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THE EVALUATION OF PORTAL DOSIMETRY FOR IMRT PATIENT QA
IMRT: T REATMENT VERIFICATION AND QA
mized with the collapsed cone convolution algorithm. Completed plans were copied and recalculated to a calibrated water-equivalent cube phantom. A 180 single beam planar dose was calculated. The phantom treatment plan was programmed into a VarianTM 2100 C/D treatment unit using the Dynamic Arc modality. The treatment was delivered to a cube phantom with a film at isocenter to capture the dose image. Dose was measure via an ionization chamber 1 cm below the isocenter of the cube phantom. Cube phantom calculated and measure doses were compared as well as the film and planar image comparison results. Results: Point-of-Interest (POI) dose comparison results revealed agreement within ± 3% on all trials. Film and Planar images vertical and horizontal profiles agreed within ± 3%. All trials distance to agreement was within ± 3 mm (Table 1). Multiple MLC errors occurred at arc distances of 300 and 360 degrees at dose rates of 500 and 600 MU/min. MLC tolerance limits were set at 0.5 cm instead of the default 0.2 cm. Noteworthy of mention is the vast improvement of optimization calculations (≈ 80 to ≈ 11.5 minutes), when compared to the previous clinical version of software on the Sun Blade 2300 platform. Conclusions: Large arc distance with their small field shapes and higher dose rates result in MLC errors. It is concluded that the errors occur because of the gantry speed at higher dose rates causing MLC tolerance faults. Utilizing dose rates that do not exceed 400 MU/min with the 300 and 360 degree arc distance resulted in favorable results. The new hardware/software platform improved optimization calculation time by almost 700%. 594 poster (Physics Track) TAKING INTO ACCOUNT THE UNCERTAINTY OF THE IONISATION CHAMBER POSITIONING IN IMRT VERIFICATIONS J. Richart Sancho1 , J. Perez-Calatayud1 , S. Rodriguez Villalba1 , M. Santos1 , F. Ballester3 , D. Granero4 , F. Lliso2 , V. Carmona2 1 H OSPITAL C LINICA B ENIDORM, Radiotherapy Department, Benidorm, Spain 2 H OSPITAL U NIVERSITARIO L A F E, Radiotherapy Department, Valencia, Spain 3 U NIVERSITY OF VALENCIA -IFIC, Valencia, Spain 4 H OSPITAL G ENERAL, Radiotherapy Department, Valencia, Spain
Purpose: The existing recommendations state that each IMRT plan should be checked prior to delivery. A widely extended approach is to translate each plan to a phantom and to re-calculate it for this new geometry. The dose distribution inside the phantom is verified in one or several planes by means of film dosimetry and the dose at a point with an appropriate ionisation chamber.The comparison of dose distributions (measured/calculated) is usually performed: dose differences, distance-to-agreement (DTA), and a combination of these two parameters, the gamma evaluation method. When comparing the measured absorbed dose at a point the criterion is only to obtain the dose difference without taking into account the uncertainty in the chamber positioning.The purpose of this work is to present a simple method that accounts for the ionisation chamber displacements and its application to 229 IMRT plans. Materials: Dynamic IMRT treatments are delivered in a Clinac 2100-CD (Varian) linear accelerator. At our Centre IMRT verifications are performed twofold: A) for each beam the absolute fluence is verified with the EPID which software is also used to carry out the analysis, the comparison is evaluated with the gamma criterion being the reference value 3%/3mm, admitting 5%/5mm is small isolated regions at the limits of the beam; B) the dose delivered by the global plan is checked at a point with a pin-point chamber (PTW), being the reference value for the difference, ±3%. When it is possible the chamber is allocated in a low dose gradient region.Deviations in the chamber position have been evaluated as follows:In the calculated plan a set of 6 points are generated around the measuring point in the 3 principal axes, 3mm apart in each direction. The measured dose is compared with the calculated dose at these 7 points; the minimal difference is selected as the result of the comparison. We have compared this method with the conventional difference with respect to the central value in 229 cases. Results: a) Comparison with the central value: the mean value of the differences has been -0.8% (s=1.4%, k=1). In 18 cases (7.9%) the difference exceeded ±3%.b) Comparison with the minimum value: the mean value of the differences has been -0.6% (s=1%, k=1). In 6 cases (2.6%) the difference exceeded ±3%.c) Taking into account the difference between the central value and the minimum, the average has been 0.5% (s=0.5%, k=1). In 25 cases (11%) the difference exceeded ±3%. Applying one of the methods or the other means having discrepancies greater than ±1%. Conclusions: We present a simple method that accounts for the uncertainty in the ionisation chamber positioning, in order to perform the comparison with similar criteria than the dose distribution with films, EPIDs or 2D arrays. When it has been applied to the studied plans it is shown that for a significant fraction this value should be considered for this purpose.
