In recent years, small fields have come to play a key role in advanced radiotherapy, yet protocols to perform dosimetry under small field conditions are still in their infancy. In 2008, the IAEA and AAPM published a formalism [Med. Phys. 35, 5179-5186] recommending the use of point-dose correction factors. This thesis uses Monte Carlo simulations to demonstrate that the values of these correction factors depend strongly on both detector design and field size, as well as other variables such as detector off-axis position and detector azimuthal angle. Mass density is found to be the principal determinant of detector water non-equivalence. Furthermore, it is shown that it is possible to compensate for the mass-density of a detector cavity by incorporating additional components of contrasting mass-density into that detector’s design. For small cavities, such design modifications enable the detector’s small- to large- field response ratio to be matched to that of a “point-like” water-structure: ideal detector performance can be achieved across a variety of irradiation conditions. For existing commercial detectors, a Dose Area Product (DAP) formalism is also developed and shown to be much more robust than the point-dose correction factor approach. In conclusion, correction factor values for existing detector designs depend on a host of variables and their calculation typically relies on the use of time-intensive Monte Carlo methods. This thesis indicates that future moves towards density-compensated detector designs or DAP-based protocols can simplify the methodology of small field dosimetry.

Accurate dosimetry is a fundamental requirement for the safe and efficient use of radiation in medical applications. International Codes of Practice, such as IAEA TRS-398 (2000) for radiotherapy beams and IAEA TRS-457 (2007) for diagnostic radiology beams, provide the necessary formulation for reference and relative dosimetry and the data required for their implementation. Research in recent years has highlighted the shortage of such data for radiotherapy small photon beams and for surface dose estimations in diagnostic and interventional radiology, leading to significant dosimetric errors that in some instances have jeopardized patient’s safety and treatment efficiency. The aim of this thesis is to investigate and determine key data for the reference and relative dosimetry of radiotherapy and radiodiagnostics beams. For that purpose the Monte Carlo system PENELOPE has been used to simulate the transport of radiation in different media and a number of experimental determinations have also been made. A review of the key data for radiotherapy beams published after the release of IAEA TRS-398 was conducted, and in some cases the considerable differences found were questioned under the criterion of data consistency throughout the dosimetry chain (from standards laboratories to the user). A modified concept of output factor, defined in a new international formalism for the dosimetry of small photon beams, requires corrections to dosimeter readings for the dose determination in small beams used clinically. In this work, output correction factors were determined, for Varian Clinac 6 MV photon beams and Leksell Gamma Knife Perfexion 60Co gamma-ray beams, for a large number of small field detectors, including air and liquid ionization chambers, shielded and unshielded silicon diodes and diamond detectors, all of which were simulated by Monte Carlo with great detail. Backscatter factors and ratios of mass energy-absorption coefficients required for surface (skin) determinations in diagnostic and interventional radiology applications were also determined, as well as their extension to account for non-standard phantom thicknesses and materials. A database of these quantities was created for a broad range of monoenergetic photon beams and computer codes developed to convolve the data with clinical spectra, thus enabling the determination of key data for arbitrary beam qualities. Data presented in this thesis has been contributed to the IAEA international dosimetry recommendations for small radiotherapy beams and for diagnostic radiology in paediatric patients. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 6: Manuscript.</p>