250 poster workshop Pre-treatment verification of the patient dose with an amorphous silicon EPID

Poster workshop

Tuesday, October 26, 2004 S l 1 3

with approx. 200x200 pixels) to 2-3 seconds. Additionally, the software summarizes the y-values statistically (maximum and mean value, cumulative histogram). Finally, dose profiles and dose difference maps were extracted for each plane. For all films the total percentage of the y-values > 1 in the ydistribution was extracted. Results: In 3/24 films y-values exceeding 1 were observed in large areas, with 17-19% of points with y > l . The y-angle distribution indicated that these areas were dominated by dose differences. From an analysis of film planes with respect to IMRT segments we concluded that these areas correlated with slices where the tongue & groove effect might have an important influence. The maximum y-value was o n a v e r a g e 2.1 (SD 0.8) for all films evaluated, the mean value was only 0.4 (SD 0.09). From the histogram distribution analysis an acceptance criteria for our IMRT-equipment has been defined as follows: an IMRT plan is accepted if the percentage of points with y > l is not exceeding 10%. Discussion: It is useful to make statistical analysis of the ydistribution and to use the y-angle distribution for the interpretation of deviations. From the additional information of the angle one can conclude whether the y-value is driven by dose difference or distance to agreement. The resolution with respect to DTA agreement criteria is important to avoid artifacts in the evaluation. 248 poster workshop Correction factors for accurate use .of MOSFETs entrance in-vivo dosimetry in photon beams

for

E. Bloemen-van Gurp, C. Wulms, P. Lambin, B. Mijnheer, A. Minken U.H. Maastricht, Radiation Oncology (MAASTRO clinic), Maastricht, The Netherlands Background and aim: MOSFETs are more and more used for in-vivo dosimetry. In contradiction to diodes little is known about correction factors for very accurate in-vivo dosimetry. In this study we wanted to establish correction factors for the use of MOSFETs in entrance measurements for patient in-vivo dosimetry. Material & Methods: MOSFETs were calibrated at 100 cm SSD for a 10xl0cm field size with 6 MV photons, with MOSFETs at surface under a slab of build-up and ionisation chamber at Dmax. We compared MOSFETs and ionisation chamber readings in various phantom experiments under different clinically relevant conditions. We varied the energy: 6 and 10 MV photons. For both energies the direction of incidence was investigated in full scatter conditions and in a cylindrical mini phantom. Further study showed the influence of SSD, field size and wedge. Apart from this the effect of beam entrance through the carbon fibre table top was established. An extra correction was given for the fact that the calibration was done under slab build-up while in-vivo measurements were performed under a half-spherical build-up. All experiments were performed on Elekta SL15 machines. We used a standard Baldwin chamber. For the MOSFET measurements we used an auto sense TN-RD-60 system that was a loan of ThomsonNielsen. To get statistically relevant results all measurements were repeated 10 times. Results: No influenpe was found for: energy, and direction of incidence (in both circumstances). For 10 MV beams a slight dependence of SSD (0.997/5 cm) and field size (Corr=0.0008*FS+0.9916) was found, the influence of these parameters by 6 MV was negiglible. The correction for the wedge was 3.7% in both energies, for carbon fibre in 6 MV: 3.0% and in 10 MV: 1.2%. For the difference in build-up the correction was 1.2% in 6 MV and 3.0% IN 10 MV.

Conclusion: Accurate in-vivo dosimetry with MOSFETs cannot be performed without a proper commissioning of the MOSFETs. The influence of clinically relevant parameters was so large that we introduced correction factors for each patient treatment. These experiments could not be carried out in a reasonable time without the auto sense system that we used in the experiments. When these precautions are taken into account invivo dosimetry with MOSFETs is a good alternative for in-vivo measurements with TLD and diodes. 249 poster workshop MU calculations for irregular electron fields

