399 Dosimetric verification with portal imaging

Posters beam spectrum and hence from measurement depth, several correction are used and described. In particular, detector linearity is discussed and separated in two components (primary and transmitted radiation). Additionally a "sliding aperture" detector response is modelled for dynamic fields to properly account for actual field sizes during delivery. All calibration tests will be reported for a Clinac 6EX. Algorithm validation will be reported comparing measurements against calculations and against conventional film dosimetry by means of profile and gamma index evaluation on clinical cases. Results: results will be presented for four different calibration conditions (at 0.8, 1.5, 3.8, 10 cm depth in solid water). A strong correlation between films and PV-aS500 will be proved allowing the possibility to replace highly demanding film based dosimetry with a more integrated and efficient quality assurance system with electronic imaging. Robustness of the algorithm will be proved by showing cases were simpler algorithms would fail and by applying the GLAaS algorithm to the simultaneous verification of split fields. With calibration conditions set at 0.8 or 1.5 cm depth it is possible t o grant the applicability of PV-aS500 dosimetry to any gantry angle, solving one of the most important and common limitations of IMRT verification with conventional tools. Future extension of the GLAaS algorithm will incorporate general machine QA (e.g. wedges or dynamic wedges verification) and preliminary tests will be shown. Conclusion: Pre-treatment verification of intensity modulated treatments has been routinely tested for the PVaS500 portal imager and film dosimetry will be replaced by portal dosimetry. Figure 1: example of pre-treatment QA with films (left) and right with PV-aS500 and GLAaS (right)

397 Combination of different I M R T c o m p o n e n t s - features and problems

B. Doblec F. Lorenz, M. Polednik, H. Wertz, D. Wolff, F. Wenz, F. Lohr Mannheim Medical Center, Radiation Oncology, Mannheim, Germany Aim: For IMRT planning and delivery a reliable chain of Treatment Planning System (TPS), Record and Verify System (R&V), and Linear Accelerator (Linac) is required. Different combinations of different systems lead to different features and problems. The differences regarding quality assurance (QA) are presented here based on the experience gathered in Mannheim using 3 different TPS, 2 types of linacs and 3 MLC types. Material and methods: Based on standard QA methods for IMRT as proposed by the AAPM (2003), the TPS Corvus (Nomos) and KonRad (Siemens) were commissioned for the Linacs KD2 (Siemens) and Synergy (Elekta). The TPS PrecisePlan (Elekta) was commissioned for Synergy only. As R&V system Multi-Acces (Impac/Etekta) was used. In addition, the use of the Multi-Vane-Collimator MIMIC

$175 (Nomos) was investigated in combination with the TPS Corvus and the Linac Siemens KD2. The systems were compared regarding accuracy of dose calculation and robustness against malfunction. Results: The combination of components has a strong influence on the quality of the treatment: In the comparison of calculated to measured dose, KonRad gave good results for the Toshiba MLC incorporated in the Siemens KD2 but showed problems in combination with the Elekta MLC concerning the coverage of the leaf gap. Corvus showed good results for both MLCs, but was less efficient in the creation of MLC-segments for the Elekta MLC, requiring a higher number of monitor units (MU). The R&V system Multi-Access initially encountered communication problems with both the Elekta and the Siemens Linac, which led to incorrect recording of the treatment in several instances. For plans with multiple collimator angles per beam, Multi-Access failed to assign the collimator angle correctly. All problems mentioned here are currently addressed by the manufacturers. Conclusion: For the commissioning of IMRT systems, the whole chain from the TPS to the Linac has to be investigated. Components that passed the commissioning in another clinical environment can compromise treatment delivery when used in a new environment. Therefore not only single components but the whole chain from planning to delivery has to be evaluated in commissioning and checked regularly for QA. 398 Calibration of t h e true leaf positions in Monte Carlo simulations of an MLC