S 217
595 poster (Physics Track) THE EVALUATION OF PORTAL DOSIMETRY FOR IMRT PATIENT QA C. Meehan1 R OYAL M ARSDEN H OSPITAL, PHYSICS, London, United Kingdom
1
Purpose: To investigate the feasibility of using a Varian ASi500 imager, whilst maintaining current levels of accuracy with clinical IMRT protocols. Materials: Tests were performed to evaluate the dose detected on the EPID compared to ion chamber and film measurements for small gap sizes, linearity of acquired image readout with MU, and accuracy in low dose areas of a modulated dmlc field. The whole area of the detector was assessed using two methods of dosimetric calibration. Once the EPID evaluation was completed, the evaluation tools within Eclipse were investigated for patient IMRT fields to ensure that the accuracy of the current clinical IMRT protocols can be maintained. Results: The cassette was able to detect doses to within 3% for dmlc fields of sweeping gap sizes from 1mm to 10mm gaps. Acquired images were linear for values >5MU, and agreed to within 2% of the predicted images for MU>30. For evaluation of the accuracy in a modulated dmlc field, a test pattern was delivered with dose levels of 100%, 30% and 20%. Dose levels of 30% (0.3Gy), PD was within 1.5% of the calculated dose, rising to 4.2% for dose levels of 20% (0.2Gy). When assessing the different calibrations techniques, acquired open fields were compared to predicted images for the whole cassette. The first method, using a diagonal profile, only the centre 15cm2 achieved an accuracy of 2% for doses >0.3Gy. Using the second method of a 2D image the whole cassette can be used to same accuracy of 2% for doses >0.3Gy. Looking at the evaluation tools within Eclipse a dose difference map of a 5% induced dose error for the acquired image against the predicted image of the correct dose identifies an error of 4% and above over the whole field. Using the gamma evaluation with the criteria of 3%/2mm of the same images this difference is only seen in the high dose region. The time taken to perform a patient IMRT QA using PD is on average 15mins per patient on the treatment machine, compared to 50mins for our current methods. Conclusions: Portal Dosimetry is suitable for patient IMRT treatment verification with the capabilities of detecting an absolute dose to within 3% of the delivered dose, although lower accuracy is expected at low dose regions and low MUs. Care should be taken when using the calibration technique of a diagonal profile. To maintain consistency with of our current practice a gamma criteria of 3%/2mm can be used with Eclipse evaluation tools with the tolerance that no more than 5% of the field has a gamma >1. However, for modulated fields care should be taken when looking at dose levels of less than 60% of the maximum level in that field. The advantage of using PD for IMRT patient QA is that it saves time needed on the treatment machine. As the accuracy obtainable depends heavily on the level of dose, one improvement would be the capability to add together the segments of the delivered field. 596 poster (Physics Track) THE RESPONSE CURVE OF GAFCHROMIC EBT FILMS FROM THE PERCOLATION THEORY POINT OF VIEW F. Del Moral1 , A. Lopez Medina1 , J. A. Vazquez1 , J. J. Ferrero1 , A. Teijeiro1 , M. Salgado Fernandez1 1 H OSPITAL DO M EIXOEIRO, Medical Physics, Vigo (Pontevedra), Spain
Purpose: Modern radiotherapy uses complex treatments that need more complex quality assurance procedures. As a continuous medium, Gafchromic EBT films offer suitable characteristics for this kind of verifications. However, its sensitometric curve is not fully understood in terms of classical theoretical models. In fact, a significant difference is found between measured optical densities and those predicted by the classical models. This difference increases systematically as wider dose ranges are employed. So, IMRT accuracy requirements are not achievable by classical methods.In that way, experimental parameterizations, such as polynomial fits, are replacing the phenomenological expressions in modern investigations. However, empirical parameterizations neither increase our knowledge about the mechanics of the GafChromic film interaction with radiation nor mean something about their internal structure. In fact, polynomial parameterizations are, strictly speaking, not well defined. They predict infinite optical densities for infinite doses, whereas it is known that optical density saturates with dose. This work focuses on finding a new theoretical way to describe sensitometric curves, as well on checking the quality of the fits for the proposed model. Materials: A flat-bed scanner based method was chosen for the film analysis. Different tests, such as consistency of the numeric results for the proposed model and double examination using data from independent researchers, was performed. Results: Results show that our percolation theory based model provides the best theoretical explanation for the sensitometric behaviour of Gafchromic films. The different sizes of aggregates of aligned monomer crystals are on the basis of this model (figure 1), so it allows to obtain information about the internal structure of the films. Values for the mean size of the active centres were obtained in accordance with technical specifications. In this
