Modern radiation therapy techniques, such as stereotactic radiotherapy and tomotherapy, deliver the total treatment dose to the patient using many small fields. This helps to construct a dose distribution which delivers a high dose to the tumour, while minimizing the dose to normal tissue. When considering a single small field, the dose resulting from that small field is usually reported as a relative dose factor (RDF). The RDF is the percentage of dose in a small field as compared to a large reference field. Task group report #51 from the American Association of Physicists in Medicine (AAPM) provides the protocol according to which absolute dose measurements using an ion chamber must be taken, all with respect to measurements at a reference point in a large reference field. In the case of RDF measurements several assumptions are made which allows for the simplification of the TG-51 protocol, but attention is not necessarily paid to the fact that the conditions in a large reference field differ considerably from those ina small field. In this study we have used experimental measurements with plane-parallel ion chambers and theoretical simulations (Monte Carlo BEAMnrc code) to investigate the validity of the assumptions made in the determination of RDFs for small fields. Four custom-made plane parallel ion chambers, as well as other common high resolution radiation dosimeters, were used to measure the dose response in circular 6MV stereotactic radiotherapy fields. BEAMnrc was then used to simulate the experimental conditions and determine the validity of the assumptions mentioned previously. The assumptions were determined to be invalid for use in small radiation fields. Corrections


were calculated from Monte Carlo results and applied to the experimental measurements. Care needs to be taken when applying existing protocols to non-reference conditions