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T. Landberg et al. / Medical Physics in the Baltic States 7 (2009) 43 - 51 Fig. 10. 51 Reference Point should be reported. When reporting the treatments in scientific journals, it is recommended that the prescribed CTV and PTV and corresponding doses be illustrated in an isodose distribution chart, giving the total absorbed doses in Gy. References 1. ICRU. (1978) International Commission on Radiation Units and Measurements. Dose Specification for Reporting External Beam Therapy with Photons and Electrons. ICRU Report 29 (Bethesda, MD: ICRU). 2. ICRU. (1993) International Commission on Radiation Units and Measurements. Prescribing, Recording, and Reporting Photon Beam Therapy. ICRU Report 50 (Bethesda, MD: ICRU). 3. ICRU. (1999) International Commission on Radiation Units and Measurements. Prescribing, Recording, and Reporting Photon Beam Therapy, Supplement to ICRU Report No. 50. ICRU Report 62 (Bethesda, MD: ICRU). 4. ICRU. (2004) International Commission on Radiation Units and Measurements. Prescribing, Recording, and Reporting Electron Beam Therapy. ICRU Report 71 (Bethesda, MD: ICRU).
MEDICAL PHYSICS IN THE BALTIC STATES 7 (2009) Proceedings of International Conference “Medical Physics 2009” 8 - 10 October 2009, Kaunas, Lithuania MODELING OF THE RESPONSE OF IONIZATION CHAMBERS IN RADIOTHERAPY FIELDS WITH DINAMIC INTENSITY MODULATION Sandija PLAUDE * , Sergey POPOV * , Arturs MEIJERS * , Albert MILLER ** , Yuri DEKHTYAR *** * Latvian Oncology Centre of Riga Eastern Clinical University Hospital, Riga, Latvia ** Vilnius University Oncology Institute, Vilnius, Lithuania *** Riga Technical University, Riga, Latvia Abstract: Ionization chamber (IC) is the most common and trusted radiation detector in Radiotherapy (RT) dosimetry. However, due to its finite size detector volume have an impact on point dose measurements especially in a penumbra and regions of high dose gradients inside Intensity Modulated Radiotherapy (IMRT) fields. The aim of this work is to make a comparison of measured and modeled point doses in the case of dynamic radiotherapy using IC`s of different active volumes, to evaluate magnitude of the chamber volume effect on the discrepancy between the calculated and the measured dose. Keywords: Point dose measurements, ionization chamber, IMRT 1. Introduction Although there are many different radiation detectors used in IMRT dosimetry, the IC is still the gold standard in point dose measurements [1]. In three-dimensional (3D) conformal radiotherapy and in IMRT beam’s central axis (CAX) is usually used for absolute dose measurements. In the case of conformal radiotherapy usually there is a uniform dose distribution and low dose gradients inside the field, while in the intensity modulated field there could be a region of a high dose gradient as well as areas of low dose. Moran et al. within the framework of their study created a dose-gradient analysis tool for IMRT quality assurance (QA) that helps to avoid dose measurements in high dose gradients and low dose regions [2]. However in the case of IMRT there are low dose regions and high dose gradients inside the radiation field moreover inside the Planning Target Volume (PTV), so it is very important to check the dose also in these areas. Many authors suggest that in case of IC over – responses or under – responses this effect will smooth out when all treatment fields are analyzed together [3]. Although in the previously mentioned study in almost all situations when IC over or under – responses were identified, this can be used only to understand a behaviour of IC in certain treatment conditions but not to analyze a treatment plan for the patient. One of the most common sources of errors in dose 52 determination in case of IMRT is the variation of the measurement conditions from the reference ones [4, 5]. Sánchez – Doblado et al. found a general correlation between the IC position relative to a segment and the derived correction factor, c, that indicate the difference between reference and measurement conditions. Almost in all studies about IMRT dosimetry Monte – Carlo (MC) simulation is used as a model for theoretical dose calculation. From the other hand there are attempts to introduce in clinical routine methods of absolute dosimetry in IMRT fields, employing simple chamber volume Dose-Volume Histogram based correction, in that way correcting chamber volume effect [6]. Aim of present study is to verify correctness of dose modelling using simple geometrical model for evaluation of the chamber volume effect. 2. Material and Methods 2.1. Accelerator and collimator Measurements were performed on linear accelerator Clinac 2100C/D (Varian Medical Systems, USA). This is a dual photon energies accelerator equipped with a multileaf collimator (MLC). The MLC has 40 opposed leaf pairs. Each MLC leaf is 6 cm thick made from wolfram with leaf transmission 1.5% - 2.0% for 6MV photon energy. A width of each MLC leaf is 1 cm at the level of the isocenter.
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