Characterization of small megavoltage photon beams for radiography

English: Introduction The landscape of radiation treatment techniques is ever evolving in pursuit of improved target coverage. The latest techniques such as IMRT, SBRT, SRS and VMAT, provide improved target coverage by controlling the intensity of the given dose through the use of multiple small fields in contrast to large fields in conventional treatments. The advantage of using these large fields is that, their characteristics are fully understood. The introduction of small fields leads to improved coverage, but the physics of these fields are not fully understood. So, when used in patient treatment, it resulted in unaccounted radiation exposure due to inaccurate commissioning and inaccurate absolute dose calibration at these field sizes. The errors were due to incorrect detectors used for data collection, and incorrect application of factors when performing absolute dose calibration. This report investigated the characteristics of these small fields using different detectors whilst varying the SSD and the incident photon beam energy. The measurements included beam profiles, percentage depth dose (PDD) curves as well as the relative output factors (ROF). Materials and Methods The photon energies, 6 MV, 10 MV and 15 MV were delivered using the Synergy LINAC, which is equipped with Agility multileaf collimators (MLCs). The detectors that were investigated were the CC01 ion chamber, EFD-3G diode, PTW60019 microdiamond, EBT2 radiochromic film and the EDR2 radiographic film. Measurements were carried out using water as a medium for the CC01 ion chamber, EFD-3G diode and the PTW60019. Films were placed in between water equivalent RW3 phantom slabs. These measurements were carried out at 90 cm, 95 cm, 100 cm and 110 cm source to surface distances (SSD). The field sizes that were investigated were 1×1 cm², 2×2 cm², 3×3 cm², 4×4 cm², 5×5 cm² and 10×10 cm², these fields sizes were set using Jaws and MLCs. The 10×10 cm² field size was included as a reference field. Results and Discussion The results showed that the beam profiles were insignificantly different at the various SSDs for the detectors. The EBT2 film showed the sharpest penumbra, with the EDR2 and the CC01 showing broad penumbrae, but the difference was negligible. The PDD measurements showed that the difference between the detectors after Depth of maximum dose (Dmax) were insignificant. The films differed significantly at shallower depths, and this can be attributed to setup, as well as the artefacts that showed up when the films were being analyzed. The PDD measurements indicated that the setup used for the films was not adequate for measuring the 1 cm square field sizes and below. Dmax was used to compare the detectors, though it did not vary greatly for the detectors, it was shown that there is a change in the manner in which this factor changes with field size. Below a certain field size, 2 cm for the 6 MV and 10 MV and 3 cm for the 15 MV, the Dmax would start shifting back to the surface instead of moving deeper as expected. The relative output factor (ROF) increased with energy, and this is true for all the fields which had lateral electronic equilibrium (LEE). This relation broke down as the field sizes decreased due to the onset of lateral electronic disequilibrium (LED). The high-density detector, PTW60019 gave the highest ROF for the different energies, with the less dense CC01 giving the lowest ROFs. This showed that the density of the detector had an effect on the output factor measured. Conclusion The fields were characterized with the different detectors, barring the artefacts experienced with film measurements in some instances, these detectors can be used safely for the small fields. The ROFs can be measured at longer SSDs as they showed little variation due to increased SSDs.