L. Parent I P. Evans I, D. Dance 2, J. Seco I, A. Fielding3 IInstitute of Cancer Research, Joint Department of Physics, Sutton, UK 2Institute of Cancer Research, Joint Department of Physics, London, UK 3School of Physical and Chemical Sciences, Physics Department, Brisbane, Australia I n t r o d u c t i o n : IMRT treatments often consist of the delivery of small segments or include small leaf separations in the segments. An accurate model of the MLC in Monte Carlo simulations is crucial for these situations, as a small variation of the leaf positions will have a large impact on the dose delivered. A method is proposed to calibrate the true leaf positions in the Monte Carlo model. Methods: The standard 40 leaf MLC calibration consists of placing the central leaf pair at two calibration points and interpolating between them. The other leaves are then calibrated by applying an offset correction. This calibration method was reproduced in the Monte Carlo model and tested. Results were compared with a 2, 4 and 6 points calibration method for each leaf. Results: Reproducing the MLC calibration in the Monte Carlo simulations allows predicting the leaf positions within l m m at the isocentre only for the most central leaves (leaf numbers 14 to 26). Differences of up to 5ram at the isocentre are observed for the most outer leaves. By applying the two point calibration method for each leaf, the leaf position is predicted within 1 mm at the isocentre. Using more points during the calibration does not improve the precision of the leaf position prediction. Conclusion: It is recommended to apply a calibration of the true leaf positions in Monte Carlo simulations to accurately model the MLC. Its implementation is made easier by the acquisition of EPID images allowing quick and simple automatic processing. 399 Dosimetric verification with portal imaging

E. W~hlin, B. Sorcini, S. Axelsson Karolinska University Hospital, Department of Hospital Physics, Stockholm, Sweden

Posters

$176 I n t r o d u c t i o n : An Electronic Portal Imaging Device (EPID), together with appropriate software, offers the possibility for absolute dosimetric pre-treatment verification of radiotherapy treatment fields. This is especially useful in IMRT, where the complexity of the fields enhances the need for formal verification. However, to utilize the EPID for absolute dosimetry requires, in addition to proper calibration and configuration, that the dosimetric properties of the detector are known. More specifically, the ability of the EPID to give reproducible results must be known as well as any sources of artifacts, such as memory effects. Our department was chosen as a beta site for a new model of amorphous silicon EPID, the aS1000 T M (Varian Medical Systems). The accelerator on which the aS1000 was mounted was also equipped with a OBI (On Board Imager). This study reports the result from studies on the aS1000, used together with the Portal Dosimetry T M (Varian Medical Systems) software, with respect to mainly the dosimetric properties of the EPID. Materials and methods: The ability of the aS1000 to give reproducible results has been measured over a period of a few months. Measurements have also been made to examine the occurrence of memory effects (ghosting) or saturation effects, gravity effect (constancy with gantry angle), linearity with dose and constancy with dose rate. The investigation into possible saturation effects was performed by changing the dose rate setting of the accelerator, as well as by changing the SDD (Source to Detector Distance). Acquired pre-treatment dosimetric images from IMRT treatment fields were compared with predicted images, calculated with the PDC (Portal Dose Calculation) algorithm in the Eclipse dose planning system. The comparison was made with the gamma evaluation method1. With dosimetric images acquired during treatment, comparison was made with images from different fractions. Results: The dose value of the detector has been shown to be stable over a period of a few months. For a field with 200 MU, all values are within about 2 % from the average, which is consistent with results reported for the similar aS5002. It's linearity with dose has been found to be good. Ghosting effects has been detected, but are low enough to be considered negligible for clinical application. No saturation effects could be detected. A gravity effect of up to about 1 % has been detected. For the comparison between predicted and acquired images, using a distance-to-agreement criterion of 3 mm and a relative dose criterion of 3%, the gamma value is less than unity for 97 % of the image. Conclusions and discussion: The aS1000 EPID has been shown to have suitable characteristics for acquisition of integrated dosimetric images. The PortaIDosimetry system together with the aS1000 has shown promise as a tool for pre-treatment verification of radiotherapy treatment fields. 4OO T w o years of m e a s u r e m e n t s with a 2D system diodes for dose distribution of m o d u l a t e d beams.

