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494 A new Collimator insert system for stereotactic radiotherapy
Posters
$212 >6cm in a single dimension. A protocol for planning and treatment was devised. Localisation is with virtual simulation. The GRID fraction is planned using a single direct field to tumour alone avoiding vital structures e.g. spinal cord. This field receives 15Gy to mid tumour point. A parallel opposed pair is planned concurrently with conventional fields and doses. At least one fraction of the conventional RT must be given the day following the GRID fraction prior to any breaks e.g. a weekend. A GRID was constructed suitable for an Elekta linac with IOMV photons. It consists of a hexagonal matrix of 6.5mm diameter holes, spaced such that 50% of the beam is shielded and 50% passes through the holes drilled in an 8cm thick lead block, accommodating field sizes _<15cm square. The holes projected are 12 mm at the isocentre. Because of the close spacing of the holes, scattered radiation produces a background level under the shielding of 25% of the maximum dose. This has been quantified with measurements carried out using diodes, film, TLDs and MAGIC dosimetry gels. (Trapp) The first patient was treated in Sept 2004 and to date 3 patients have completed treatment out of the required 10 for feasibility study. The GRID has been implemented safely into the radiotherapy department.
Stereotactic treatment 494 A new Collimator Radiotherapy
Insert
System
for
Stereotactic
T. Paschalis i, p. Sand/los 1,2 p. Karaiskos 2, H. Chatzigiannis
1,
E. Koutsouveli 2, K. Dardoufas 1, L. Vlachos 1 iAretaieion University Hospital of Athens, Greece 2Hygeia Hospital, Medical Physics Department, Greece
Athens,
Purpose: The aim of the study is to develop a linac based stereotactic irradiation system for clinical use. All dosimetry parameters were measured to establish a dos/metric quality assurance program. M a t e r i a l and Methods: A prototype collimator insert system was designed and developed in order to simulate stereotactic irradiation. 3 cones made of alloy were constructed and mounted to the wedge-tray of a Siemens MX 6 MV linac to produce circular fields from 1 - 2,5cm in diameter. Collimator concentricity test was performed to ensure that the central axis coincides with the isocentre of the treatment unit. Multi profile measurements were made for each cone, along with PDD calculations and other beam parameters such as TMR,
off-axis ratios and output factors, to implement in a TPS. For in vivo verification of the planned dose distribution TLD-100 rods and Kodak EDR-2 films were used in a humanoid phantom. Results: Collimator concentricity test showed a variation of not more than 0,5mm, which is acceptable for Stereotactic Radiotherapy. All dos/metric parameters examined demonstrate high accuracy in dose distribution for each one of the developed stereotactic cones. Non-coplanar arcs of various angles were performed to indicate that the absorbed dose of organs at risk was in good agreement compared to the dose provided by the TPS. Conclusions: This work illustrates the high reliability and reproducibility of the proposed treatment process, in terms of dose calculation precision, As a result it can be implemented to irradiate intracranial tumors.
495 Accuracy in o p h t h a l m i c imaging, d o s i m e t r y
radiosurgery
? eye f i x a t i o n ,
J. Novotny Jr. I, J. Novotny 1, R. Liscak 2, V. Spevacek3, J. Hrbacek 3, P. Dvorak3, T. Cechak 3, M. Schmitt 2, D. Tlachacova 2, J. Tintera2, J. VymazaP INa Hornolce Hospital, Medical physics, Prague, Czech Republic 2 Na Homolce Hospital, Stereotactic and radiation neurosurgery, Prague, Czech Republic 3CTU in Prague, Faculty of Nuclear Science and Physical Engineering, Dosimetry and application of ionizing radiation, Prague, Czech Republic Purpose of the study: Following issues concerning the accuracy of the ophthalmic radiosurgery were addressed: stability and reliability of eye fixation, accuracy of stereotactic imaging, absolute and relative dosimetry. M e t h o d and Materials: Stability and reliability of eye fixation was checked by two subsequent magnetic resonance imaging (MRI) performed in at least two hours interval, Accuracy of stereotactic imaging was assessed by a special phantom for image distortion evaluation. Since patient is usually treated in prone position comparison between MRI done in prone and supine patient position was done as well. Special water filled head phantom was used for measurements to assess accuracy of absolute and relative dosimetry in eye lesion. Phantom with selected dosimeter was fixed in the Leksell stereotactic frame and underwent imaging, treatment planning and treatment according to normal patient procedures, Altogether six different treatments were simulated and evaluated, Polymer-gel dosimeter evaluated by nuclear magnetic resonance was used to assess the accuracy of relative dosimetry. Small ion chamber was used to assess the accuracy of absolute dosimetry. Results and conclusion: Clinically important inaccuracies were observed neither in eye fixation nor in MRI of the eye. Typical image distortion for selected MRI sequence was not higher than 1,0 mm (mean value 0,5 mm), Deviations observed between two subsequent MRIs done in at least two hours interval as well as for MRIs done in prone and supine patient position were typically within 0,5 mm, A comparison of calculated dose profiles from the treatment planning system and those measured by the polymer-gel dosimeter in all three axes demonstrated a very good agreement. Typical deviations between measured and calculated absolute dose were within 5 % for depths larger than 10 mm. For depths smaller than 10 mm there was observed up to 15-20% deviation compare to the treatment planning system calculations. Further improvement of absolute dose calculation accuracy can be probably reached by employing a proper built up tissue equivalent material applied on eye.
