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FILMLESS QUALITY ASSURANCE FOR HELICAL TOMOTHERAPY WITH A CYLINDRICAL STAIR-LIKE ALUMINIUM PHANTOM
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P ROFFERED PAPER
and neck, thoracic or pelvic region. The latter were treated in prone position. The final positioning prior to treatment, was carried out by means of the co-registration of a MV-CT scan, generated from the TomoTherapy device with the planning Kv-CT.At our department, radiation technologists are responsible for the decision making in the correct positioning of patients prior to treatment on TomoTherapy. Instructions must be followed and a good cooperation between the different disciplines (physicians, physicists and RTT’s) is essential to perform this IGRT procedure accurately. Results: The mean overall treatment time is 23,3 minutes. The mean overall preparation time, i.e. positioning-, scan-, registration- and repositioning time takes 13,4 minutes. In 4.1 percent of the total population, extreme long time measurements exceeding 36,9 minutes, have been observed. Comparison between time measurements performed after clinical implementation and time measurements performed one year later to examine the learning curve, showed no differences.Table 1 shows the mean and stdev for all measurements of the translational and rotational adjustments per region. Similar as well as significant differences between both arms has been noticed (data not shown).
T UESDAY, S EPTEMBER 1, 2009
Materials: A set of RapidArc machine QA plans were delivered on three beam-matched Varian Clinac iXs, and images were acquired during delivery with an aS1000 EPID. For the analysis of the images three MatLab scripts were produced. The scripts automatically analyze each leaf pair separately, resulting in a number of leaf pairs outside a set tolerance level. The results are also presented graphically, showing which leaf pair is out of tolerance.(1) MLC leaf position testA picket fence pattern is delivered, and the position of each leaf pair is compared with the mean position for the whole bank, outputting leaf pairs with deviation from the mean position. Sensitivity was tested by introducing an intentional displacement of 0.5 mm for one of the MLC leaf pairs.(2) Dose rate and gantry speed testThe plan for testing dose rate and gantry speed consists of seven segments, the same dose is delivered in each segment but with different gantry speed and dose rate. The segments in each image were compared with each other by a calculation of the deviation of each segment level from the mean level. (3) MLC leaf speed testTo test the MLC leaf speed, three segments are given the same dose, with the MLCs moving at different speeds. The segments were compared with each other by a calculation of the deviation of each segment level from the mean level. In the last two tests the images were normalized with an open field image to remove the influence of asymmetry and nonflatness.A more detailed description regarding the test plans has been presented previously (Ling et al, 2008). Results: The scripts were easily run and the total analysis of each test took less than half a minute. Each script has been tested on eight QA images acquired on each of the three accelerators. Results from the eight measurements on each accelerator are summarized in the table showing the maximum deviation from mean and standard deviation for each of the tests.In the test of sensitivity of the MLC leaf position test, the mean deviation of the displaced leaf pair varied between 0.47 mm and 0.53 mm with a mean of 0.50 mm.
Conclusions: Treating a patient on TomoTherapy takes approximately 25 minutes, yielding 19 patients to be treated within eight hours. Investigation on translational and rotational positioning adjustments prior to treatment, shows that IGRT procedure is crucial to treat patients correctly, despite of the extra time this procedure takes. It also increases the responsibilities of all radiation technologist involved.
Treatment and patient safety in advanced techniques 248 oral ANALYSIS OF EPID IMAGES ACQUIRED DURING DELIVERY OF RAPIDARC QA PLANS A. Fredh1 , S. Korreman1 , P. Munck af Rosenschöld1 1 T HE F INSEN C ENTER - R IGSHOSPITALET, Department of Radiation Oncology, Copenhagen, Denmark
Purpose: In treatments with the RapidArc technique, dose is delivered during the rotation of the gantry with variable MLC positions, dose rate and gantry speed. The purpose of this study is to develop and evaluate methods for automated analysis of EPID images acquired during delivery of RapidArc machine QA plans, which include tests of MLC position, MLC speed, dose rate and gantry speed.