of

S. Marci~ I, E. Charpiot 1, R. Bensadoun 2, B. Serrano 3, E. Franchisseur 3 1Centre Antoine-Lacassagne, Unit~ de Physique, Nice, France 2Centre Antoine-Lacassagne, Unit~ de Radioth~rapie, Nice, France 3Universit~ de Nice, LPES-CRESA, Nice, France Purpose: 1 2

Since 2003

a 2D system of diodes for the

Low, D.A., Harms, W.B., Mutic, S., Purdy, J.A. "A technique for the quantitative evaluation of dose distributions", Medical Physics, Vol 25, No 5, pp 656-661, 1998. Van Esch, A., Depuydt, T., Huyskens D.P. "The use of a aSi-based EPID for routine absolute dosimetrie pre-treatment verification of dynamic IMRT fields", Radiotherapy and Oncology, Vol 71, No 2, pp 223-234, 2004.

verification of modulated beams is used. We presented the results of this experience. Materials and methods: Measurements were performed with energy of 6 MV and a 2D system with 445 diodes. The depth of measurements was equivalent to 5 cm of tissue. Calculations were performed on a TPS from Nucletron (Helax TMS). 242 fields were analysed. At each session of measurements, a study of a reference field (10x10 cm 2) is done. Comparisons between calculated and measured values were performed with the software of the 2D system on the basis of discrepancies in dose and in distance in an area of 10x10 cm 2 centred on the axis beam. Results: During the 2 years we have not observed a change in the response of diodes. Discrepancies between calculated and measured doses in points of low gradient areas were within 4 %. The percentage of points of measurements (110 to 240) which have discrepancies with calculated dose within 4 % in dose and 4 mm in distance was 95 %. Higher modulated is the beam, lower is the number of points satisfying criteria. Profiles are also shown to localize points in a high or low dose gradient. Conclusion: This system is easy to use and reliable. The absolute dose measurement, the profiles and the number of points with discrepancies in dose and in distance are tools to quickly validate the dose distribution. 401 I M R T plan verification using a 2D ion c h a m b e r a r r a y

E. Spezi, A.L. Angelini, F. Romani, A. Ferri Policlinico S. Orsola-Malpighi, Servizio di Fisica Sanitaria, Bologna, Italy The clinical implementation of IMRT demands a careful verification of planned treatments. The level of complexity reached by current clinical treatments require an accurate, reliable and above all practical verification system. Films are widely used for IMRT testing and QA. They have been also used for the verification of IMRT beams 1. However their use a precise absolute and/or relative dosimeter require significant efforts. Furthermore the time required to develop and process the film may be very time consuming and therefore not feasible for routine IMRT verification. A number of planar detectors have been recently developed for the dosimetric verification of IMRT beams and are being introduced in the clinical practice 2,3,4 . In this work we investigate the performance of a 2D ion chamber array. The device is a PTW 2D Array Seven29T M consisting on a matrix of 729 ionization chambers of (0.5×0.5×0.5) cm 3 size and equally spaced of 1 cm center-tocenter. Following an accurate device characterization, involving repeatability, output factor and linearity tests the 2D Array was used for the pre-treatment verification of a number of IMRT research studies. IMRT beam verification was carried out by coping to phantom the optimized treatment plans, as shown in Figure la. All the treatment parameters remained unchanged, except the gantry angle, which was set to 0 °. The beam sequence was then delivered on a Siemens Oncor Impression IMRT Plus equipped with a OPTIFOCUST M MLC and operating at 6 MV. The planar dose distribution was recorded and analysed as depicted in Figure lb and c. In this work we present the results of the comparison between 2D Array measurements, TPS calculations and independent experimental datasets. Detector resolution and efficiency are also investigated and discussed.