Ph D Thesis: Key Data for the Reference and Relative Dosimetry of Radiotherapy and Diagnostic and Interventional Radiology Beams
Wednesday 22 April 2015
to 16:30 at
Hamza Benmakhlouf (Stockholm University, Department of Physics)
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 nonstandard
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.