Conclusions: A set of methods for analyzing RapidArc QA plans has been developed and programmed in MatLab scripts. The methods have been used successfully for machine QA of three Clinacs. 249 oral FILMLESS QUALITY ASSURANCE FOR HELICAL TOMOTHERAPY WITH A CYLINDRICAL STAIR-LIKE ALUMINIUM PHANTOM V. Althof1 , D. Kramer1 , R. Westendorp1 , P. van Haaren2 , M. Wispelweij1 , A. Minken1 1 RISO, Medical Physics, Deventer, Netherlands 2 ACADEMIC M EDICAL C ENTER, Medical Physics, Amsterdam, Netherlands
T UESDAY, S EPTEMBER 1, 2009
Purpose: To implement filmless QA for the Tomotherapy Hi-Art unit specifically designed for the QA of helical treatment delivery mode. Materials: The Tomotherapy machine considerably differs from conventional linear accelerators , making QA for this unit different in a number of ways. Initially a stair-like aluminium phantom (supplied by Tomotherapy) was applied to perform QA with the gantry at a fixed position. Though a number of relevant machine parameters could be monitored with this phantom, there was a need for a method which enables the monitoring of parameters specific for the helical treatment mode. We designed a stair-like aluminium cylindrical phantom with lead markings (StepWedge) that enables the measurement of QA items in helical treatment mode. The QA procedure utilizes the signals from the systems’ on-board MegaVoltage CT-detector (MVCT). This detector consists of 640 channels which are sampled with a frequency of 30 Hz. A number of QA items could be deduced from analysis of the MVCT detector data with the phantom: the position of the sagittal and transversal green lasers, beam energy constancy, width of the 1 cm slit, table velocity and accuracy of abutment during a completion procedure after a treatment interruption. The cylindrical phantom in helical mode provides dynamic parameters like the position of the rotating gantry, gantry speed variations, output in helical mode, synchronisation of MLC and gantry, synchronisation of table and gantry.
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sensitivity of the MVCT-detector, which has to be accounted for in (future) in-vivo dosimetry with the MVCT-detector.
Conclusions: We developed a tool that provides insight in the machine characteristics of Tomotherapy treatment machines. Our three years experience with the machines and the analysis tools enable us to change from patient QA to more specific machine related QA. The jump in monitor chamber sensitivity shows that output measurements based on monitor chamber read out, should be verified by ionization chamber measurements on a regular basis. Monitoring machine behaviour using the MVCT-detector is useful to decrease unexpected downtime by predicting machine failures and plan preventive maintenance. Extensive measurements and analysis were necessary as the characteristics of Tomotherapy machines differ considerably from classical linear accelerators. 251 oral ABSORBED DOSE AND DOSE RATE USING THE VARIAN OBI 1.3 AND 1.4 CBCT SYSTEM A. Palm1 , E. Nilsson1 , L. Herrnsdorf2 1 S AHLGRENSKA U NIVERSITY H OSPITAL, Dept of Medical Physics and Biomedical Engineering, Göteborg, Sweden 2 RTI E LECTRONICS, Product Manager, Mölndal, Sweden
Figure: cylindrical stepwedge for QA in helical mode Results: From the StepWedge procedure the position of the transversal and sagittal lasers can be checked with an accuracy of ~0.5mm, the field width with an accuracy of ~0.5mm, beam energy consistency with an accuracy of ~1%, and couch movement and velocity with an accuracy of ~0.1mm resp. ~0.5%. The accuracy of the completion procedure, in terms of a possible additional couch shift, is ~0.1mm. MLC synchronization with gantry rotation can be checked with an accuracy of ~0.1◦ . Conclusions: The StepWedge combined with the built in MVCT detector offers a fast, accurate and filmless QA program, allowing evaluation of almost all relevant parameters of the helical delivery mode of Tomotherapy. 250 oral ANALYSIS OF MVCT-DETECTOR AND MACHINE PARAMETER DATA FOR DAILY TOMOTHERAPY MACHINE QA D. Kramer1 , V. Althof1 , R. Westendorp1 , M. Wispelweij1 , A. Minken1 1 RISO, Medical Physics, Deventer, Netherlands
Purpose: To analyse the status of two Tomotherapy machines based on the built in MVCT-detector and the machine parameter file, on a daily basis, without the need for extensive extra QA measurements. Materials: During three years we conducted extensive preventive QA on our two Tomotherapy machines, which includes the acquisition and analysis of the detector data files from about 2500 rotational variation (ROTVAR) measurements. The ROTVAR procedure is part of each morning start-up procedure and is the daily output check. Machine-specific items as water flow through critical parts are also monitored in this procedure. Besides the daily output check, also the long-term stability of the machines was evaluated. Among other items, changes were monitored in the lateral profile, in MVCT-HU and in sensitivity of monitor chambers and MVCT-detector channels. The latter two items were related to measurements performed with an ionisation chamber on a weekly basis. The weekly measurements also include MLC related items as latency and output per leaf. All data are visualized in detail and trending graphs, for day-by-day inspection and long-term analysis. Results: Trending data for a three-year period is shown (figure). Our data reveal target deterioration and seem to predict target failure. Drift in MVCT HU is observed, which has implications for the use of treatment planning based on the MVCT. We have found effects up to 10%. This stresses the importance of periodic calibration of the detector. On one machine we found an inexplicable jump in monitor chamber sensitivity. We also found slow changes in
