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90 oral First clinical results for DMLC and IMRT verification with a 2D iixel ionisation chamber
Proffered papers
Introduction: For treatments with dynamic IMRT, the mechanical setting of the MLC is crucial. It affects both the dose distributions and the optimization of the main parameters characterizing the MLC: the transmission (T) and the Dosimetric Leaf Separation (DLS). This last parameter is a correction factor introduced by Varian to take into account the rounded leaf end transmission. The aim of this study is to present the effects of the MLC adjustment on measured dose. The influence on optimized T and DLS values, to have the best agreement between calculations and measurements, is also studied. Material and Methods: Tests are performed with a Varian 23EX linac operated at 20MV and equipped with a 120 leaves millenium dynamic collimator. The treatment planning system is CadPlan / Helios (v6.3.6). The study consists in comparing measurements obta!ned before and after an MLC maintenance. Dose distributions and absolute dose measurements achieved for patients treated with IMRT are compared before and after MLC adjustment. The influence of the MLC maintenance on tests based on ion chamber measurements performed in phantom or in a block fixed to the collimator for a dynamic sliding slit, has also been investigated. Such checks have been previously developed to assess the Iinac stability in dynamic mode. They may also be used to derive the optimized T and DLS values. Results: The 'sliding slit' tests demonstrated that a 9% difference exists on measured absolute dose before and after the MLC calibration. Consequently, the T and DLS values selected before the new MLC adjustment were not any longer optimal after the setting. Although a large difference on dose measurements is observed for these tests, no such difference exists for patient dose distributions (around 1 to 2%). In that case, for the same T and DLS values the agreement between calculations and measurements is better after the MLC adjustment. Conclusion: The study shows that a small variation of MLC calibration (0.8 mm in our case) generates a large difference for the 'sliding slit' test, whereas for the patient dose distribution the consequences are smaller but still significant. This test allows to detect small variations in leaf adjustment and to ensure consistency between calculation and actual dose delivered to patients. 89 oral H i g h photon beam attenuation by the treatment couch and (commercial) i m m o b i l i s a t i o n d e v i c e s in I M R T f i e l d s w i t h " u n c o n v e n t i o n a l " a n g l e s o f incidence
S. Vieira, R.S.J.P. Kaatee, M.L.P. Dirkx, B.J.M. Heijmen ErasmusMC- Daniel den Hoed Cancer Center, Physics, Rotterdam, The Netherlands Introduction: In our institution, a comprehensive individualised dosimetrical quality assurance protocol for intensity modulated radiotherapy (IMRT) is being implemented. This protocol includes weekly dosimetrical measurements with our fluoroscopic electronic portal imaging device (EPID) for all IMRT fields, while the patient is being irradiated. For some of the first patients enrolled in this protocol, significant beam attenuation by the treatment couch and immobilisation devices was observed, especially for gantry angles that were uncommon for non-IMRT treatments. These findings led to a thorough investigation to quantify this attenuation and to define measures to avoid resulting underdosages in the patient plan. Materials and methods: Our treatment unit (Clinac 2300C/D) is equipped with an Exact treatment couch with two translatable rails and a couch insert, both made from carbon fibre. For immobilisation of head and neck cancer patients, a carbon fibre baseplate and a carbon fibre head support (Sinmed BV) are used. For precise assessment of the beam attenuation, EPID images were acquired for all clinical 6 MV IMRT beams, in absence of the patient, both with the immobilisation devices and treatment couch as positioned during treatment, and without any attenuating devices in the beam. Results: For the clinical setups (however without the patient in the beam) beam attenuation by the treatment couch of up to 15 % were observed. For the head and neck immobilization device, the EPID images revealed beam attenuation of about 2.5%, 5% and 15% by the baseplate, the head support, and the pin used to attach the mask, respectively. To avoid underdosages in the patient, new tools and procedures for planning and treatment delivery have been developed. Conclusions: The EPID was successfully used to detect and assess significant beam attenuation (up to 15%) by the different components of the treatment couch and immobilisation devices for clinical beam configurations.
