SU‐E‐T‐266: New Improved Methods to Read Output Factors for Small Fields

Abstract

Linear Accelerators represent big challenges in measuring output factors for small fields. Output factors are crucial for dose calculations in intensity modulated radiation therapy (IMRT) planning. The challenge comes from charged particle non‐equilibrium conditions and high dose gradients, type, and the detectors used. We analyze all available detectors and the output factors obtained with them. Based on this we come with recommendations in a protocol for accurately measuring the output factors for commissioning any linac that will perform IMRT. Advances in therapy equipment implicitly encourage the use of smaller treatment field sizes on equipment originally designed for treatments based on conventional, broad photon fields.Challenges at small fields: penumbra size vs. detector size, lack of lateral electronic equilibrium (LEE), photon energy and spectrum dependence on detector response, finite detector volume, detector density effect, detectors' polarity effects, physics at small fields: field perturbation, fluence. Method: Initial data gathered have led to necessity of using small detectors for small fields. We propose to use different detectors to measure the output factors for microMLC linacs using three detectors: a diode detector and two types of micro ionization chambers (for corroborative measurements and consistency). We compared the performance of the Edge detector, PTW pinpoint and Exradin A1 ionization chamber. The output factors are taken at three different depths: (dmax, 2cm, and 5.0cm) in SAD setup. Comparison of these data lead to precise measurement of the correction factor to provide 1%–2% variation in calculated doses for IMRT with the Treatment Planning System. DISCUSSION: From TG‐155 recommendations, dosimetric measurements should be taken with more than one detector system. Small detectors should be used that has minimum energy, dose and dose rate dependence. If field size is small compared to the detector, measurements should be taken at greater source to surface distance with proper correction. Advanced Radiation Physics Inc