Purpose: According to published data, the absorbed dose used for a CBCT image acquisition with Varian OBI v1.3 can be as high as 100 mGy, which likely limits the number of CBCTs that can be performed during the patient’s course of treatment. In 2008 Varian released a new OBI version (v1.4), which promised to reduce the imaging dose. In this study, absorbed doses used for CBCT image acquisitions with the default irradiation techniques of Varian OBI v1.3 and v1.4 are measured. Materials: TLDs are used to derive dose distributions at three planes inside a female Alderson anthropomorphic phantom; in the head, thorax, and pelvic region. In addition, point doses and dose profiles inside a ’stack’ of three CTDI body phantoms are measured using a new solid state detector, the ’CT Dose Profiler’ (RTI Electronics AB). With the ’CT Dose Profiler’ the individual pulses from the x-ray tube is also studied. To verify the absorbed dose measured with the ’CT Dose Profiler’ it is compared to TLD. For both OBI versions, the image quality is evaluated using a Catphan phantom. Results: In OBI v1.3, doses measured in transverse planes of the Alderson phantom range between 64 mGy and 144 mGy. The average dose used for default imaging modes is 100 mGy for a Full-fan Head scan, and 90 mGy for a Half-fan Pelvis scan. In OBI v1.4, doses measured in transverse planes of the Alderson phantom range between 1 mGy and 51 mGy. The three Head scan modes result in average doses of 2.5 mGy, 5 mGy, and 25 mGy per scan, respectively. The 200 degree scan option is shown to reduce the imaging dose to the lens by a factor of 4. The Low-dose Thorax mode gives an average dose of 12 mGy per scan, and a CBCT scan of the Pelvis, 25 mGy to 35 mGy. ’CT Dose Profiler’ data agree with TLD measurements in a CTDI phantom within the uncertainty of the TLD measurements (estimated SD ±10%). Instantaneous dose rate at the periphery of the phantom can be higher than 20 mGy/s, which is 10 times the dose rate at the center. The spatial resolution in v1.4 is not as high as in v1.3, for corresponding CBCT modes. Conclusions: Measurements show that the imaging doses for default modes in Varian OBI v1.4 CBCT system are significantly lower than in v1.3. In v1.3 the mean dose is around 100 mGy (maximum measured dose 150 mGy). In v1.4 mean doses range between 3-35 mGy (maximum measured dose 50 mGy) depending on CBCT mode. The ’CT Dose Profiler’ is proven fast and accurate for measurements in CTDI phantoms. 252 oral LOWERING WHOLE-BODY RADIATION DOSES IN PAEDIATRIC IMRT THROUGH THE USE OF UNFLATTENED PHOTON BEAMS. J. Cashmore1 1 U NIVERSITY H OSPITAL B IRMINGHAM, Radiotherapy Physics, Birmingham, United Kingdom
Purpose: IMRT is an established technique used to concentrate dose into a target region/s whilst sparing normal tissues. IMRT delivery, however, is wasteful of monitor units leading to an increase in leakage radiation to the patient and therefore unwanted whole-body doses. There may therefore be an increased risk of radiation induced secondary cancers associated with
P ROFFERED PAPER
and neck, thoracic or pelvic region. The latter were treated in prone position. The final positioning prior to treatment, was carried out by means of the co-registration of a MV-CT scan, generated from the TomoTherapy device with the planning Kv-CT.At our department, radiation technologists are responsible for the decision making in the correct positioning of patients prior to treatment on TomoTherapy. Instructions must be followed and a good cooperation between the different disciplines (physicians, physicists and RTT’s) is essential to perform this IGRT procedure accurately. Results: The mean overall treatment time is 23,3 minutes. The mean overall preparation time, i.e. positioning-, scan-, registration- and repositioning time takes 13,4 minutes. In 4.1 percent of the total population, extreme long time measurements exceeding 36,9 minutes, have been observed. Comparison between time measurements performed after clinical implementation and time measurements performed one year later to examine the learning curve, showed no differences.Table 1 shows the mean and stdev for all measurements of the translational and rotational adjustments per region. Similar as well as significant differences between both arms has been noticed (data not shown).
