684 Poster Analysis of two-year daily quality control measurements of megavoltage radiotherapy equipment

Posters

Wednesday/Thursday, 18-19 September 2002 $207

ures have been experienced, neither on the client nor on the server side. Lack in R&V communications due to hospital's net problems were resolved isolating via hardware the R&V LAN. Conclusion: Generation 6 R&V system has proven, over the last three years, to be a useful tool for the radiotherapy daily practice, as it increased the control on patient daily treatment without a significant added workload for most of the staff. In particular there was no problem in IMRT's fields management and the EPID images proven to be a valuable instruments in everyday check of IMRT's patients positioning. The R&V offers a valuable help to personnel managed with RT treatments, reducing errors and assuring records of events, but it is absolutely mandatory a clear and fitted installation and management of the applications, otherwise all the benefits could be lost. 681

Poster

In vivo dosimetry with diodes. Technical and physical cor-

rectJons J.M.J. Chdstensen, L. Hoffmann Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark Dose measurements with diodes must be corrected according to field size and dose per pulse, To obtain an adequate build up thickness the diodes are covered by a radiological equivalent layer of metal. The corresponding filtration of the photons is the reason for the correction according to the field size. The sensitivity of a diode depends on the dose per pulse. Changing the dose rate by changing the distance from source to diode or by introducing a conventional wedge demands a correction, To determine these corrections measurements are made on a solid water phantom at room temperature and compared with corresponding measurements with a plane parallel ion chamber. The correction due to temperature is neglected, The physical corrections are related to scattered radiation in the phantom, Doses calculated by use of the pencil beam algorithm or depth dose tables will differ from the corresponding measurements. The reason is that the calculations are based on data measured in a 50 x 50 x 50 cm3water phantom and they dO not take into account the influence of missing back scatter in a smaller phantom or in a patient, The effect is appreciable for the exit dose but will also have some influence on the entrance and central dose. A correction proportional with NPSF and decreasing with the distance to the exit surface modifies the calculated doses and causes a good agreement with the measurements. The corrected calculations are compared with collapsed cone calculations, Finally these corrections will give rise to reconsideration concerning the calibration of diodes, 682

Poster

Q u a l i t y assurance in radiation therapy: a tool for human

anatomy learning and contouring comparison V. Va/entini 1, N. Dinapo/i 1, S. Nofi2, A.G. Morganti 1, G.C. Mattiucci 1, F. Deodato 1, D. Maronta 1, N. Cellini I 1Cattedra di Radioterapia,/stituto di Radio/ogia, Rome, Italy 2/stituto di Anatomia Umana Norma/e, Rome, Italy Background: Advances in radiotherapy as conformal radiation therapy and intensity modulated radiotherapy require considerable attention in quality assurance procedures, in order to favor higher standard of care. One of the most critical steps in newest radiotherapy procedures is the contoudng on patients images: several issues in literature arose about the need of standardizing and optimizing this important step but at the moment no comprehensive solution for this problem has been proposed. We realized a software, called TIGER (Tutorial for Image Guided External Radiotherapy), to help radiation oncologists to focus on contouring procedures problems, Materials and methods: TIGER project consists of a software that allows physicians to examine anatomical and CAT scans images obtained from the Visible Human Project~ database of the National Library of Medicine. These images are anatomical axial sections of two human subjects (a male and a female one) and are associated with corresponding CAT scans that were graphically adapted by us to be as best as possible overlappable to the corresponding sections. TIGER software allows the user to display this kind of images and, furthermore, it offers the possibility to trace and show contours of anatomical structures traced over the images and stored in one appropriate database by members of TIGER board. TIGER software has also at disposal the possibility to test the skill of users by compariog con-

tours traced on images with the database stored ones: this comparison is obtained by calculation of the percentage of overlapping areas between the two different polygonal surfaces of the given contour and the user traced one. Conclusions: We think that the possibilities provided by TIGER software could result helpful in human anatomy learning focused on the contouring procedures and, at the same time, they could supply radiation oncologists with a tool to define a first system for standardizing one of the most critical and variable elements in modern radiation therapy procedures. 683

