Dosimetry for new radiotherapy modalities: Towards improved traceablility for scintillator dosimetry in small MV photon beams

Abstract

For over a century, radiotherapy treatments have been used as weapon against cancer. The main objective of radiotherapy is to deliver high dose to the tumor and spare the adjacent healthy tissues, and accuracy is of vital importance when delivering these treatments. The inclusion of multi-leaf collimators and image guidance in megavoltage linear accelerators has enabled complex shaping of the beam with irradiation field sizes smaller than 1×1 cm2. These improvements in treatment techniques pose several dosimetric challenges, and measurement of the absorbed dose in such small fields using conventional guidelines and detectors developed for larger fields with nearly full charge-particle equilibrium was found to lead to a large spread in results. The effect of this inconsistency in dose would potentially result in an over- or under-treatment of patients, and hospitals was therefore not be able to use the new treatment technology to its full potential before these problems were resolved. In 2017, the International Atomic Energy Agency (IAEA) and the American Association of Medical Physics (AAPM) published a first code of practice (TRS-483)for determining the absorbed dose under small field conditions for MV photon beams. Since primary standards for small field dosimetry are not well established, the code of practice could not be directly based on such standards. The code of practice therefore derived traceability to the gray (Gy) in the internaternational system of units (SI) using standards for conventional large fields combined with Monte-Carlo computed correction factors for small fields. However, the code of practice highlights the potential of using fiber-coupled organic plastic scintillators for direct measurements of output factors as these detectors (i) are practically water equivalent, and (ii) have small sensitive volumes. One issue of concern, however, is that scintillating detectors suffer from signal loss during irradiation, called ionization quenching. The importance of this effect was not directly addressed in the TRS-483 code of practice, and a main objective of the present study therefore was to assess the importance of ionization quenching in MV photon beam dosimetry with organic plastic scintillators, in particular for measurements of field output factors. A Monte Carlo-based method was developed to evaluate the importance of ionization quenching in organic plastic scintillators during MV photon dosimetry. The method accounts for dose deposition by secondary electrons based on a modified version of Birks law. Ionization quenching was found to have a small but statistically significant influence on two relevant applications: (0.6± 0.2) % for the field output factor measurements between 0.5×0.5 cm2 and 10×10 cm2 and about (2± 0.4) % for the ionization chamber kQ-factor measurements for beams between 4 MV and 15 MV. The modelling results were in agreement with experimental measurements. The results support that the ionization quenching effect has a small effect on field-output factor measurements and it can probably be neglected during clinical measurement conditions. Finally, a new dosimetric system based on graphite calorimetry and a scintillator transfer detector for direct measurement of traceable field output factors was proposed. Monte-Carlo computations support that the designed calorimeter is suitable for establishing an alternative, more direct route to traceable measurements of absorbed doses in small field sizes down to 1×1 cm2 (less than 3 % correction). An important feature of the new scintillator transfer detector is that it provides an improved blinding-technique for separating the scintillator signal from the stem signal (Cerenkov light and fluorescence produced in the optical fiber cable by the primary beam and scattered radiation). The proposed dosimetric system supports the existing TRS-483 code of practice and it provides an alternative, more direct route to traceable field output factor measurements. The work in this thesis therefore is a step towards improvements in radiotherapy treatments.