Tuesday, 16 September 2003 $37
90 oral F i r s t c l i n i c a l r e s u l t s f o r D M L C a n d I M R T v e r i f i c a t i o n w i t h a 2D
iixel ionisation chamber R. Cirio 1, M. Donetti 1,2, S. Giordanengo 1, E. Madon 3, U. Nastasi 4, D. Santuari3, A. Sardo 3, G. Scielzo 5, M. Stasi 5, E. Trevisiol4 l lstituto Nazionate di Fisica Nucleate, Torino, Italy 2TERA Foundation, Novara, Italy 301RM S.Anna, Todno, Italy 4S.Giovanni A.S., Torino, Italy 51RCC, Candiolo (TO), Italy Introduction: One of the major problems for the widespread use of intensity modulated techniques of radiotherapy is the clumsy and time-consuming verification phase. One would like to have a simple device that allows dose (and eventually dose rate) measurements in a short time and with a reasonable granularity. Method: In the last years we have designed and built a detector based on the ionisation chamber architecture. It is made of several cylindrical ionisation chambers placed in a PMMA phantom. The sensitive part (named Pixet Ionisation Chamber) is a parallel plate ionisation chamber with one of the two electrodes segmented into pixeis. The sensitive area is 24x24cm ~ and the total number of independently read channels is 1024. The readout is performed with custom-designed Very Large Scale Integration electronics chips, each with 64 recycling integrator channels, dead-time free read-out, 3 microA maximum input current and charge resolution variable between 100fC and 800fC. The total time needed to read-out the 1024 channels can be as fast as 100ms, and the read-out operations are performed without introducing any dead-time in the measurement; this allows dose-rate measurements. Results and Discussion: The detector has been used with the 6 MV photon beams of two hospitals. One accelerator uses the 3D-Line Dynamic MultiLeaf Collimator with 48 4.71 mm leaves, the other hospital uses the Varian Millenium 120 5 mm leaves MultiLeaf Collimator. The Pixel Ionisation Chamber, used in a solid water phantom, was used to get the 2D dose distributions. The results were compared with commercial verification systems. Results for head&neck, prostate and breast cancer pathologies treated with dynamic IMRT and DMLC arc therapy showed good agreement between the Pixel Ionisation Chamber and the other systems. In particular, dose profiles were found to be equal within 2% in the flat regions and 5% in the high dose gradient regions. The isodose curve difference was always better than 3 mm. However these measurements suggested that careful optimisation in the geometrical parameters of the Pixel lonisation Chamber are needed to fully exploit its capabilities. 91 oral N o r m a l i z e d s e n s i t o m e t r i c c u r v e s for the v e r i f i c a t i o n o f h y b r i d I M R T t r e a t m e n t p l a n s w i t h m u l t i p l e energies
D. Georo. B. Kroupa, P. Winkler, R. POtter AKH Vienna, Dept, Radiotherapy and Radiobiology, Vienna, Austria Introduction: With the clinical implementation of time-variable dose patterns and IMRT film dosimetry has regained popularity, especially for patient specific quality assurance (QA). A common QA procedure is to verify so-called hybrid IMRT plans, i.e. the patient specific treatment plan with unmodified fluence patterns recalculated in a phantom. For such applications the sensitometric curve should not depend on beam energy, field size, depth, or film orientation. Materials and Methods: The influence of beam energy (6,10,25MV), field size (5x5-20x20cm2), depth (5-20cm), and film orientation on sensitometric curves is investigated for Kodak X-Omat V and EDR-2 films. After introducing a normalization of sensitometric curves the dependences of sensitometric are rechecked. Results: When normalizing sensitometric curves to the dose value necessary to obtain a net OD=I for that respective geometry and energy the large energy dependence of sensitometric curves can be almost eliminated. By doing so, one unique sensitometric curve can be used for the three beam qualities investigated. This holds for both Kodak X-Omat V films and EDR2 films. For doses smaller than 0.8Gy for X-Omat V and doses smaller than 3Gy for EDR-2 films the field size variation of normalized sensitometric curves is smaller than 3% for fields up to 20x20cm 2. For X-Omat V films all differences between sensitometric curves determined at depths of 5, 10, and 15cm are smaller than 3%. For EDR-2 films deviations larger than 3% are only observed at net OD<0.25. The dependence of film orientation (parallel versus perpendicular) on normalized sensitometric curve is found to be not critical. However, processing conditions have the largest influence and can result in differences up to 20 % for sensitometric curves derived from
