External radiotherapy dosimetry on nonstandard fields

Resumo

The dosimetry of the small and intensity modulated fields employed in radiotherapy, with high dose gradients involved, is a quite demanding task. The need for reliable measurements in these beams responds not only to quality assurance requirements, but also to the legal regulations of Radiotherapy (in Spain, Real Decreto 1566/1998 sobre Criterios de Calidad en Radioterapia, and also EURATOM 97/43). The complexity of modern radiotherapy techniques led to an extensive incorporation of thorough treatment dosimetric verification in the hospital quality assurance programs. This verification, previous to the treatment, is performed in order to check that the dose distributions delivered by the radiotherapy machine match the corresponding planned dose distributions within the required tolerances. One work performed in this thesis project consists in the study of different commercial detector arrays, devices widely employed for dosimetric treatment verification. The response of the detectors involved in these devices is determined in order to study the impact of the detector size, technology and layout on the measurement of intensity modulated dose distributions. The capabilities of detector arrays for the detection of fluence variations is also studied, as this is one of the main objectives of treatment verification. On the other hand, also related with the quality assurance of radiotherapy, a new dosimetry protocol is studied and applied to two modern radiotherapy machines, TomoTherapy and CyberKnife, for the determination of absorbed dose to water. Until now, dosimetry protocols ensured the traceability of dose to water through the measurement of 10 cm x10 cm radiation fields under charged particle equilibrium, following the recommendations of conventional dosimetry codes of practice, for example the TRS 398 of the International Atomic Energy Agency (IAEA). Modern radiotherapy techniques involve the use of small radiation fields and intensity modulated fluencies to achieve higher conformation of the dose to the tumour volume. Additionally, there was an increase in the radiotherapy machines involving this type of radiation fields that cannot reproduce the 10 cm x10 cm standard reference field. This situation increases the uncertainty associated to the determination of absorbed dose to water, compromising the quality of treatment planning in these machines. This prompted the creation of a Working Group of reference dosimetry on nonstandard fields through the collaboration of IAEA and the American Association of Physics in Medicine, which published recommendations for the development of a new dosimetry protocol: "A new formalism for reference dosimetry of small and non-standard fields" Medical Physics Volume 35, Issue 11, p. 5179-5186 (November 2008). The application of the new protocol to the above mentioned machines requires the definition of intermediate calibration fields and the measurement and simulation of correction factors associated to different ionization chambers under these radiation fields, being one of the scopes of this doctoral thesis. Finally, alanine dosimetry is a secondary standard of absorbed dose to water in the therapy dose range (1 to 20 Gy), offered by primary laboratories like the National Physical Laboratory (NPL) in UK, the National Institute of Stantards and Technology (NIST) in US, or the Physikalisch-Technische Bundesanstalt (PTB) in Germany. This dosimetry system, which is tissue equivalent and exhibits small energy dependence, involves the use of small detectors, being widely used for small fields dosimetry and the calculation of ionization chamber correction fac- tors. Another piece of work addressed in this thesis consists on the development of an alanine dosimetry system, unique in Spain, through the quantification of the dosimeters signal by electron spin resonance and with traceability to the secondary standard of absorbed dose to water of the Radiation Physics Laboratory at the Universidade de Santiago de Compostela.