Conceptual Improvements and Limitations in Non-Standard Beam Reference Dosimetry

During the past decade, advances in technology have considerably transformed radiation therapy treatment techniques. New treatment modalities improve target dose conformity compared to conventional methods, but increase the complexity of dosimetry procedures because deliveries are composed of treatment fields using a combination of small fields. The complexities of reference dosimetry of deliveries involving non-standard beams have triggered efforts by the IAEA and AAPM towards the development of a new protocol applicable to these beams. This paper summarizes three studies in the scope of the development of the new protocol. The first part of the paper concerns the fundamentals of non-standard beam dosimetry, developing theoretical aspects and summarizing a study of ionization chamber perturbation factors in modulated beams. Conceptual solutions are proposed and a theoretical expression of correction factors is obtained for ideal non-standard reference fields (PCSR). Limitations in the calculation of the correction factors are discussed. A second part of the paper discusses the improvement in kQ experimental measurements in regard to direct absorbed dose to water measurements. Levels of uncertainty of the order of 0.3% are achieved with radiochromic film and show great potential in non-standard beam dosimetry. A third aspect of the paper represents experimental uncertainties to also be considered. A method of evaluation of uncertainties induced by positioning errors is summarized. Results suggest that experimental uncertainties in non-standard beam can be higher than for standard conditions. This is an important issue to consider both during daily QA routine and reference dosimetry, and could be a limiting factor in the new generation of protocols. (author)