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1444 poster ROUTINE QUALITY ASSURANCE OF RAPIDARC (ROTATIONAL IMRT THERAPY) TREATMENT PLANS
QA FOR IMRT AND ROTATIONAL THERAPY TECHNIQUES
1444 poster ROUTINE QUALITY ASSURANCE OF RAPIDARC (ROTATIONAL IMRT THERAPY) TREATMENT PLANS A. esmail1 , H. James2 1 S UFFOLK O NCOLOGY C ENTRE T HE I PSWICH H OSPITAL NHS T, Radiotherapy Physics, Ipswich Suffolk, United Kingdom 2 S UFFOLK O NCOLOGY C ENTRE T HE I PSWICH H OSPITAL NHS T, Ipswich Suffolk, United Kingdom Purpose: This study describes our clinical experience of pre-treatment QA of RapidArc treatment plans performed within our centre over the past 12 months. The Varian RapidArc system was commissioned in December 2009. Pre-treatment dose plane verification ensures dosimetric accuracy of plans before delivery to the patient. A commercially available arc therapy QA tool, the Octavius Phantom, along with the Seven29 2D ion chamber array and Verisoft Analysis software (all supplied by PTW) were commissioned and evaluated. Materials: Initially, the QA of RapidArc treatment plans was carried out by performing dose point verification in a perspex block using a calibrated pinpoint chamber positioned at a fixed point. An independent check of the planned Monitor Units (MU) was also performed using Radcalc (Sun Nuclear Systems) with a tolerance set at ±2%. The perspex block method meant only one dose point could be measured at a time. The 2D-Array allows for the evaluation of a possible 729 dose points in a single plane. Two orthogonal planes were acquired in the Octavius Phantom. A model of the treatment couch, provided in the TPS, was included in the verification. Verisoft software was used to compare the isodose data from the TPS with the measured data. A Gamma index algorithm, incorporating dose difference and distance to agreement (DTA), was used for dose evaluation Results: Initial confidence of the RapidArc system was attained with all Perspex block measurements being within 2% of the expected value obtained from the TPS (average absolute deviation of 0.74%±1.09%) and all Radcalc MU checks being within the tolerance of ±2%. For the first 15 RapidArc patients using the Octavius System, the measured dose planes had an average Gamma Index result of 97.5%±2.4%, using a DTA of 3%/2mm with reference to Local dose. Only 4 out of the 30 dose planes had a result less than 95% but all were greater than 90%. Looking at the Gamma maps, areas of failure were in low-dose regions outside the PTV and delivered dose was lower than planned. These plans were deemed clinically acceptable and proceeded to treatment. Verification plan set-up takes 40 minutes per plane, 10 mins is required to set the phantom up on the Linac and perform a dose calibration. Delivery of each arc takes 1-3 mins, and at least 5 mins is required to reorientate the phantom for the next dose plane. Verisoft evaluation of each dose plane takes no more than 2 mins. Therefore at least 100 minutes total QA time is required. Conclusions: The use of the Octavius phantom with the 2D Array and Verisoft software has given us a high level of confidence in the RapidArc planning and delivery systems. Our tolerance of at least 95% of the evaluated dose points passing the Gamma criteria of 3%/2mm DTA, with reference to Local dose, is met in the majority of cases. Dose plane measurements in Octavius is now routine individual pre-treatment patient QA for all our RapidArc plans.