$212 >6cm in a single dimension. A protocol for planning and treatment was devised. Localisation is with virtual simulation. The GRID fraction is planned using a single direct field to tumour alone avoiding vital structures e.g. spinal cord. This field receives 15Gy to mid tumour point. A parallel opposed pair is planned concurrently with conventional fields and doses. At least one fraction of the conventional RT must be given the day following the GRID fraction prior to any breaks e.g. a weekend. A GRID was constructed suitable for an Elekta linac with IOMV photons. It consists of a hexagonal matrix of 6.5mm diameter holes, spaced such that 50% of the beam is shielded and 50% passes through the holes drilled in an 8cm thick lead block, accommodating field sizes _<15cm square. The holes projected are 12 mm at the isocentre. Because of the close spacing of the holes, scattered radiation produces a background level under the shielding of 25% of the maximum dose. This has been quantified with measurements carried out using diodes, film, TLDs and MAGIC dosimetry gels. (Trapp) The first patient was treated in Sept 2004 and to date 3 patients have completed treatment out of the required 10 for feasibility study. The GRID has been implemented safely into the radiotherapy department.
Stereotactic treatment 494 A new Collimator Radiotherapy
Insert
System
for
Stereotactic
T. Paschalis i, p. Sand/los 1,2 p. Karaiskos 2, H. Chatzigiannis
1,
E. Koutsouveli 2, K. Dardoufas 1, L. Vlachos 1 iAretaieion University Hospital of Athens, Greece 2Hygeia Hospital, Medical Physics Department, Greece
Athens,
Purpose: The aim of the study is to develop a linac based stereotactic irradiation system for clinical use. All dosimetry parameters were measured to establish a dos/metric quality assurance program. M a t e r i a l and Methods: A prototype collimator insert system was designed and developed in order to simulate stereotactic irradiation. 3 cones made of alloy were constructed and mounted to the wedge-tray of a Siemens MX 6 MV linac to produce circular fields from 1 - 2,5cm in diameter. Collimator concentricity test was performed to ensure that the central axis coincides with the isocentre of the treatment unit. Multi profile measurements were made for each cone, along with PDD calculations and other beam parameters such as TMR,
off-axis ratios and output factors, to implement in a TPS. For in vivo verification of the planned dose distribution TLD-100 rods and Kodak EDR-2 films were used in a humanoid phantom. Results: Collimator concentricity test showed a variation of not more than 0,5mm, which is acceptable for Stereotactic Radiotherapy. All dos/metric parameters examined demonstrate high accuracy in dose distribution for each one of the developed stereotactic cones. Non-coplanar arcs of various angles were performed to indicate that the absorbed dose of organs at risk was in good agreement compared to the dose provided by the TPS. Conclusions: This work illustrates the high reliability and reproducibility of the proposed treatment process, in terms of dose calculation precision, As a result it can be implemented to irradiate intracranial tumors.
495 Accuracy in o p h t h a l m i c imaging, d o s i m e t r y
radiosurgery
? eye f i x a t i o n ,
J. Novotny Jr. I, J. Novotny 1, R. Liscak 2, V. Spevacek3, J. Hrbacek 3, P. Dvorak3, T. Cechak 3, M. Schmitt 2, D. Tlachacova 2, J. Tintera2, J. VymazaP INa Hornolce Hospital, Medical physics, Prague, Czech Republic 2 Na Homolce Hospital, Stereotactic and radiation neurosurgery, Prague, Czech Republic 3CTU in Prague, Faculty of Nuclear Science and Physical Engineering, Dosimetry and application of ionizing radiation, Prague, Czech Republic Purpose of the study: Following issues concerning the accuracy of the ophthalmic radiosurgery were addressed: stability and reliability of eye fixation, accuracy of stereotactic imaging, absolute and relative dosimetry. M e t h o d and Materials: Stability and reliability of eye fixation was checked by two subsequent magnetic resonance imaging (MRI) performed in at least two hours interval, Accuracy of stereotactic imaging was assessed by a special phantom for image distortion evaluation. Since patient is usually treated in prone position comparison between MRI done in prone and supine patient position was done as well. Special water filled head phantom was used for measurements to assess accuracy of absolute and relative dosimetry in eye lesion. Phantom with selected dosimeter was fixed in the Leksell stereotactic frame and underwent imaging, treatment planning and treatment according to normal patient procedures, Altogether six different treatments were simulated and evaluated, Polymer-gel dosimeter evaluated by nuclear magnetic resonance was used to assess the accuracy of relative dosimetry. Small ion chamber was used to assess the accuracy of absolute dosimetry. Results and conclusion: Clinically important inaccuracies were observed neither in eye fixation nor in MRI of the eye. Typical image distortion for selected MRI sequence was not higher than 1,0 mm (mean value 0,5 mm), Deviations observed between two subsequent MRIs done in at least two hours interval as well as for MRIs done in prone and supine patient position were typically within 0,5 mm, A comparison of calculated dose profiles from the treatment planning system and those measured by the polymer-gel dosimeter in all three axes demonstrated a very good agreement. Typical deviations between measured and calculated absolute dose were within 5 % for depths larger than 10 mm. For depths smaller than 10 mm there was observed up to 15-20% deviation compare to the treatment planning system calculations. Further improvement of absolute dose calculation accuracy can be probably reached by employing a proper built up tissue equivalent material applied on eye.