T UESDAY, S EPTEMBER 1, 2009
Materials: A set of RapidArc machine QA plans were delivered on three beam-matched Varian Clinac iXs, and images were acquired during delivery with an aS1000 EPID. For the analysis of the images three MatLab scripts were produced. The scripts automatically analyze each leaf pair separately, resulting in a number of leaf pairs outside a set tolerance level. The results are also presented graphically, showing which leaf pair is out of tolerance.(1) MLC leaf position testA picket fence pattern is delivered, and the position of each leaf pair is compared with the mean position for the whole bank, outputting leaf pairs with deviation from the mean position. Sensitivity was tested by introducing an intentional displacement of 0.5 mm for one of the MLC leaf pairs.(2) Dose rate and gantry speed testThe plan for testing dose rate and gantry speed consists of seven segments, the same dose is delivered in each segment but with different gantry speed and dose rate. The segments in each image were compared with each other by a calculation of the deviation of each segment level from the mean level. (3) MLC leaf speed testTo test the MLC leaf speed, three segments are given the same dose, with the MLCs moving at different speeds. The segments were compared with each other by a calculation of the deviation of each segment level from the mean level. In the last two tests the images were normalized with an open field image to remove the influence of asymmetry and nonflatness.A more detailed description regarding the test plans has been presented previously (Ling et al, 2008). Results: The scripts were easily run and the total analysis of each test took less than half a minute. Each script has been tested on eight QA images acquired on each of the three accelerators. Results from the eight measurements on each accelerator are summarized in the table showing the maximum deviation from mean and standard deviation for each of the tests.In the test of sensitivity of the MLC leaf position test, the mean deviation of the displaced leaf pair varied between 0.47 mm and 0.53 mm with a mean of 0.50 mm.
Conclusions: Treating a patient on TomoTherapy takes approximately 25 minutes, yielding 19 patients to be treated within eight hours. Investigation on translational and rotational positioning adjustments prior to treatment, shows that IGRT procedure is crucial to treat patients correctly, despite of the extra time this procedure takes. It also increases the responsibilities of all radiation technologist involved.
Treatment and patient safety in advanced techniques 248 oral ANALYSIS OF EPID IMAGES ACQUIRED DURING DELIVERY OF RAPIDARC QA PLANS A. Fredh1 , S. Korreman1 , P. Munck af Rosenschöld1 1 T HE F INSEN C ENTER - R IGSHOSPITALET, Department of Radiation Oncology, Copenhagen, Denmark
Purpose: In treatments with the RapidArc technique, dose is delivered during the rotation of the gantry with variable MLC positions, dose rate and gantry speed. The purpose of this study is to develop and evaluate methods for automated analysis of EPID images acquired during delivery of RapidArc machine QA plans, which include tests of MLC position, MLC speed, dose rate and gantry speed.