S. Daniel 1, A. Nevelsk S, R. Bar-Deroma 1'2,A. Kuten ~'2 ~Rambam Medical Center, Oncology, Haifa, Israel ~Technion liT, Faculty of Medicine, Haifa, Israel Rationale: Small electron beams are used clinically to treat skin and subcutaneous lesions in the chest wall, head and neck regions, scar tissues and the regions under blocked photon beams. Electron dosimetry is machine dependent, because it is affected by the design of the different applicators used and by the design of the accelerator head. This imposes the need for measurements to be done one each accelerator in each department, although some general conclusions and rules can be derived. In the clinic MU for each treatment are usually calculated based on output measurements done for each cutout and patient. The purpose of this work was to find a simple and practical algorithm to calculate output and MU to be used in the clinic. Methods and Materials: An algorithm was derived based on the "Clarkson Integration" idea. Each field is divided in sectors and the output contribution of each sector is calculated. The contribution of each sector was derived from a series of measurements done using circular cutouts of different radii for several energies. Measurements were performed in the Varian Clinac 1800, using a parallel plane chamber in water. Based on the algorithm, a code was written and run on a PC connected to a digitizer. The different field shapes were read into the program and the MU / output were calculated. Results: MU and output were obtained for field shapes and for energies 6,9,12 and SSD=100cm setup. Comparison between measured results showed differences in the depending on the field size and shape. Future to introduce corrections for different SSD's.

different electron 16MeV using a calculated and range of 1 -3%, work will be done

250 poster workshop Pre-treatment verification of the patient dose with an amorphous silicon EPID

M. Wendlin.q, L.N. McDermott, R.J.W. Louwe, M.B. van Herk, B.J. Mijnheer The Netherlands Cancer Institute, Department of Radiotherapy, Amsterdam, The Netherlands Introduction: Due to the increasing complexity of radiotherapy treatments, dose verification becomes more and more important. Amorphous silicon electronic portal imaging devices (aSi EPIDs) are promising for dosimetry. In the back-projection method applied in our institute the absolute 2D dose distribution for each beam inside the patient can be reconstructed from portal images. After careful determination of the parameters for this approach a clinical plan of a conformal 3-field prostate treatment is evaluated. Materials & Methods: All measurements were carried out with an Elekta aSi EPID at an SDD of 157 cm using 18 MV photon beams. First, the EPID was calibrated and the parameters of the back-projection model were determined by fitting the

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Tuesday, October 26, 2004

phantom midplane dose reconstructed from EPID images which were acquired for fields of different size, without and with slab geometry phantoms of several thicknesses in the beam with the EPID behind the phantoms - with appropriate ionisation chamber measurements. Secondly, a clinical plan for a 3-field prostate treatment was replanned with our clinical treatment planning system (TPS) UMplan for irradiation of a 20 cm thick polystyrene slab geometry phantom. Portal images of all beams at a gantry angle of 0 degrees were acquired with and without the phantom in the beam. Results: Profiles of square fields of 3x3 to 15x15 cm 2 measured with an ionisation chamber in a (water-) phantom, which were used to determine the parameters for the back projection, are reproduced from the portal images within 2% on the central beam axis and within 2 mm in the penumbra. For the prostate treatment, the doses calculated with the TPS at the isocentre agree within 2% with the doses reconstructed from the EPID images for the open beams, while the difference is about 5% for the wedged fields. In the penumbra a steeper profile is obtained with the EPID than is calculated with the TPS. This difference is larger than the accuracy of the backprojection method in the penumbra region (2 mm). This suggests that the back-projection method performs better than the TPS in the high dose gradient regions. Conclusions: The accuracy of the back-projection method is 2% of the dose in the central beam axis region and 2 mm in the penumbra, the latter being important to correctly estimate the dose to organs at risk. We demonstrated that with the backprojection method the dose inside a phantom can be determined with high accuracy to verify a clinical plan. 251 poster workshop Determination of fluence scaling factors for plastic water for high-energy electron beams using IAEA TRS-398 code of practice