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Leipzig applicators montecarlo simulations: results and comparison with experimental and manufacturer's data M. Trooeano 1,2, F. Foppiano 1, P. Franzone 1, M.G. Pia 3, S. Guatelli2,3, F. Bisi 1 1National Cancer Research Institute of Genoa, Radiotherapy, Genoa, Italy 2physics Department, University of Genoa, Genoa, Italy 31NFN,Genoa, Italy At National Cancer Research Institute of Genoa, the Leipzig applicators designed by Nucletron for brachytherapy surface treatments (especially skin and intraoral) are used, with a HDR unit, to give the optimum isodose and treat to the correct depth without causing excessive skin reaction. To study the dose distribution produced by the different applicators, we have performed Monte Carlo simulations with Geant4, a software toolkit developed at CERN (European Organisation for Nuclear Research). Geant4 is a toolkit for the simulation of the passage of the particles through the matter. It provides a complete set of tools for all the domains of the sireulation. In particular, Geant4 permits the representation of all the geometdcat elements, their materials and electronics properties, together with visualisation attributes. An abundant set of physics processes handles the diverse interaction of particles with matter across a wide energy range (for electrons and photons down to 250 eV). In our simulations, we have exactly reproduced all the geometry and material attributes of the Leipzig applicators and of the 1921r source, computing the energy deposition in a water box with a voxel grid with cells of 1 mm3. From this dose distribution, we have extrapolated the dose along the Iongitudinal axes of the applicator. To compare and validate the results of the simulations, some measurements have been performed. In particular, to evaluate the dose distribution in water along the longitudinal axes of the applicator, we have used a Markus Ionisation Chamber with a PTW Dosimentor. Isodose distributions in water have been produced with a statistical error, in the firsts 3 cm, less than 3%. The dose deposition along the longitudinal axes of the applicator has been compared with experimental data showing a good agreement (within the statistical error) also with the manufacturer's data (Nucletron). For the daily radiotherapy work, we have produced, for all the six applicators, isodose distributions and tables showing the percentage depth dose: these information will be used by the radiotherapist to choose the applicator more suitable for the treatment. 684

Poster

A n a l y s i s o f two-year daily quality control measurements of

megavoltage radiotherapy equipment K. Hatziioannou 1, O. Kimoundri 1, G. Komisopou/os 1, K. Badiavas f, V. Giannouzakos 2, D. Missailidou 2 1papageorgiou Hospital, Medical Physics Department, Thessaloniki, Greece 2papageorgiou Hospital, Radiotherapy Department, Thessaloniki, Greece Present day practice in radiation therapy requires high doses of radiation to be delivered with increased accuracy. This requires the minimization of the overall uncertainties in radiation dosimetry and a carefully designed and implemented program of Quality Control (QC) tests which will ensure the stability, accuracy and safety of the equipment used for treatment delivery. In the present work, a detailed daily QC protocol is presented, which is applied for the performance check of the two Linear Accelerators (Varian CL600C and CL2100C), which are installed in the Radiation Oncotogy Department of Papageorgiou General Hospital in Thessaloniki. The daily QC of the LINACS consists of checking some major mechanical and geometrical parameters as well as the characteristics of all the radiation beams used. The geometrical parameters control was performed using a dedicated phantom (ISO-ALIGN, MED TEC), and included SSD, field size and laser alignment accuracy. Radiation beam characteristics were moni-

$208 Wednesday/Thursday, 18-19 September 2002

tored using the PTW MultiDOS QC6 and appropriate software (QCWin). The daily quality control included field homogeneity, X and Y symmetry and quality of all radiation beams used. Results. The only geometrical parameter that deviated twice from the tolerance limits during the past two years was laser alignment. On every occasion, the Biomedical Engineering Dept. of the Hospital preceded promptly to all necessary corrective actions. The analysis of the radiation beam measurements showed that the mean value of the inhomogeneity of all beams ranged from 1.14=L-0.25% (CL2100C/16 MeV e-) to 2.67-.L-0.35% (CL2100C/20 MeV e-). Mean symmetry values vatled between 0.62±0.66% (CL2100C/9 MeV e-) and 1.89-+0.70% (CL2100C/18 MV x-rays) for the X-axis and 0.64±0.57% (CL2100C/16 MeV e-) and 1.73i-0.85% (CL2100C/6 MV x-rays) for the Y-axis respectively. The above values are well below the tolerance limits set for each case. Only once did field symmetry exceed tolerance. Beam Quality Factors were 80.15±0.41% (CLG00C/6 MV), 79.2±0.33% (CL2100C/6 MV) and 90.57:L'0.29% (CL2100C/18 MV). The daily QC procedure is a valuable tool for the early diagnosis and prompt repair of geometrical and dosimetric deviations, ensures the stability, accuracy and safe performance of the equipment and contributes significantly to the accuracy and reproducibility of the treatment. 685

Poster

Surface and peripheral dose properties of virtual and physical wedge N. Kucuk 1, A. Kili~ 1, G. Kemikler?., L. Ozkan 1, K. Engin I 1Department of Radiation Oncology, Uludag University Medical Collegel Bursa 21.U. Oncology Institute, Radiation Oncology, Istanbul Purpose: surface and peripheral dose differences between Virtual Wedge (VW) and Physical Wedge (PW) systems for 6 and 25 MV are discussed in this work. Methods and materials: surface and peripheral dose measurements were carried out using Markus-type parallel-plate ion chamber in a plastic water phantom for different field sizes (10x10.15x15 and 20x20cm 2) and SSDs (85,90,100cm). The measurements were performed using Siemens Mevatron-KD2 linear accelerator. For normalization depth, i.e. the depth of maximum dose, 1.5 and 3.5 cm were chosen for 6 and 25 MV photon beams, respectively. The relative ioniztion values were corrected to zero-chamber volume absorbed dose values by using Gerbi and Khan's reported data for surface dose measurements. Peripheral dose measurements were taken distances outside the field (2 and 5 cm). Results: there is a strong relationship between field size and skin dose. As the field size increased the skin dose increased for open, VW and PW fields. The VW skin dose values were very similar to those of open fields, PW skin dose values were lower than VW and open field skin dose values. The skin dose increased as SSD decreased especially in larger field sizes and high energy (25 MV) for open, VW and PW fields. VW and open field peripheral dose values are nearly similar for 15° , 30 ° and 45 ° wedge angles. We measured higher peripheral dose values for 60 ° VW fields. On the other hand, PW may have a significantly higher peripheral dose especially for large wedge angles. When distances from field edge increase, peripheral dose significantly decreses for both wedge systems. Conclusion: PW and VW systems surface and peripheral dose values significantly change from one set-up to another. There fore, it's important to know dose distribution of normal tissue in or outside the field before treatment because of possible biological complications of high skin and peripheral doses, such as desquamation, erythema and fibrosis that occur in radiotherapy treatments. 686

