1086 Poster Portal image verification of a software option for improving MLC resolution on a siemens primus linac

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Results: Portal films and simulator radiographs of 14 patients who were irradiated in the pelvic area were evaluated with a standard four-field-box technique. During this investigation 178 films were scanned and analysed according to the procedure described above. Simulator radiographs of the latera!, fields could not be evaluated due to their poor image quality. The standard deviation of the verification films with 0 ° gantry position was calculated to 3.6 mm in lateral and 3.2 mm in cranio-caudal direction. For the lateral (90 °) gantry position we obtained 4.3 mm (ventro-dorsal) and 3.4 mm cranio-caudal. Standard deviations of the X-radiographs amounted to 2.9 mm (lateral shift), 3.3 mm (cranio-caudal shift) and 1.1 ° for rotations around the coronar axis. Furthermore the analysis of the histograms of the Euclidiz~n distance from the translations resulted in a maximum between 3.5 and 4.5 mm. The mean deviation of positioning between simulator and therapy unit is corresponding to the systematic error, which was assessed to be -1.3 mm lateral, 1.6 mm cranio-caudal for ventral geometries, Conclusion: An image based correlation technique was developed to determine the set-up errors of patients in radiotherapy. The method is readily applicable and allows the evaluation of standard deviations of pelvic patients. In particular it enables the comparison of the patient set-ups between a simulator and the linear accelerator. 1083 Poster Off-line set-up corrections during 3-dimensional conformal r a d i o t h e r a p y in patients with pelvic tumours

A. Vila, C. Lainez, E. Femdndez-Vefilla, A. Pedro, F. Sanchiz Clfnica Plat6, Radiotherapy, Barcelona, Spain Purpose: Treatment setup in pelvic tumors has proven to be relatively inaccurate. To improve the accuracy and reduce the margins of target volumes from CTV to PTV, off-line set-up corrections were investigated, Methods and Materials: anterior-posterior and lateral 6MV X-ray portal film images of 114 patients (Rectum 36%, Prostate 34%, Gynecologic 18%, bladder 12%) were acquired. The images were analysed off-line on the verification day and every week during irradiation fractions. The set-up deviation was stablished by matching center landmark and MLC with DRR. If any deviation > 5 mm was found, the position was corrected for the next session. If more ~han 8 mm were found, the correction was carried out immediatly to obtain the final set-up accuracy < 3 mm before the next fraction. Results: The mean and standard deviation (SD) of the distribution before corrections the verification day was 0.2 _+3.8 mm on left-right direction, -0.6 + 5.4 mm craneo-caudal, and &2 ± 3.3 mm antero-posterior, The mean set-up error found was 0 in prostate (SD < 3.3 mm) before corrections and 82 patients (72%) were correctly positioned (accurate coeft, near to 1). Isocentric set-up procedure was found accurate (coeff: 0.85) in relation to the isometric method (coeft: 0.61). The mean random deviation after corrections was < 1.12 mm The matching between Linac 6 MV X-ray port films and DRR procedure was sufficiently accurate and added an average of 5 rain to the treatment time and 7 min to oft-line comparison. The craneo-caudal set-up corrected deviation was found statistically significant in supine rectum patient under radical surgery (14) and in a group of 40 obese, claustrophobic, muscular deficiencies and protesics (p< 0.001, Int 3.5-11.5 mm). Also, the antero-posterior set-up corrected deviation was statistically significant for gynecological (Int 0.5-1.3 mm), prostate (Int: 1.6-3.8 mm) and in the group of these 40 patients previously mentioned (p< 0.001). The application of oft-line corrections justifies a CTV-to-PTV margin reduction to a mean of 5 mm in craneo-caudal dimension and 2.5 mm A-P in radical irradiation patients, Conclusions: 1) Off-line set-up corrections significantly improve the positioning accuracy in the mispositionated patients, 2) Corrections increase treatment time but might be used effectively in the majority of centers as a routine test and starting point to assure quality control in radiation fractionated therapy, 1084 Poster A software tool for verification of couch shifts prescribed by

decision rules in portal imaging P.W. Koken 1, 2, E. Van Beckhoven 2, D. Vink 1 1VU medica/ center, Radiation Onco/ogy, Amsterdam, The Netherlands 2VU medica/center, C/inica/Physics and/nformatics, Amsterdam, The Netheflands Purpose: Off-line decision protocols are available to reduce the systematic deviation between actual and planned patient set-up, which result in a couch~ shift to be performed ,for the remaining treatment fractions. We ,describe a software tool developed in order to perform the couch shift correctly.