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ball-bearing and the MV field centre. Results: The displacement of the isocentre calculated from pointer position data in the vertical, lateral and longitudinal directions (IEC 61217) was found to be 0.26 ± 0.02 mm, 0.01 ± 0.02 mm, and 0.04 ± 0.02 mm, respectively. The displacement measured using the ball-bearing phantom was found to be 0.32 ± 0.02 mm, 0.00 ± 0.03 mm, and 0.08 ± 0.06 mm. The effect of the X-ray tube and XVI panel positions was found to be less than the uncertainty in the measurements. Conclusions: The displacement found using the two techniques was similar and small. A correction for the displacement becomes reasonable in stereotactic radiotherapy with small field sizes. An automatic correction requires that the XVI system is calibrated with the Apex DMLC attached. Treatment tables are not designed to control movements in the sub-millimetre range. Therefore, a correction is not achievable by table movements. A potential method for correction is to incorporate the displacement in the dose planning system. 1446 poster THE EFFECT OF IONIZATION CHAMBER IN DOSE CALCULATION AND MEASUREMENT H. H. Lee1 2 , K. Son3 2 , S. H. Shin2 , K. Hee-Joung1 , H. Jung3 2 , Y. H. Ji3 2 , M. S. Kim3 2 , K. B. Kim3 2 1 YONSEI U NIVERSITY, Seoul, Korea Republic of 2 KOREA I NSTITUTE OF R ADIOLOGICAL AND M EDICAL S CIENCES, Seoul, Korea Republic of 3 U NIVERSITY OF S CIENCE AND T ECHNOLOGY, Seoul, Korea Republic of Purpose: It is generalized to use a Computed Tomography (CT) image in radiation therapy planning (RTP). In the image, the existence of the ionization chamber can cause an image artifact and can effect on a dose computation. In this study, we evaluate the effect of the ionization chamber in dose calculation and measurement. Materials: Two phantoms were used in this study. The one was universal solid water phantom (RW3, ρ = 1.18 g/cm3 , PTW) with ionization chamber (TM31010, 0.125 cc, PTW). Each side of the phantom was length 30 cm and the total thickness of the phantom was 20 cm. The ionization chamber was located at 10 cm below the anterior surface. Another one was customized water phantom as "house phantom (tunnel-shaped phantom)" with ionization chamber(CC13, IBA, 0.13 cc). The dimension of RW3 was 25 cm x 20 cm x 25 cm. The phantom had a hole on top of it, and we located a bended tube (J-shaped tube) through the hole. We acquired two images of RW3 phantom and house phantom with and/or without ionization chamber, respectively. We used these four images on RT and the planning methods were applied to 2D, 3D, conventional IMRT, and Rapid Arc radiotherapy techniques. These plans were calculated for prescribed dose; 2 Gy, and then these two phantoms were irradiated with a 6 MV x-ray from ClinaciX (Varian, USA) and the delivered dose was measured at the effective point of ionization chamber.
1445 poster THE DISPLACEMENT OF THE ISOCENTRE OF AN ELEKTA SYNERGY ACCELERATOR CAUSED BY THE APEX ADD-ON DYNAMIC MULTILEAF COLLIMATOR S. J. Zimmermann1 , H. L. Riis1 , M. Hjelm-Hansen1 1 R ADIOFYSISK L ABORATORIUM, Odense University Hospital Purpose: Stereotactic radiotherapy demands small field sizes and smoothly shaped contours to cover planning target volumes and limit irradiation of normal tissues. The newly developed Apex add-on dynamic multileaf collimator (DMLC) with leaf width of 2.5 mm at isocentre and maximum field size of 142 cm2 may meet this demand. The Apex DMLC is adding a weight of 50 kg to the radiation head which causes a displacement of the accelerator isocentre. Two methods for measurement of this displacement are proposed. Materials: The measurements were carried out on an Elekta Synergy accelerator with an internal multileaf collimator having leaf width of 1.0 cm at the isocentre. The displacement was measured using a theodolite and a pointer attached to the radiation head. The Apex DMLC was simulated by four weights attached to the radiation head. The centre of mass of the weights was approximately at the same position as that of the Apex DMLC. The measurements were carried out with and without the Apex weight attached in both clockwise (CW) and counter-clockwise (CCW) rotation combined with mutual opposed radiation head angles. Pointer positions were measured in steps of 10 over a full gantry rotation of 360. Also, the radiation isocentre was investigated using a ball-bearing phantom. Images of the ball-bearing phantom were acquired by a 10 0 cm2 field size for two mutual opposed collimator angles at each of the four cardinal gantry angles. The gantry angles were reached by CW rotation only. All eight images were analysed using the Elekta cone beam CT (XVI) software to calculate the average position of the