mized with the collapsed cone convolution algorithm. Completed plans were copied and recalculated to a calibrated water-equivalent cube phantom. A 180 single beam planar dose was calculated. The phantom treatment plan was programmed into a VarianTM 2100 C/D treatment unit using the Dynamic Arc modality. The treatment was delivered to a cube phantom with a film at isocenter to capture the dose image. Dose was measure via an ionization chamber 1 cm below the isocenter of the cube phantom. Cube phantom calculated and measure doses were compared as well as the film and planar image comparison results. Results: Point-of-Interest (POI) dose comparison results revealed agreement within ± 3% on all trials. Film and Planar images vertical and horizontal profiles agreed within ± 3%. All trials distance to agreement was within ± 3 mm (Table 1). Multiple MLC errors occurred at arc distances of 300 and 360 degrees at dose rates of 500 and 600 MU/min. MLC tolerance limits were set at 0.5 cm instead of the default 0.2 cm. Noteworthy of mention is the vast improvement of optimization calculations (≈ 80 to ≈ 11.5 minutes), when compared to the previous clinical version of software on the Sun Blade 2300 platform. Conclusions: Large arc distance with their small field shapes and higher dose rates result in MLC errors. It is concluded that the errors occur because of the gantry speed at higher dose rates causing MLC tolerance faults. Utilizing dose rates that do not exceed 400 MU/min with the 300 and 360 degree arc distance resulted in favorable results. The new hardware/software platform improved optimization calculation time by almost 700%. 594 poster (Physics Track) TAKING INTO ACCOUNT THE UNCERTAINTY OF THE IONISATION CHAMBER POSITIONING IN IMRT VERIFICATIONS J. Richart Sancho1 , J. Perez-Calatayud1 , S. Rodriguez Villalba1 , M. Santos1 , F. Ballester3 , D. Granero4 , F. Lliso2 , V. Carmona2 1 H OSPITAL C LINICA B ENIDORM, Radiotherapy Department, Benidorm, Spain 2 H OSPITAL U NIVERSITARIO L A F E, Radiotherapy Department, Valencia, Spain 3 U NIVERSITY OF VALENCIA -IFIC, Valencia, Spain 4 H OSPITAL G ENERAL, Radiotherapy Department, Valencia, Spain
Purpose: The existing recommendations state that each IMRT plan should be checked prior to delivery. A widely extended approach is to translate each plan to a phantom and to re-calculate it for this new geometry. The dose distribution inside the phantom is verified in one or several planes by means of film dosimetry and the dose at a point with an appropriate ionisation chamber.The comparison of dose distributions (measured/calculated) is usually performed: dose differences, distance-to-agreement (DTA), and a combination of these two parameters, the gamma evaluation method. When comparing the measured absorbed dose at a point the criterion is only to obtain the dose difference without taking into account the uncertainty in the chamber positioning.The purpose of this work is to present a simple method that accounts for the ionisation chamber displacements and its application to 229 IMRT plans. Materials: Dynamic IMRT treatments are delivered in a Clinac 2100-CD (Varian) linear accelerator. At our Centre IMRT verifications are performed twofold: A) for each beam the absolute fluence is verified with the EPID which software is also used to carry out the analysis, the comparison is evaluated with the gamma criterion being the reference value 3%/3mm, admitting 5%/5mm is small isolated regions at the limits of the beam; B) the dose delivered by the global plan is checked at a point with a pin-point chamber (PTW), being the reference value for the difference, ±3%. When it is possible the chamber is allocated in a low dose gradient region.Deviations in the chamber position have been evaluated as follows:In the calculated plan a set of 6 points are generated around the measuring point in the 3 principal axes, 3mm apart in each direction. The measured dose is compared with the calculated dose at these 7 points; the minimal difference is selected as the result of the comparison. We have compared this method with the conventional difference with respect to the central value in 229 cases. Results: a) Comparison with the central value: the mean value of the differences has been -0.8% (s=1.4%, k=1). In 18 cases (7.9%) the difference exceeded ±3%.b) Comparison with the minimum value: the mean value of the differences has been -0.6% (s=1%, k=1). In 6 cases (2.6%) the difference exceeded ±3%.c) Taking into account the difference between the central value and the minimum, the average has been 0.5% (s=0.5%, k=1). In 25 cases (11%) the difference exceeded ±3%. Applying one of the methods or the other means having discrepancies greater than ±1%. Conclusions: We present a simple method that accounts for the uncertainty in the ionisation chamber positioning, in order to perform the comparison with similar criteria than the dose distribution with films, EPIDs or 2D arrays. When it has been applied to the studied plans it is shown that for a significant fraction this value should be considered for this purpose.
