Master of Science- Research
Department of Engineering
Warrener, Kirbie Jaye, Output Factors for Small Field Megavoltage Photon Beams, Master of Science- Research thesis, Department of Engineering, University of Wollongong, 2012. http://ro.uow.edu.au/theses/3818
Treatment planning systems and beam modelling are built upon and validated by experimentally measured parameters, both in water and in air. Accurate dose measurements are challenging in small radiation fields because there is no commercially available real-time dosimeter that can accurately measure output factors without the use of specific small field correction factors.
This study supplements the measurements made with a range of commercially available dosimeters with those made using a waterequivalent fibre optic dosimeter (FOD), validated with radiochromic film. The results for in-water output factors, Sc,p , and in-air output factors, Sc , were measured for field sizes down to 4mm.
The in-water output factor, Sc,p , measurements using a diamond, an unshielded diode and a FOD agree to within 2.6% for field sizes between 10 and 30 mm. For smaller fields, the values for Sc,p measured using the various dosimeters begin to diverge, reaching a maximum of 24.5% for a field size of 4 mm. After applying volume averaging correction factors for the three dosimeters, there remains an 13.6% difference in output factors between detectors. Since the FOD is virtually dosimetrically water equivalent, it is assumed that the FOD readings provide accurate measurements in small fields. Small field correction factors can therefore be calculated using a ratio of Sc,p readings from the diamond to the FOD and the diode to the FOD. The correction factors presented are valid only for the specific physical conditions used in this study.
The in-air output factor, Sc , was measured using brass miniphantoms. These miniphantoms were of a unique design with either a cap (with sidewalls) or top (no sidewalls) design for a range of heights and widths. For all dosimeters Sc agreed to within 1% for fields larger than 10 mm. For smaller fields, the agreement was within 2.3%. Unlike the case for measurement of Sc,p , the selection of dosimeter type for measurement of Sc is not critical provided that the active volume is small with respect to the field size. The miniphantom design critically affects the measured values in air. The dominant contribution to the dose measured by the dosimeter is scatter from the miniphantom, while the dosimeter housing makes a relatively small contribution.