Introduction: For treatments with dynamic IMRT, the mechanical setting of the MLC is crucial. It affects both the dose distributions and the optimization of the main parameters characterizing the MLC: the transmission (T) and the Dosimetric Leaf Separation (DLS). This last parameter is a correction factor introduced by Varian to take into account the rounded leaf end transmission. The aim of this study is to present the effects of the MLC adjustment on measured dose. The influence on optimized T and DLS values, to have the best agreement between calculations and measurements, is also studied. Material and Methods: Tests are performed with a Varian 23EX linac operated at 20MV and equipped with a 120 leaves millenium dynamic collimator. The treatment planning system is CadPlan / Helios (v6.3.6). The study consists in comparing measurements obta!ned before and after an MLC maintenance. Dose distributions and absolute dose measurements achieved for patients treated with IMRT are compared before and after MLC adjustment. The influence of the MLC maintenance on tests based on ion chamber measurements performed in phantom or in a block fixed to the collimator for a dynamic sliding slit, has also been investigated. Such checks have been previously developed to assess the Iinac stability in dynamic mode. They may also be used to derive the optimized T and DLS values. Results: The 'sliding slit' tests demonstrated that a 9% difference exists on measured absolute dose before and after the MLC calibration. Consequently, the T and DLS values selected before the new MLC adjustment were not any longer optimal after the setting. Although a large difference on dose measurements is observed for these tests, no such difference exists for patient dose distributions (around 1 to 2%). In that case, for the same T and DLS values the agreement between calculations and measurements is better after the MLC adjustment. Conclusion: The study shows that a small variation of MLC calibration (0.8 mm in our case) generates a large difference for the 'sliding slit' test, whereas for the patient dose distribution the consequences are smaller but still significant. This test allows to detect small variations in leaf adjustment and to ensure consistency between calculation and actual dose delivered to patients. 89 oral H i g h photon beam attenuation by the treatment couch and (commercial) i m m o b i l i s a t i o n d e v i c e s in I M R T f i e l d s w i t h " u n c o n v e n t i o n a l " a n g l e s o f incidence
S. Vieira, R.S.J.P. Kaatee, M.L.P. Dirkx, B.J.M. Heijmen ErasmusMC- Daniel den Hoed Cancer Center, Physics, Rotterdam, The Netherlands Introduction: In our institution, a comprehensive individualised dosimetrical quality assurance protocol for intensity modulated radiotherapy (IMRT) is being implemented. This protocol includes weekly dosimetrical measurements with our fluoroscopic electronic portal imaging device (EPID) for all IMRT fields, while the patient is being irradiated. For some of the first patients enrolled in this protocol, significant beam attenuation by the treatment couch and immobilisation devices was observed, especially for gantry angles that were uncommon for non-IMRT treatments. These findings led to a thorough investigation to quantify this attenuation and to define measures to avoid resulting underdosages in the patient plan. Materials and methods: Our treatment unit (Clinac 2300C/D) is equipped with an Exact treatment couch with two translatable rails and a couch insert, both made from carbon fibre. For immobilisation of head and neck cancer patients, a carbon fibre baseplate and a carbon fibre head support (Sinmed BV) are used. For precise assessment of the beam attenuation, EPID images were acquired for all clinical 6 MV IMRT beams, in absence of the patient, both with the immobilisation devices and treatment couch as positioned during treatment, and without any attenuating devices in the beam. Results: For the clinical setups (however without the patient in the beam) beam attenuation by the treatment couch of up to 15 % were observed. For the head and neck immobilization device, the EPID images revealed beam attenuation of about 2.5%, 5% and 15% by the baseplate, the head support, and the pin used to attach the mask, respectively. To avoid underdosages in the patient, new tools and procedures for planning and treatment delivery have been developed. Conclusions: The EPID was successfully used to detect and assess significant beam attenuation (up to 15%) by the different components of the treatment couch and immobilisation devices for clinical beam configurations.
