Thesis (Ph.D.) - Cairo University - Faculty of Science - Department of Biophysics

This work aims at providing a comparison between three important detectors (Pinpoint, Semiflex and Markus) that are frequently used in radiation dosimetry in radiotherapy. This is carried out through the employment of these detectors for quantitative estimation of the change in out-of-field dose with important dosimetric parameters such as field size (from 5 x 5 cm² to 30 x30 cm²) and depth (from 1.5 cm to 30 cm) at two different energies (6 MV and 15 MV) and two different collimator angles (0{u00B0} and 90{u00B0}). The change in out-of-field dose with Source-Skin- Distance (SSD) is also studied using these detectors. Results show that, the Pinpoint and Semiflex detectors both reported an increase in out-of-field dose with field size, depth, energy and SSD. In almost all measurements, Pinpoint detector reported considerably higher out-of-field dose values compared to Semiflex. For 6 MV and 0{u00B0} collimator angle, the out-of-field dose at field size of 30 x 30 cm² and at a depth of 1.5 cm is 7.3% for Pinpoint detector compared to 4.3% for Semiflex. At collimator angle of 90{u00B0}, the out-of-field dose is 6.5% for Pinpoint detector compared to 5.5% for semiflex. The outof- field dose for a depth of 30 cm and field size of 10 x 10 cm2 is 7.9% for Pinpoint detector compared to 5.9% for Semiflex. For 15 MV and 0{u00B0} collimator angle, the out-of-field dose at field size of 30 x 30 cm² and at a of depth of 1.5 cm is 7.5% for Pinpoint detector compared 5.1% for Semiflex. At 6 MV, field size of 10 x 10 cm2 and depth of 1.5 cm, the out-of-field dose at SSD 115 cm is 3.7% for Pinpoint detector compared to 3.4% for Semiflex. The considerably higher out-of-field dose values reported by Pinpoint detector compared to Semiflex may be attributed to the relatively higher sensitivity of Pinpoint detector for low doses (such as out-of-field doses). Results also show that there is a high degree of agreement between Semiflex and Markus ionization chamber detectors in the measured out-field doses for all cases. This work also aims to investigate the measured and calculated out-of-field dose for the previously mentioned dosimetric parameters

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