Cobalt teletherapy small field dosimetry

A research report submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in the field of Medical Physics. 2017Aim The aim of this research was to contribute to clinical implementation of the small field dosimetry Code of Practice (CoP) that is due to be published by the International Atomic Energy Agency (IAEA) in collaboration with the American Association of Physicists in Medicine (AAPM) (1). A 6 cm × 6 cm virtual machine-specific reference (fmsr) field was established in a clinical 60Cobalt teletherapy beam used for conventional radiotherapy at Charlotte Maxeke Johannesburg Academic Hospital, and relative output factors were measured down to a set field size of 1 cm × 1 cm using three different models of Physikalisch-Technische Werkstatten (PTW) small field ionization chambers. Materials and Methodology The measurements were all performed on a Cobalt teletherapy unit (MDS Nordion Equinox, S/N 2009) in a PTW MP3 water phantom. The small field ionization chambers that were used were a PTW 31016 3D pinpoint 0.016 cm3, a PTW 31006 pinpoint 0.015 cm3 and a PTW 31010 semiflex 0.125 cm3. A calibrated PTW 30013 Farmer 0.6 cm3 ionization chamber was used to provide traceability for the cross calibration. A ―daisy chain‖ methodology was used to perform the cross calibration in a virtual fmsr field of 6 cm × 6 cm and then establish the absolute dose rate in a 4 cm × 4 cm field. Relative output factors as a function of field size were measured with each small field ionization chamber and then compared to published results. Results Small square fields from a 60Cobalt beam were created using the secondary collimators integrated into the unit. Equivalent square fields were calculated using the profiles obtained by the three ionization chambers during scanning and were in agreement with the ones that were programmed into the console. The coincidence of the central axis of the beam and the point of measurement for each detector was determined from the beam profiles. The cross calibration and daisy chain measurements resulted in a consistent dose rate of within + 0.6% in the 4 cm × 4 cm field when measured with the four different ionization chambers. For 6 cm × 6 cm, 4 cm × 4 cm, 3 cm × 3 cm and 2 cm × 2 cm field sizes, relative output factors obtained from the uncorrected detectors’ response agreed to within + 0.8 % between the three small field ionization chambers. The variation in the 1 cm × 1 cm field size was + 8.1 %. When compared to published data, large differences in field size correction factors were obtained. Conclusion Small field dosimetry in a 60Cobalt photon beam using three different PTW small field ionization chamber models was investigated. A cross calibration in a virtual msr field was done followed by a daisy chain process to determine the dose rate in a small field. Dose profiles and relative output factors were then measured and compared. The lack of lateral charge particle equilibrium and volume averaging effect was evident when using the PTW 31010 semiflex chamber in a 1 cm × 1 cm field. The PTW 31006 pinpoint and 31016 3D pinpoint were in close agreement for field sizes down to 1 cm × 1 cm with the 3D pinpoint performing as the best detector in this study. The optimal positioning of a detector should be determined from beam profile scans and not the engineering diagrams. The PTW 31016 3D pinpoint and PTW 31006 pinpoint are recommended for the determination of output factors in small field sizes. However, field output correction factors are required for both detectors in field sizes under 2 cm × 2 cm. Small field data published in the British Journal of Radiology (BJR) Supplement 25 (2) should not be used to benchmark dosimetry in modern 60Cobalt teletherapy units.MT 201