Using films, sensitive to high energy photons, for tumour dose determination by densytmetric technique 60 co

ABSTRACT USING FILMS, SENSITIVE TO HIGH ENERGY PHOTONS, FOR TUMOUR DOSE DETERMINATION BY DENSYTOMETRIC TECHNIQUE IN 60Co Adem Ünal KIZILDA? M.Sc. in Physics Adviser: Asst.Prof.Dr. Şerafettin Yaltkaya Second Adviser: Dr. Nina Tunçel 2004, 47 Pages The goal of radiotherapy is to deliver the prescribed dose to the specified tumors according to treatment protocols without exceeding tolerance dose limits of the normal tissues. To protect the normal tissues from the high dose these regions are blocked or the radiation intensity is arranged. So the tumor-target receives the prescribed dose which was planned. The sharp dose gradients are created around the irradiated region. The treatment efficacy is achieved if the dose error at the tumor is around 3-4% of the prescribed dose. The prescribed treatment could be accepted as successful if achieved tumor dose error is less than 3-4% in the target volume. On the other hand the treatment quality assurance is depend on the accuracy of the dose delivered to a patient. The better way for controlling the dose received by the patient on treatment course could be done by direct invivo dose measurements. The suitable technique is entrance and exit dose measurements and than calculating the midplane (tumor) doses, invivo dose measurements are suggested by manynational and international organizations (ICRU, AAPM) to achieve accuracy of dose delivery on the treatment. In radiotherapy, the treatment accuracy depends on two important parameters. One of these parameters is the control of the delivered dose and the other is the control of irradiated field localization. The irradiated field localization control is achieved by obtaining portal films on treatment course. In this project the necessary processes and required equations was developed for calculating and controlling the delivered dose from the portal films or other film and film systems obtained during treatment course. The dosimetric methods for determining the midplane dose for different compositions of treatment parameters such as absorber thickness, source-axis, source-skin, source-film and film-exit surface distances are ionometric dosimetry technique by using ion chambers and densitometric dosimetry technique by films. In this project, the relationship between the treatment parameters and midplane doses was investigated. This method was examined with five different films or film systems as ready pack or cassette set-ups for different phantom thickness. Kodak X-Omat V, Kodak EDR and Kodak EC films were used in Kodak EC-V cassette. Kodak X-Omat V and Kodak EDR films were ready pack.. Formulations and procedures were found and arranged for calculating midline doses via measuring transit doses. Midline dose equation contained phantom thickness parameter, linear attenuation coefficient (ju.), inverse square law correction factor and midline-exit transmission ratios obtained from ionometric measurements. These calculations were performed for all films or film systems in different phantom thickness. The difference between calculated and prescribed midline doses for Kodak X- Omat V and Kodak EDR ready pack films were found ±%3.8 and ±%3.2 respectively. These differences were in acceptable limits (±%5). VIThe difference between calculated and prescribed midline doses for Kodak EC film with Kodak EC-V cassette would be attained as ±%5.7. The results of this study were not applicable to inhomogeneous material as in radiotherapy. In patient treatment the transit dose would be affected from inhomogeneous material. Thus the differences between calculated and prescribed midline dose would be more than expected. Application of this method needs at least one CT slice of patient which contains information about inhomogeneity and thickness of tissues on the beam central axis. So for midline dose calculation, linear attenuation coefficients and thickness of these tissues could be considered in the formula. KEY WORDS :.Film dosimetry, radiotherapy, tumor dose calculation. COMMITTEE : Asst.Prof. Dr. Şerafettin YALTKAYA (Adviser) Dr. Nina TUNÇEL (Second Adviser) Prof. Dr. Nuri Ünal Prof. Dr. Suphi Kormalı Prof. Dr. Piraye Yargıçoğlu Vll