Conclusions: A set of methods for analyzing RapidArc QA plans has been developed and programmed in MatLab scripts. The methods have been used successfully for machine QA of three Clinacs. 249 oral FILMLESS QUALITY ASSURANCE FOR HELICAL TOMOTHERAPY WITH A CYLINDRICAL STAIR-LIKE ALUMINIUM PHANTOM V. Althof1 , D. Kramer1 , R. Westendorp1 , P. van Haaren2 , M. Wispelweij1 , A. Minken1 1 RISO, Medical Physics, Deventer, Netherlands 2 ACADEMIC M EDICAL C ENTER, Medical Physics, Amsterdam, Netherlands
T UESDAY, S EPTEMBER 1, 2009
Purpose: To implement filmless QA for the Tomotherapy Hi-Art unit specifically designed for the QA of helical treatment delivery mode. Materials: The Tomotherapy machine considerably differs from conventional linear accelerators , making QA for this unit different in a number of ways. Initially a stair-like aluminium phantom (supplied by Tomotherapy) was applied to perform QA with the gantry at a fixed position. Though a number of relevant machine parameters could be monitored with this phantom, there was a need for a method which enables the monitoring of parameters specific for the helical treatment mode. We designed a stair-like aluminium cylindrical phantom with lead markings (StepWedge) that enables the measurement of QA items in helical treatment mode. The QA procedure utilizes the signals from the systems’ on-board MegaVoltage CT-detector (MVCT). This detector consists of 640 channels which are sampled with a frequency of 30 Hz. A number of QA items could be deduced from analysis of the MVCT detector data with the phantom: the position of the sagittal and transversal green lasers, beam energy constancy, width of the 1 cm slit, table velocity and accuracy of abutment during a completion procedure after a treatment interruption. The cylindrical phantom in helical mode provides dynamic parameters like the position of the rotating gantry, gantry speed variations, output in helical mode, synchronisation of MLC and gantry, synchronisation of table and gantry.
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sensitivity of the MVCT-detector, which has to be accounted for in (future) in-vivo dosimetry with the MVCT-detector.
Conclusions: We developed a tool that provides insight in the machine characteristics of Tomotherapy treatment machines. Our three years experience with the machines and the analysis tools enable us to change from patient QA to more specific machine related QA. The jump in monitor chamber sensitivity shows that output measurements based on monitor chamber read out, should be verified by ionization chamber measurements on a regular basis. Monitoring machine behaviour using the MVCT-detector is useful to decrease unexpected downtime by predicting machine failures and plan preventive maintenance. Extensive measurements and analysis were necessary as the characteristics of Tomotherapy machines differ considerably from classical linear accelerators. 251 oral ABSORBED DOSE AND DOSE RATE USING THE VARIAN OBI 1.3 AND 1.4 CBCT SYSTEM A. Palm1 , E. Nilsson1 , L. Herrnsdorf2 1 S AHLGRENSKA U NIVERSITY H OSPITAL, Dept of Medical Physics and Biomedical Engineering, Göteborg, Sweden 2 RTI E LECTRONICS, Product Manager, Mölndal, Sweden
Figure: cylindrical stepwedge for QA in helical mode Results: From the StepWedge procedure the position of the transversal and sagittal lasers can be checked with an accuracy of ~0.5mm, the field width with an accuracy of ~0.5mm, beam energy consistency with an accuracy of ~1%, and couch movement and velocity with an accuracy of ~0.1mm resp. ~0.5%. The accuracy of the completion procedure, in terms of a possible additional couch shift, is ~0.1mm. MLC synchronization with gantry rotation can be checked with an accuracy of ~0.1◦ . Conclusions: The StepWedge combined with the built in MVCT detector offers a fast, accurate and filmless QA program, allowing evaluation of almost all relevant parameters of the helical delivery mode of Tomotherapy. 250 oral ANALYSIS OF MVCT-DETECTOR AND MACHINE PARAMETER DATA FOR DAILY TOMOTHERAPY MACHINE QA D. Kramer1 , V. Althof1 , R. Westendorp1 , M. Wispelweij1 , A. Minken1 1 RISO, Medical Physics, Deventer, Netherlands