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Technique: An EPID and commercially available software are used to acquire and to process the images obtained from the actual treatment fraction. Images are matched oft-line with a reference image, which results in mismatch values that are stored in a central database. We developed 2 dedicated software tools: one to calculate the couch shift and another to verify that the shift is done correctly. Both tools are based on commercially available products. After the matching procedure the technologist enters the mismatch values manually into the developed software. After 3 treatment fractions a couch shift, that needs to be performed with respect to the original patient set-up origin for all following fractions, is calculated. The software has been designed such that another technologist must enter the same data independently, if the data are equal a couch shift is calculated. Otherwise the data need to be entered again or overruled. The second software tool is used simultaneously in the console room and in the treatment room to meet a correct couch shift. The procedure is as follows: in the treatment room the desired patient is selected. Subsequently, the calculated shift is read from the database. Once the patient has been set up, the actual couch parameters are entered manually in the treatment room into the software tool. Then the desired couch positions are derived. Verification of the entered couch positions is done by the technologist in the console room on a monitor, which shows a copy of the information on the monitor in the treatment room. If correct, the technologist in the treatment room positions the couch according to the desired positions. The technologist in the console room verifies whether or not the actual and desired couch positions agree. If so, the patient is treated accordingly. If not, the couch is shifted again until the desired positions are obtained. Conclusion: We developed a software tool to assist in the verification of couch shifts obtained from a decision portal imaging protocol. It can be implemented relatively easy, does not need electric couch position signals and therefore is couch-independent. 1085 Poster E v a l u a t i o n o f set up errors during irradiation by use o f p o r t a l

verification image G. Wozniak 1, W. Sasiadek2, E. Fidarova 1, L. Miszczyk 1, A. Wygoda 1 1Institute of Oncology, Department of Radiotherapy, Gliwice, Poland 2Institute of OncoIogy, I Clinic of Radiotherapy, Gliwice, Poland Purpose of study: An estimation of radiotherapy quality according to the results of mesurements of geometric set up errors, using Portal Vision Images and an evaluation of the impact of CTV location and izocentric point(in head and neck cancer), field size and in vivo dosimetry. Material and method : Geometric set up errors for 225 fields of 160 patiens were analyzed. The mesurements of shifts on X and Y axis on portal verifications images in relation to simulation images were performed by use of Varian's Soma Vision application. All cases were divided into 8 groups according to locations of CTV. Additionally head and neck cancer patiens were divided into two subgroups: one has izocentric point above angle of mandible and second one below it. 124 measurements of in vivo dosimetry were performed. Correlation between parameters was performed using Spearman analysis and logistic regression. Results: The mean X shift for all location was 2,76 mm (SD - 2,64), mean Y shift was 2,47 mm (SD - 3~88) The mean value of difference between planed and measured dose was 0,19% (SD - 2,23). The Spearman anaiysis shows statistically significant correlation (p=0.02), between location and Y shift. There were no correlations between X and Y shifts and DIV results, location of izocentric point and field size. Logistic regression proved that value of Y shift, strongly depends on irradiation field location. For abdomen, pelvis and prostate fields, shifts on Y axis are more significant than in head and neck fields. Results of the analysis sugests that immobilization of the lower parts of the body such as abdomen and pelvis is not as recordable as in head and neck regions especially in vertical direction. Conclusions: The obtained results allow to form the conclusion that Y shifts strongly depend on irradiation fields location and that Portal Vision Image system is very useful and important method of geografic errors assesment during irradiation and it let us improve the radiotherapy quality. 1086 Poster Portal image verification o f a software option for improving

