479 Ionometric and fricke electron and gamma dosimetry for the implementation of waste water treatment by radiation in Portugal

Posters

$206 Heterogeneous media is generally a problem in dosimetry and radiotherapy, as absolute dose standards and dose planning algorithms usually are based on measurements in water. Therefore, it is important to investigate the effects caused by heterogeneities in order to improve the current standards. The free radicals stabilized in polycrystalline alanine following irradiation have been utilized in EPR (Electron Paramagnetic Resonance) dosimetry for almost 40 years. Lately, thin films coated with alanine have been developed for EPR dosimetry. In the current work, alanine films (130 microns thick) have been used for dose determinations in heterogeneous media following irradiation with 6 and 15 MV photon beams. Comparisons are made with measurements using a PTW Advanced Markus plane-parallel ion chamber, Monte Carlo simulations using the EGSnrc code and calculations performed by the treatment planning system Oncentra Treatment Planning (OTP). In the first part of this study, alanine film measurements of the dose distribution near the interface between dissimilar materials (e.g. PMMA and aluminum) are compared with ion chamber measurements and Monte Carlo simulations• In the second part, the depth-dose characteristics in a cylindrical, heterogeneous phantom were evaluated by all the aforementioned methods. The cylindrical phantom consisted of PMMA and Styrofoam. Quite good agreement (usually within 4-3 %) was found between alanine film measurements, ion chamber measurements and Monte Carlo simulations. In the case of the cylindrical phantom, some calculations performed by the treatment planning system OTP deviated significantly from the measurements and Monte Carlo simulations. The reasons for these discrepancies will be discussed. The results show that alanine film dosimetry is a useful method for determining doses in heterogeneous media. 479 I o n o m e t r i c and Fricke Electron and g a m m a D o s i m e t r y for t h e I m p l e m e n t a t i o n of Waste W a t e r T r e a t m e n t by Radiation in Portugal

P. Ferreira 1, K. Jacob I, M. Rosa 1, R. Galh6s 1, F. Serra 1, S. Oliveira I, N. Teixeira 2, M. Rarnalho 3 1Medical Consult - HC Descobertas, Lisboa, Portugal 2Medical Consult - ESTeSL, Lisbon, Portugal 3Medical Consult, Lisbon, Portugal Application of radiation as an efficient process to reduce the microbial levels in materials (food, environmental, health care products .••). Irradiation of organic and inorganic compounds with electron and gamma rays can modify the micro organisms, depending on the energy, dose rate, and others variables related to the irradiation conditions• Anthropogenic sources have been responsible for introducing high levels of chemical elements and compounds into the environment, many of this through the industrial waste waters. This is an issue of global concern, with potential impact on human health and the environment. IAEA recognizes the potentiality of radiation technology to clean up waste discharges, and since 2003 IAEA participate in this project• Although this is a multidisciplinary project, by the physical point of view the main objective is to characterize the most efficient electron (by intercomparison with gamma) beam, obtaining the better irradiation geometry, to implement waste water (originated in hospitals, food industry ...) treatments. Until now, we've characterized electron beams with Ionometric and fricke dosimetry at non-standard irradiation conditions, in commercial Linacs for radiotherapy purposes. We used a Varian Clinac 2100 CD, with the special TBI ETRAY conditions and nominal dose rate of 1000 UM/min, and

an Elekta Digital Precise with nominal dose rate of 400 UM/min. The better dose rates obtained in water were approximately equal to 1.4 kGy/h and 0.7 kGy/h, respectively• The impact of the radiation has been studied microbiologically, toxicologically and chemically, and the evaluation between this kind of radiation and cobalt gamma radiation is also carried out at the same time. The results obtained until know and the continuous support of the IAEA experts in the project, indicates that this project will contribute to a better quality of the Portuguese industrial or hospital complexes, minimising the impact of pollutants on the environment 480 Collimator scatter factors for small photon t r e a t m e n t fields

J. Byrne Newcastle General Hospital, Newcastle upon Tyne, UK

Regional

Medical

Physics,

Collimator scatter constitutes 5-10% of dose to patient in external beam photon radiotherapy by linear accelerator and varies across the field size range by between 5% and 17%, depending on linac head design. With increasing use of small treatment fields and segments, there is an increasing requirement to measure relative collimator scatter factor (Sc) for collimator settings down to 2x2cm or l x l c m . We have designed, produced and used a number of miniphantoms/build-up caps to measure Sc for small fields. Sc has been measured on Siemens, Elekta and GE Saturne linacs using Tungsten, Gold, Aluminium, Brass and WT1 phantoms with ionisation chambers (small and pinpoint), PTW diamond detectors and diodes• Special problems are found when using pinpoint ionisation chambers due to the small signals produced and the then relatively large cable and stem signals which required special shielding to obtain useful Sc measurements (see Figure 1). The PTW diamond detector was used in an Aluminium miniphantom. I t is demonstrated however, that the discshaped diamond detector cannot be considered a small detector for the purposes of Sc measurements because of the limitation of having only 1 small dimension. Measurements show clear electron contamination due to the closeness of the detector to the detector outside wall in its widest dimension. Further work is being done using small miniphantoms with diode detectors to measure Sc for small fields as an improvement over small ionisation chambers and wide diamond dectectors. Results from these additional measurements will be presented• Primus6MVSc

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Figure 1: Variation in cable signal with cable length when compared to PTW 31002 0.125cc chamber