Results: The calculated dose and prescribed dose at the state of with and/or without ionization chamber were shown in Table 1. The different ratios between the calculated dose and prescribed dose were 0.4% and 0.0%, in the cases of 2D and 3D, respectively. These results are so small that we can ignore the effect of ionization chamber. However, the result for the conventional IMRT method was larger than outcomes of 2D and 3D. The different ratios for the conventional IMRT were 0.9% in RW3 phantom and 6.3% in house phantom, respectively. Lastly, the calculated doses for Rapid Arc were 8.9% in RW3 phantom and 9.7% in house phantom, respectively. Dose differences were bigger than 2D and 3D methods. We evaluated the results of
1444 poster ROUTINE QUALITY ASSURANCE OF RAPIDARC (ROTATIONAL IMRT THERAPY) TREATMENT PLANS A. esmail1 , H. James2 1 S UFFOLK O NCOLOGY C ENTRE T HE I PSWICH H OSPITAL NHS T, Radiotherapy Physics, Ipswich Suffolk, United Kingdom 2 S UFFOLK O NCOLOGY C ENTRE T HE I PSWICH H OSPITAL NHS T, Ipswich Suffolk, United Kingdom Purpose: This study describes our clinical experience of pre-treatment QA of RapidArc treatment plans performed within our centre over the past 12 months. The Varian RapidArc system was commissioned in December 2009. Pre-treatment dose plane verification ensures dosimetric accuracy of plans before delivery to the patient. A commercially available arc therapy QA tool, the Octavius Phantom, along with the Seven29 2D ion chamber array and Verisoft Analysis software (all supplied by PTW) were commissioned and evaluated. Materials: Initially, the QA of RapidArc treatment plans was carried out by performing dose point verification in a perspex block using a calibrated pinpoint chamber positioned at a fixed point. An independent check of the planned Monitor Units (MU) was also performed using Radcalc (Sun Nuclear Systems) with a tolerance set at ±2%. The perspex block method meant only one dose point could be measured at a time. The 2D-Array allows for the evaluation of a possible 729 dose points in a single plane. Two orthogonal planes were acquired in the Octavius Phantom. A model of the treatment couch, provided in the TPS, was included in the verification. Verisoft software was used to compare the isodose data from the TPS with the measured data. A Gamma index algorithm, incorporating dose difference and distance to agreement (DTA), was used for dose evaluation Results: Initial confidence of the RapidArc system was attained with all Perspex block measurements being within 2% of the expected value obtained from the TPS (average absolute deviation of 0.74%±1.09%) and all Radcalc MU checks being within the tolerance of ±2%. For the first 15 RapidArc patients using the Octavius System, the measured dose planes had an average Gamma Index result of 97.5%±2.4%, using a DTA of 3%/2mm with reference to Local dose. Only 4 out of the 30 dose planes had a result less than 95% but all were greater than 90%. Looking at the Gamma maps, areas of failure were in low-dose regions outside the PTV and delivered dose was lower than planned. These plans were deemed clinically acceptable and proceeded to treatment. Verification plan set-up takes 40 minutes per plane, 10 mins is required to set the phantom up on the Linac and perform a dose calibration. Delivery of each arc takes 1-3 mins, and at least 5 mins is required to reorientate the phantom for the next dose plane. Verisoft evaluation of each dose plane takes no more than 2 mins. Therefore at least 100 minutes total QA time is required. Conclusions: The use of the Octavius phantom with the 2D Array and Verisoft software has given us a high level of confidence in the RapidArc planning and delivery systems. Our tolerance of at least 95% of the evaluated dose points passing the Gamma criteria of 3%/2mm DTA, with reference to Local dose, is met in the majority of cases. Dose plane measurements in Octavius is now routine individual pre-treatment patient QA for all our RapidArc plans.