Poster

The comparison of the dose delivery accuracy during total body irradiations performed on the two types of therapeutic units. The analysis of the 560 fields G. Kosicka 1, A. Strugala 1, J. Wachowiak 2, J. Malicki 1 1Great Poland Cancer Centre, Medical Physics, Poznan, Poland 2University Hospital, Clinic of Paediatric Haematology and Oncology, Poznan, Poland Background and Purpose. The aim of this work was to check if the total body irradiation performed on the different treatment units (different energy) provides the similar dose accuracy. Material and Methods. Two groups of patients treated at the Cobalt-60 and with 15 MV photons were included into the study. The accuracy of irradiation wasanalysed through the comparing the measured doses with those calculated. Total number of fields checked were 336 (Co-60) and 224 (15

Posters

MV), respectively. For the each field the doses were determined for the points assigned to particular body parts (on- and off-axis). Total number of these points was 7940 (entrances and exits). The semiconductor Scanditronix diodes equipped with suitable cups providing electronic equilibrium were used for the measurements. Results. The mean ratio of measured to calculated doses at the entrance was 1,023 (Standard Deviation = 0,021 ) for the 15 MV and 1,032 (0,034) for the Co-60. Respectively, such defined dose ratio at the exits was 1,014 (0,035) for 15 MV and 0,992 (0,039) for the Co-60. The breakdown of the dose results into the on- and off-axis points showed that standard deviation for the on-axis points was smaller by 0,006 and 0,008 than for the off-axis, respectively for 15 MV and Co-60. Conclusion. There was no significant difference observed in the dose delivery accuracy between treatments performed at Co-60 and with 15 MV photons. 687

Poster

The application o f H E R M E S system for Quality Control P. Cernohubv, L. Frencl Institute of Radiation Oncology, 1st Faculty of Medicine, Charles University & University Hospital Na Bulovce, Institute for Postgraduate Medical Education, Prague, CZ Our experience with the quality control system HERMES from PRECITRON are presented. This system consists of the solid phantom with 10 semiconductor detectors connected each to independent electrometer and build up plates for the depths up to 30 mm. The steel cylinders set is supplied for the depth dose detector. Optoisolated RS 232 interface to PC and manufacturer's program for calibration, measuring and storage of the data is also available. This phantom was used as a "dosimetric multimeter" in our QA system for some years. The beams in the basic and main gantry positions combined with main collimator rotations were checked. The large scale of data for analysis was so obtained. Three modes for the calibration of HERMES were compared. First mode of calibration is recommended by manufacturer. Only substandard dosimeter with a chamber is necessary for comparison and calibration of the absolute dose. Second mode is advanced calibration in which some basic parameters of the analysing beam are directly measured. So basic parameters of the beam must be known first before the other measurements. Third mode is direct attachment to the water phantom measurement. The calibration must be done closely after beam analysis in water phantom. The special programs were developed to create different tables for analysis of large quantity of data. We have demonstrated which tables are more suitable for the different purposes. We would like to report some limitations regarding to our experiments with this system.

IMAGING FOR TARGET VOLUME DEFINITION 688

Poster

The reasons for discrepancies in target volume delineation : I, Physical part R. Moeckli 1, W. Jeanneret Sozzi 2, J.F. Valley 1, R.O. Mirimanoff2 1University Institute of Applied Radiophysics, Lausanne, Switzerland 2CHUV, Radio-oncology department, Lausanne, Switzerland Objective : To understand the reasons of the observed discrepancies in the delineation of the target volume between physicians. Materials and Methods : Eleven Swiss radio-oncology centres were asked to delineate volumes according ICRU 50 recommendation and to answer a questionnary for a prostate and a head and neck case. The physics part of the study is based on three different techniques for the evaluation of discrepancies : 1) Comparison between the maximum and the minimum extensions of the target volume in the three main axes. 2) Comparison between the volumes obtained by the union and the intersection of the target volumes. 3) Comparison between the mapping of the isocentre-to-eurface in a theta ? phi plan. The comparison of each target volume delineation to all others leads to a so called "correlation matrix" between centres. It is helpful to find groups of centres which have the same strategie for the delineation. The physicals correlations are matched to the clinical correlations to explain