S 217
595 poster (Physics Track) THE EVALUATION OF PORTAL DOSIMETRY FOR IMRT PATIENT QA C. Meehan1 R OYAL M ARSDEN H OSPITAL, PHYSICS, London, United Kingdom
1
Purpose: To investigate the feasibility of using a Varian ASi500 imager, whilst maintaining current levels of accuracy with clinical IMRT protocols. Materials: Tests were performed to evaluate the dose detected on the EPID compared to ion chamber and film measurements for small gap sizes, linearity of acquired image readout with MU, and accuracy in low dose areas of a modulated dmlc field. The whole area of the detector was assessed using two methods of dosimetric calibration. Once the EPID evaluation was completed, the evaluation tools within Eclipse were investigated for patient IMRT fields to ensure that the accuracy of the current clinical IMRT protocols can be maintained. Results: The cassette was able to detect doses to within 3% for dmlc fields of sweeping gap sizes from 1mm to 10mm gaps. Acquired images were linear for values >5MU, and agreed to within 2% of the predicted images for MU>30. For evaluation of the accuracy in a modulated dmlc field, a test pattern was delivered with dose levels of 100%, 30% and 20%. Dose levels of 30% (0.3Gy), PD was within 1.5% of the calculated dose, rising to 4.2% for dose levels of 20% (0.2Gy). When assessing the different calibrations techniques, acquired open fields were compared to predicted images for the whole cassette. The first method, using a diagonal profile, only the centre 15cm2 achieved an accuracy of 2% for doses >0.3Gy. Using the second method of a 2D image the whole cassette can be used to same accuracy of 2% for doses >0.3Gy. Looking at the evaluation tools within Eclipse a dose difference map of a 5% induced dose error for the acquired image against the predicted image of the correct dose identifies an error of 4% and above over the whole field. Using the gamma evaluation with the criteria of 3%/2mm of the same images this difference is only seen in the high dose region. The time taken to perform a patient IMRT QA using PD is on average 15mins per patient on the treatment machine, compared to 50mins for our current methods. Conclusions: Portal Dosimetry is suitable for patient IMRT treatment verification with the capabilities of detecting an absolute dose to within 3% of the delivered dose, although lower accuracy is expected at low dose regions and low MUs. Care should be taken when using the calibration technique of a diagonal profile. To maintain consistency with of our current practice a gamma criteria of 3%/2mm can be used with Eclipse evaluation tools with the tolerance that no more than 5% of the field has a gamma >1. However, for modulated fields care should be taken when looking at dose levels of less than 60% of the maximum level in that field. The advantage of using PD for IMRT patient QA is that it saves time needed on the treatment machine. As the accuracy obtainable depends heavily on the level of dose, one improvement would be the capability to add together the segments of the delivered field. 596 poster (Physics Track) THE RESPONSE CURVE OF GAFCHROMIC EBT FILMS FROM THE PERCOLATION THEORY POINT OF VIEW F. Del Moral1 , A. Lopez Medina1 , J. A. Vazquez1 , J. J. Ferrero1 , A. Teijeiro1 , M. Salgado Fernandez1 1 H OSPITAL DO M EIXOEIRO, Medical Physics, Vigo (Pontevedra), Spain
Purpose: Modern radiotherapy uses complex treatments that need more complex quality assurance procedures. As a continuous medium, Gafchromic EBT films offer suitable characteristics for this kind of verifications. However, its sensitometric curve is not fully understood in terms of classical theoretical models. In fact, a significant difference is found between measured optical densities and those predicted by the classical models. This difference increases systematically as wider dose ranges are employed. So, IMRT accuracy requirements are not achievable by classical methods.In that way, experimental parameterizations, such as polynomial fits, are replacing the phenomenological expressions in modern investigations. However, empirical parameterizations neither increase our knowledge about the mechanics of the GafChromic film interaction with radiation nor mean something about their internal structure. In fact, polynomial parameterizations are, strictly speaking, not well defined. They predict infinite optical densities for infinite doses, whereas it is known that optical density saturates with dose. This work focuses on finding a new theoretical way to describe sensitometric curves, as well on checking the quality of the fits for the proposed model. Materials: A flat-bed scanner based method was chosen for the film analysis. Different tests, such as consistency of the numeric results for the proposed model and double examination using data from independent researchers, was performed. Results: Results show that our percolation theory based model provides the best theoretical explanation for the sensitometric behaviour of Gafchromic films. The different sizes of aggregates of aligned monomer crystals are on the basis of this model (figure 1), so it allows to obtain information about the internal structure of the films. Values for the mean size of the active centres were obtained in accordance with technical specifications. In this