Small Field Dosimetric Measurements with TLD-100, Alanine and Ionization Chambers: Preliminary Measurements

The new small and non-standard field dosimetry protocol introduced new reference field conditions for photon beam calibration. Preliminary measurements of beam quality correction factors corresponding to this formalism were performed for three different models of ionization chambers: a Farmer type ionization chamber, a thimble ionization chamber and a microchamber. Beam quality correction factor measurements were made in a cylindrical acrylic phantom and a water phantom using thermoluminescent dosimeters (TLDs) and alanine dosimeters to determine dose to water. The behaviour of the beam quality correction factor was observed as it transfers the calibration coefficient from the University of Wisconsin Accredited Dosimetry Calibration Laboratory 60Co reference beam to the small field calibration conditions of the small field formalism. There was no statistically significant change in the beam quality correction factor for the ionization chambers in static and composite small fields compared to the TG-51 reference fields. Loss of lateral charged particle equilibrium was observed with the Farmer type ionization chamber. The beam quality correction factors for the ionization chambers as determined by the TLDs had an average standard deviation of ∼1.5%, with an estimated relative combined standard uncertainty of ∼3-4% (k = 1). The alanine absorbed dose to water measurements showed an average standard deviation of 5% and a discrepancy up to 4.9% from the American Association of Physicists in Medicine Task Group 51 (TG-51) [2] determined absorbed dose to water. (author)