Small field dosimetry: experimental methods and Monte Carlo simulation in small field radiation therapy dosimetry

The goal of the thesis was to investigate, and better define, what the requirements are for accurate small field relative dosimetry. Diode detector selection and experimental techniques were evaluated. EGSnrc Monte Carlo simulations were used to predict diode detector dosimetric parameters and assist in interpreting measured data. An emerging scintillator based detector technology was also tested and methods developed to standardize the reporting of small field dosimetric data. Using careful experimental methods the relative output uncertainty for the smallest square field size of side 0.5 cm was reduced to better than ±1.00% for all detector types. Monte Carlo simulation data revealed that for the same small field size the relative output measured using unshielded and shielded diodes will be 5% and 10% greater than the actual relative output in water. Further simulation work showed that simplified diode detector models are valid for use in small field dosimetry simulations. The diode detector over-response was also shown to be insensitive to variations in the electron energy and spot size incident on the Bremsstrahlung target. Experimental methods were refined to include the definition of an effective field size, which was shown to remove much of the ambiguity in reporting small field relative output data across a population of linear accelerators. Each of the for mentioned areas of investigation have been shown to be requirements for accurate small field relative.