MLC resolution on a siemens primus linac B. Gibson 1, N. Burnet2 1Addenbrooke's NHS Trust, Medical Physicsi, Cambridge, United Kingdom 2University of Cambridge, Oncology, Cambridge, United Kingdom HD270 is a software option on Siemens Primus Linear Accelerators that

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controls the table and MLC's in such a way that the resolution of the 1 cm leaves can be improved to a virtual resolution of 2, 3, or 5 mm. Each treatment field is divided into segments and the table and MLC's are adjusted automatically between segments, so the field verification is nontrivial. A phantom study was carried out for different treatment machine setups using radioopawue markers which were subsequently registered using pipspro software. The results of the registration were compared to the expected table movements and, using the results of the registration, the various segments can be shifted such that the summed image can be viewed. The phantom study confirmed the operation of the HD270 option to within experimental uncertainty. The registration and summation of the segment's image now allows verification of the field placement. This process will now be applied to a patient study, 1087 Poster E f f e c t i v i t y of electronic portal imaging technique in the quality control of radiation treatment

M.N. Andrieu 1, C. Kurtman 1, A.Y. (~ztQrk1, C. Akfirat 1, A. HigsSnmez1, B. Dirican2 1Ankara University Medical Faculty, Radiation Oncology, Ankara, Turkey 2GQlhane Military Medical School, Radiation Oncology, Ankara, Turkey Purpose: It is aimed to use electronic portal imaging (EPI) to measure daily on-line set up errors in patients treated with lineer accelerator to determine an optimum planning target volume (PTV) margin according to set up error and organ motion. Material and method: A total of 43 electronic portal images of 5 patients (1 pelvic, 1 total cranium, 1 mantle, and 2 tangential fields for breast) collected during the course of study. Before the treatment procedure, immobilization devices; thermoplastic mask for the cranium; a-cradle for pelvic and mantle field; and inclined plane with unilateral arm pole for the breast were used for the stabilisation of patients.The electronic portal images were taken under photon exposure of Varian Clinac 2300 C/D. The first portal images after a correction of set-up errors according to the simulation films were accepted as the reference for the subsequent sessions. By comparing each portal image with the reference EPI, the deviations in lateral (x) and oranio-caudal (y) axis, the mean of displacement and standart deviations were calculated. Additionally, standart deviations after corrections were also calculated, Results: The standart deviations in x and y axis according to matching reference EPI with subsequent EPIs were 15.0mm and 9.0mm in pelvis; 6.0ram and 6.8mm in brain; 9.6mm and 11.8 mm in mantle field and 10.3 mm and 13.5 mm in tangential fields. The set up accuracy after correction was improved, the derived standart deviations were 9.3 mm and 5.2 ram; 2.6 mm and 5.3 mm; 6.4mm and 10.5 mm; 8.3 mm and 10.2 mm respectively. The ratio of the treatments with % 95 volume coverage taken by 5 mm margins of PTV were %25 and %50 for pelvis, %100 and %100 for brain, %57 and %57 for mantle, %29 and %29 for breast in x and y axis, respectively, Conclusion: Considering the standart deviations according to both set up errors and target organ motions, optimum PTV margin to ensure % 95 coyerage must be at least 10 mm for pelvic,mantle and breast treatments, 5 mm of margin is sufficient for cranial treatments. The verification of treatment planning by using EPI allows radiation encolegists to obtain an improved quality control in a faster and easier way. 1088

Poster

An automated setup control management system C. Baum. D. Buck, M. Birkner, M. A/bet, F. Ngss/in Universit~tsk/inik fgr Radioonko/ogie, Medizinische Physik, T~Jbingen,Getmany Geometrical accuracy of fractionated external beam radiotherapy is mainly limited by internal organ motion and patient setup inaccuracies. While control of internal organ motion is still infeasible in todays clinical practice, patient setup control methods are routinely applicable. However, manual acquisition, administration and evaluation of setup data as well as the application of on-line and off-line setup correction protocols often lead to an increased workload for physicians and radiotherapists. We present a software which automatically administers and registers a patient's portal images, determines the daily setup errors, and applies user-defined off-line or on-line setup correction protocols, The daily portal images are acquired by the EPID system software and automatically transfered into the system's database. The software then applies an in-house developed matching software module - based on a wavelet filtering method - to automatically detect bony structures in the portal image and to determine the setup error with respect to the planning