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ball-bearing and the MV field centre. Results: The displacement of the isocentre calculated from pointer position data in the vertical, lateral and longitudinal directions (IEC 61217) was found to be 0.26 ± 0.02 mm, 0.01 ± 0.02 mm, and 0.04 ± 0.02 mm, respectively. The displacement measured using the ball-bearing phantom was found to be 0.32 ± 0.02 mm, 0.00 ± 0.03 mm, and 0.08 ± 0.06 mm. The effect of the X-ray tube and XVI panel positions was found to be less than the uncertainty in the measurements. Conclusions: The displacement found using the two techniques was similar and small. A correction for the displacement becomes reasonable in stereotactic radiotherapy with small field sizes. An automatic correction requires that the XVI system is calibrated with the Apex DMLC attached. Treatment tables are not designed to control movements in the sub-millimetre range. Therefore, a correction is not achievable by table movements. A potential method for correction is to incorporate the displacement in the dose planning system. 1446 poster THE EFFECT OF IONIZATION CHAMBER IN DOSE CALCULATION AND MEASUREMENT H. H. Lee1 2 , K. Son3 2 , S. H. Shin2 , K. Hee-Joung1 , H. Jung3 2 , Y. H. Ji3 2 , M. S. Kim3 2 , K. B. Kim3 2 1 YONSEI U NIVERSITY, Seoul, Korea Republic of 2 KOREA I NSTITUTE OF R ADIOLOGICAL AND M EDICAL S CIENCES, Seoul, Korea Republic of 3 U NIVERSITY OF S CIENCE AND T ECHNOLOGY, Seoul, Korea Republic of Purpose: It is generalized to use a Computed Tomography (CT) image in radiation therapy planning (RTP). In the image, the existence of the ionization chamber can cause an image artifact and can effect on a dose computation. In this study, we evaluate the effect of the ionization chamber in dose calculation and measurement. Materials: Two phantoms were used in this study. The one was universal solid water phantom (RW3, ρ = 1.18 g/cm3 , PTW) with ionization chamber (TM31010, 0.125 cc, PTW). Each side of the phantom was length 30 cm and the total thickness of the phantom was 20 cm. The ionization chamber was located at 10 cm below the anterior surface. Another one was customized water phantom as "house phantom (tunnel-shaped phantom)" with ionization chamber(CC13, IBA, 0.13 cc). The dimension of RW3 was 25 cm x 20 cm x 25 cm. The phantom had a hole on top of it, and we located a bended tube (J-shaped tube) through the hole. We acquired two images of RW3 phantom and house phantom with and/or without ionization chamber, respectively. We used these four images on RT and the planning methods were applied to 2D, 3D, conventional IMRT, and Rapid Arc radiotherapy techniques. These plans were calculated for prescribed dose; 2 Gy, and then these two phantoms were irradiated with a 6 MV x-ray from ClinaciX (Varian, USA) and the delivered dose was measured at the effective point of ionization chamber.
1445 poster THE DISPLACEMENT OF THE ISOCENTRE OF AN ELEKTA SYNERGY ACCELERATOR CAUSED BY THE APEX ADD-ON DYNAMIC MULTILEAF COLLIMATOR S. J. Zimmermann1 , H. L. Riis1 , M. Hjelm-Hansen1 1 R ADIOFYSISK L ABORATORIUM, Odense University Hospital Purpose: Stereotactic radiotherapy demands small field sizes and smoothly shaped contours to cover planning target volumes and limit irradiation of normal tissues. The newly developed Apex add-on dynamic multileaf collimator (DMLC) with leaf width of 2.5 mm at isocentre and maximum field size of 142 cm2 may meet this demand. The Apex DMLC is adding a weight of 50 kg to the radiation head which causes a displacement of the accelerator isocentre. Two methods for measurement of this displacement are proposed. Materials: The measurements were carried out on an Elekta Synergy accelerator with an internal multileaf collimator having leaf width of 1.0 cm at the isocentre. The displacement was measured using a theodolite and a pointer attached to the radiation head. The Apex DMLC was simulated by four weights attached to the radiation head. The centre of mass of the weights was approximately at the same position as that of the Apex DMLC. The measurements were carried out with and without the Apex weight attached in both clockwise (CW) and counter-clockwise (CCW) rotation combined with mutual opposed radiation head angles. Pointer positions were measured in steps of 10 over a full gantry rotation of 360. Also, the radiation isocentre was investigated using a ball-bearing phantom. Images of the ball-bearing phantom were acquired by a 10 0 cm2 field size for two mutual opposed collimator angles at each of the four cardinal gantry angles. The gantry angles were reached by CW rotation only. All eight images were analysed using the Elekta cone beam CT (XVI) software to calculate the average position of the
Results: The calculated dose and prescribed dose at the state of with and/or without ionization chamber were shown in Table 1. The different ratios between the calculated dose and prescribed dose were 0.4% and 0.0%, in the cases of 2D and 3D, respectively. These results are so small that we can ignore the effect of ionization chamber. However, the result for the conventional IMRT method was larger than outcomes of 2D and 3D. The different ratios for the conventional IMRT were 0.9% in RW3 phantom and 6.3% in house phantom, respectively. Lastly, the calculated doses for Rapid Arc were 8.9% in RW3 phantom and 9.7% in house phantom, respectively. Dose differences were bigger than 2D and 3D methods. We evaluated the results of