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Conundrum of Small Field Dosimetry: What We Have Learnt?

I Das

I Das1*, O Volotskova1 , P Francescon2 , (1) NYU Langone Medical Center, New York, NY, (2) Ospedale Di Vicenza, Vicenza,


TU-FG-205-5 (Tuesday, August 1, 2017) 1:45 PM - 3:45 PM Room: 205

Purpose: Small field dosimetry is heart of advanced radiation treatment technologies that has been under rapid growth with a lot of controversial data. An evaluation of current practices, selection of best detectors and variability in microdetector data is analyzed.

Methods: Over last 12 years more than 350 research papers have been published dealing with various aspect of small field; methodology, implementation and selection of detectors. IAEA (1) has provided comprehensive data on various types of machines (source size), and detectors, with derived conclusion that tissue equivalent plastic scintillator (PSD), microdiamond (MD) and EBT films are best suited for small field dosimetry. Evaluating large published data, variation in KΩ Ξ k(fclin,fmsr, Qclin, Qmsr) factor to convert ratio of readings to dose is found to have varied degree of accuracy. An observational approach for selection of detectors and reason for variations are analyzed selectively for the best detectors (PSD and MD).

Results: KΩ is a function of machine type (source size), field size and choice of detector. Based on the data PSD, MD and EBT are found to be most suitable detectors. KΩ is found to be nearly 1.0±0.005 for most machines and fields ≥2x2 cm². For 1x1 cm², MD and PSD provide comparable data 0.995±0.008. However for MD large discrepancies (0.995±0.025; range 0.958-1.03) in 0.5x0.5 cm² is observed. It is noted that these differences could be attributed to MD design, quenching, and more importantly in centering of the detector in small fields as well as Monte Carlo simulations.

Conclusion: With selection of PSD and MD, small field dosimetry can be streamlined up to 1x1cm2 within 1% of the accuracy. For smaller fields (<1x1cm²) discrepancies needs additional research, however emerging data suggests that it can be used with caution as long as it is intercompared with published data.1. TRS-483, IAEA, 2017

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