Posters

DRRs in real-time. Having determined the patient setup error, the software applies the userdefined setup correction protocols. Firstly, if the actual setup error exceeds a user-defined limit, the system sends a warning and recommends an online setup correction. Secondly, the software uses the samples of daily setup errors from the previous course of the treatment to estimate systematic and random setup components. User-defined confidence regions and action levels for the estimated parameters (average position, standard deviation, median, etc.) determine the off-line setup protocol: neccessary offline corrections of systematic patient displacements are automatically indicated. Also, necceseary setup margin extensions can be indicated as well as possible setup margin reductions. The system offers a great flexibility in controlling the maximum number of correction steps and portal images, as well as the statistical stability of the setup error estimation. Furthermore, due to the minimal dependence on user input, the automatic setup controller minimizes the extra workload for physicians and radiotherapists. 1089

Poster

Superimposition of images in electronic media for treatment verification

N. Tungel1, A.U. Kizildag 1, A. Toy1, N. Una/2, M. Altun 1 1AkdenizUniversity School of Medicine, Radiation Oncology, Antalya, Turkey 2Akdeniz University Faculty of Science, Physics, Antalya, Turkey Aim and Introduction: Comparing Portal Films (PF) and electronic portal images (EPIs) with simulation films using naked eye may cause some errors due to subjectivity. Method of superimposition of images gives the opportunity of objective comparison eliminating subjective errors. This study aimed the superimposition of PFs or EPIs with simulation films and thus verification of treatment field. Material and Method: In our clinic PFs are taken in Co 60 machine and both PFs and EPIs are obtained in the linear accelerator. EPIs are taken with iView portal imaging device. While small spheric markers were put on field center and x-y axes on PFs, EPIs had their own electronic centers. In the simulation films field center and axes already exist. PFs and simulation films were scanned and EPIs were transferred from the imaging system to the computer on a diskette and were recorded. Field center, x-y axes and block corners were marked on port films and EPIs using different colors. In addition to this, with the aid of a special program, contrast enhancing procedures were performed on digitized images and reference anatomical regions,~olumes were marked with colored dots or lines. Moreover, the polar coordinates of marked points were determined. PFs and simulation films were superimposed in electronic media and the appropriateness of points and lines were examined. Results: Simulation films of thorax, pelvis, abdomen, head and neck and extremity sites were superimposed on PFs using centers and axes. If there was a discordance of anatomical reference points and lines, these anatomical lines and points were superimposed instead of axes and centers and it was seen that the rotation of axes and shift of centers can easily be measured. Moreover, the numerical values related with the polar coordinates of marked points on the simulation films could be compared with the values of their counterparts on the PFs or EPIs. The superimposition of centers, x-y axes and block corners on the same screen can determine systematical errors while the superimposition of reference anatomical points, regions or volumes can identify random errors objectively. The additional advantage of this method was the quantitative verification through comparison of numerical coordinate values. Conclusion: Superimposition of simulation with PFs and EPIs on the same electronic screen has the potential of supplying objective comparison 1090

Poster

Evaluating accuracy of image correction in digital simulator systems B.H.Knutsen, K. Ei/ertsen The Norwegian Radium Hospital, Medical Physics, Os/o, Norway Purpose: The purpose of this study was to develop and utilise a tool for quantifying geometric deviations between the digitised fluorescence images and a gold standard, radiographic film. Method: Images were acquired from a widely spread distribution of gantry angles and image intensifier x-, y- and z-positions. In each position a geemetrical phantom - regularly spaced steel balls in a perspex slab - was depicted on film and as a digital simulator image. The films were scanned and aligned with the corresponding digital fluoro images using the central