Purpose: To analyse the status of two Tomotherapy machines based on the built in MVCT-detector and the machine parameter file, on a daily basis, without the need for extensive extra QA measurements. Materials: During three years we conducted extensive preventive QA on our two Tomotherapy machines, which includes the acquisition and analysis of the detector data files from about 2500 rotational variation (ROTVAR) measurements. The ROTVAR procedure is part of each morning start-up procedure and is the daily output check. Machine-specific items as water flow through critical parts are also monitored in this procedure. Besides the daily output check, also the long-term stability of the machines was evaluated. Among other items, changes were monitored in the lateral profile, in MVCT-HU and in sensitivity of monitor chambers and MVCT-detector channels. The latter two items were related to measurements performed with an ionisation chamber on a weekly basis. The weekly measurements also include MLC related items as latency and output per leaf. All data are visualized in detail and trending graphs, for day-by-day inspection and long-term analysis. Results: Trending data for a three-year period is shown (figure). Our data reveal target deterioration and seem to predict target failure. Drift in MVCT HU is observed, which has implications for the use of treatment planning based on the MVCT. We have found effects up to 10%. This stresses the importance of periodic calibration of the detector. On one machine we found an inexplicable jump in monitor chamber sensitivity. We also found slow changes in
Purpose: According to published data, the absorbed dose used for a CBCT image acquisition with Varian OBI v1.3 can be as high as 100 mGy, which likely limits the number of CBCTs that can be performed during the patient’s course of treatment. In 2008 Varian released a new OBI version (v1.4), which promised to reduce the imaging dose. In this study, absorbed doses used for CBCT image acquisitions with the default irradiation techniques of Varian OBI v1.3 and v1.4 are measured. Materials: TLDs are used to derive dose distributions at three planes inside a female Alderson anthropomorphic phantom; in the head, thorax, and pelvic region. In addition, point doses and dose profiles inside a ’stack’ of three CTDI body phantoms are measured using a new solid state detector, the ’CT Dose Profiler’ (RTI Electronics AB). With the ’CT Dose Profiler’ the individual pulses from the x-ray tube is also studied. To verify the absorbed dose measured with the ’CT Dose Profiler’ it is compared to TLD. For both OBI versions, the image quality is evaluated using a Catphan phantom. Results: In OBI v1.3, doses measured in transverse planes of the Alderson phantom range between 64 mGy and 144 mGy. The average dose used for default imaging modes is 100 mGy for a Full-fan Head scan, and 90 mGy for a Half-fan Pelvis scan. In OBI v1.4, doses measured in transverse planes of the Alderson phantom range between 1 mGy and 51 mGy. The three Head scan modes result in average doses of 2.5 mGy, 5 mGy, and 25 mGy per scan, respectively. The 200 degree scan option is shown to reduce the imaging dose to the lens by a factor of 4. The Low-dose Thorax mode gives an average dose of 12 mGy per scan, and a CBCT scan of the Pelvis, 25 mGy to 35 mGy. ’CT Dose Profiler’ data agree with TLD measurements in a CTDI phantom within the uncertainty of the TLD measurements (estimated SD ±10%). Instantaneous dose rate at the periphery of the phantom can be higher than 20 mGy/s, which is 10 times the dose rate at the center. The spatial resolution in v1.4 is not as high as in v1.3, for corresponding CBCT modes. Conclusions: Measurements show that the imaging doses for default modes in Varian OBI v1.4 CBCT system are significantly lower than in v1.3. In v1.3 the mean dose is around 100 mGy (maximum measured dose 150 mGy). In v1.4 mean doses range between 3-35 mGy (maximum measured dose 50 mGy) depending on CBCT mode. The ’CT Dose Profiler’ is proven fast and accurate for measurements in CTDI phantoms. 252 oral LOWERING WHOLE-BODY RADIATION DOSES IN PAEDIATRIC IMRT THROUGH THE USE OF UNFLATTENED PHOTON BEAMS. J. Cashmore1 1 U NIVERSITY H OSPITAL B IRMINGHAM, Radiotherapy Physics, Birmingham, United Kingdom
Purpose: IMRT is an established technique used to concentrate dose into a target region/s whilst sparing normal tissues. IMRT delivery, however, is wasteful of monitor units leading to an increase in leakage radiation to the patient and therefore unwanted whole-body doses. There may therefore be an increased risk of radiation induced secondary cancers associated with