Tuesday, 16 September 2003 $37
90 oral F i r s t c l i n i c a l r e s u l t s f o r D M L C a n d I M R T v e r i f i c a t i o n w i t h a 2D
iixel ionisation chamber R. Cirio 1, M. Donetti 1,2, S. Giordanengo 1, E. Madon 3, U. Nastasi 4, D. Santuari3, A. Sardo 3, G. Scielzo 5, M. Stasi 5, E. Trevisiol4 l lstituto Nazionate di Fisica Nucleate, Torino, Italy 2TERA Foundation, Novara, Italy 301RM S.Anna, Todno, Italy 4S.Giovanni A.S., Torino, Italy 51RCC, Candiolo (TO), Italy Introduction: One of the major problems for the widespread use of intensity modulated techniques of radiotherapy is the clumsy and time-consuming verification phase. One would like to have a simple device that allows dose (and eventually dose rate) measurements in a short time and with a reasonable granularity. Method: In the last years we have designed and built a detector based on the ionisation chamber architecture. It is made of several cylindrical ionisation chambers placed in a PMMA phantom. The sensitive part (named Pixet Ionisation Chamber) is a parallel plate ionisation chamber with one of the two electrodes segmented into pixeis. The sensitive area is 24x24cm ~ and the total number of independently read channels is 1024. The readout is performed with custom-designed Very Large Scale Integration electronics chips, each with 64 recycling integrator channels, dead-time free read-out, 3 microA maximum input current and charge resolution variable between 100fC and 800fC. The total time needed to read-out the 1024 channels can be as fast as 100ms, and the read-out operations are performed without introducing any dead-time in the measurement; this allows dose-rate measurements. Results and Discussion: The detector has been used with the 6 MV photon beams of two hospitals. One accelerator uses the 3D-Line Dynamic MultiLeaf Collimator with 48 4.71 mm leaves, the other hospital uses the Varian Millenium 120 5 mm leaves MultiLeaf Collimator. The Pixel Ionisation Chamber, used in a solid water phantom, was used to get the 2D dose distributions. The results were compared with commercial verification systems. Results for head&neck, prostate and breast cancer pathologies treated with dynamic IMRT and DMLC arc therapy showed good agreement between the Pixel Ionisation Chamber and the other systems. In particular, dose profiles were found to be equal within 2% in the flat regions and 5% in the high dose gradient regions. The isodose curve difference was always better than 3 mm. However these measurements suggested that careful optimisation in the geometrical parameters of the Pixel lonisation Chamber are needed to fully exploit its capabilities. 91 oral N o r m a l i z e d s e n s i t o m e t r i c c u r v e s for the v e r i f i c a t i o n o f h y b r i d I M R T t r e a t m e n t p l a n s w i t h m u l t i p l e energies
D. Georo. B. Kroupa, P. Winkler, R. POtter AKH Vienna, Dept, Radiotherapy and Radiobiology, Vienna, Austria Introduction: With the clinical implementation of time-variable dose patterns and IMRT film dosimetry has regained popularity, especially for patient specific quality assurance (QA). A common QA procedure is to verify so-called hybrid IMRT plans, i.e. the patient specific treatment plan with unmodified fluence patterns recalculated in a phantom. For such applications the sensitometric curve should not depend on beam energy, field size, depth, or film orientation. Materials and Methods: The influence of beam energy (6,10,25MV), field size (5x5-20x20cm2), depth (5-20cm), and film orientation on sensitometric curves is investigated for Kodak X-Omat V and EDR-2 films. After introducing a normalization of sensitometric curves the dependences of sensitometric are rechecked. Results: When normalizing sensitometric curves to the dose value necessary to obtain a net OD=I for that respective geometry and energy the large energy dependence of sensitometric curves can be almost eliminated. By doing so, one unique sensitometric curve can be used for the three beam qualities investigated. This holds for both Kodak X-Omat V films and EDR2 films. For doses smaller than 0.8Gy for X-Omat V and doses smaller than 3Gy for EDR-2 films the field size variation of normalized sensitometric curves is smaller than 3% for fields up to 20x20cm 2. For X-Omat V films all differences between sensitometric curves determined at depths of 5, 10, and 15cm are smaller than 3%. For EDR-2 films deviations larger than 3% are only observed at net OD<0.25. The dependence of film orientation (parallel versus perpendicular) on normalized sensitometric curve is found to be not critical. However, processing conditions have the largest influence and can result in differences up to 20 % for sensitometric curves derived from