3 Micro Multileaf Collimator. Results: We have followed GS and HT over a period of 15 months and contrary to our expectations we measured no deterioration, with an upper limit of 20 microns. The centre of the MLC field differs from the rotational axis of the collimator by 0.3 mm. The lasers are less than 0.2 mm from the reference point. The tongue and groove effect reduces the apparent field size by 0.9 mm. Conclusions: Our device is capable of precision measurements of linac radiation position parameters. To our surprise our GS and HT do not appear to be deteriorating. We intend to include the couch rotation in future measurements. 309 oral PRELMINARY CHARACTERIZATION OF A DEDICATED IMAGEGUIDED SBRT SYSTEM M. Speiser1 , P. Medin1 , W. Mao1 , L. Papiez1 , F. Gum2 , T. Solberg1 1 2
UT S OUTHWESTERN M EDICAL C ENTER, Dallas, USA B RAIN LAB AG, Heimstetten, Germany
Purpose: Preliminary commissioning measurements and stereotactic "hidden target" tests were performed to assess dosimetric and targeting characteristics as well as clinical capabilities of a new system dedicated to Stereotactic Body Radiation Therapy (SBRT). Materials: The "Vero SBRT System" (known as "TM2000" in Japan) is a joint product of MHI (Mitsubishi Heavy Industries Ltd., Tokyo, Japan) and BrainLAB (BrainLAB AG, Feldkirchen, Germany). The Vero uses a rigid, rotating ring structure to integrate a beam delivery platform with image guidance systems. The beam delivery platform consists of a 6 MV C-band LINAC with MLC is mounted on orthogonal gimbals that are in turn built into the ring. An electronic portal imaging device (EPID) and two orthogonal kV imaging devices are also integrated into the ring. Percent depth dose curves and beam profiles were measured in a water phantom and subsequently analyzed in accordance with industry standards. In addition to symmetric fields, depth dose and off axis characteristics were measured for a 5x5 cm2 beam "tilted" 1.5 cm off the central axis using the gimbals mechanism. Additional characteristics including dose and dose rate linearity, relative output with field size, and output constancy with gantry angle were also evaluated. A commercial stereotactic phantom (LucyTM, Standard Imaging, Middleton, WI) was used to assess end-to-end localization accuracy of the image guidance system (through a "hidden-target" test) and to compare dose measurements against treatment planning calculations. Results: The essential characteristics of the Vero are commensurate with modern stereotactic requirements. Dosimetric characteristics are similar to