B. Casar1, U. Zdesar2 ~lnstitute of Oncology Ljubljana, Department of Radiophysics, Ljubljana, Slovenia Institute of Occupational Safety, Dosimetry Laboratory, Ljubljana, Slovenia Introduction: In the International Code of Practice for dosimetry TRS-398 published by International Atomic Energy Agency water is recommended as the reference medium for the determination of absorbed dose for high energy electron beams. Plastic phantoms may be used under certain circumstances for electron beam dosimetry for beam qualities Rso < 4g/cm 2 (E0 below 10 MeV). In our study water equivalency of Plastic Water@ was evaluated in order to determine fluence scaling factors hp~ for Plastic Water. Extended set of measurements in water and in Plastic Water were performed, Material and methods: Absorbed dose was determined according to IAEA TRS-398 dosimetry protocol following recommendations for all relevant parameters involved. Water equivalency of Plastic Water was evaluated for five electron beams with nominal energies from 6 MeV to 18 MeV generated by linear accelerator Varian Clinac 2100 C/D. Adequate dosimetry equipment was used throughout the measurements and reference conditions from IAEA TRS-398 were followed carefully.

Poster workshop

within previously published data. Measurements were taken over a period of 18 months within a Coordinated research project of the International Atomic Energy Agency. 252 poster workshop QA with alanine dosimetry for step and shoot IMRT of head and neck tumors

B. De Ost1, B. Schaeken I, T. Wauters 2, D. Van Gestef , A. Coelmont1, D. Van den Weyngaert/ ~ZNA AZMiddelheim, Radiotherapy, Antwerpen, Belgium 2Hogeschool Limburg, Diepenbeek, Belgium Introduction: IMRT treatments for head & neck are performed in clinical routine since October 2003. After setting up a rigid QA program for the IMRT machine related tools and treatment planning system (TPS), individual treatment plans must be verified. Materials and methods: For QA of individual IMRT treatment plans, the 'copy to phantom' tool of the TPS (Philips Pinnacle3 v6.2b) is used: all treatment parameters are copied to the Alderson phantom such that the clinical situation is simulated in the most realistic way. Measurements are carried out with in house developed alanine detectors at all points of interest: in tumor volume, at critical structures and at regions with dose gradient, to compare with calculated dose. A EPR-spectrometer (EMS104, Bruker) measures in a non-destructive way the concentration of free radicals, produced in L-c~ alanine during irradiation. The EPR-signal varies linearly with dose. The combined uncertainty on a dose measurement is 2.2% (1(~) at an absorbed dose of about 10 Gy. For a total of 97 alanine measurements on 8 patient set-ups the ratio of the calculated- to measured dose was 1.009 (1 o =4.0%). In low dose areas or dose gradients 34 measurements were performed: a mean distance to agreement was 6.0 mm (1 (~ =3.1mm). In these low dose regions the dose was overestimated by the planning system, irradiating the critical structures with a safe dose. A unique experiment was performed comparing a 1 time 20 Gy set up with a 10 times 2 Gy set-up for the same treatment. For the latter our treatment team was installing the phantom each day on the treatment table, as they would do in clinical routine, to administer 2Gy each session. In this way a total dose of 20 Gy was accumulated in the detectors. The ratio of the calculated dose to the measured dose was 0.991 (1 ~ =5.1%) versus 1.011 (1 c~ =2.8%) for the 1 x 20 Gy and 10 x 2 Gy respectively. In the 1 x 20 Gy set up 4 measuring points were found outside the 5% dose difference level, for the 10 x 2 Gy experiment this was only the case in 2 measuring points. Conclusion: To gain confidence in the IMRT treatment delivery a large number of measurements in different clinical treatment situations must be evaluated. At our department, alanine dosimetry has proven to be a technique of great value, particularly in the QA of IMRT of head and neck treatments. 253 poster workshop Accuracy of primary portal dose extraction for in-vivo dosimetry purposes

Results and discussion: Results are presented as ratios Dp~ /Dw of absorbed doses in Plastic Water and water. Upon the

W.J.C. van Elmpt, S.M.J.J.G. Nijsten, B.J. Mijnheer, A.W.H. Minken MAASTRO clinic, Radiation Oncology, Maastricht, The Netherlands

selected electron energy the ratios vary from 0.9990 - 1.0058 with combined uncertainties (1SD) of 0.46% - 0.68%. From the measured data fluence scaling factors hp~ were determined and found to be in the range from 0.9942 to 1.0010. Results are

Introduction: The derivation of dose inside a patient is the ultimate goal in in-vivo dosimetry. In portal dosimetry, the measured dose is composed of primary and scattered (patient) dose. To compute dose inside the patient, the primary and