Characterization of small megavoltage photon beams for radiography

[摘要] English: IntroductionThe landscape of radiation treatment techniques is ever evolving in pursuit of improved targetcoverage. The latest techniques such as IMRT, SBRT, SRS and VMAT, provide improved targetcoverage by controlling the intensity of the given dose through the use of multiple small fields incontrast to large fields in conventional treatments. The advantage of using these large fields isthat, their characteristics are fully understood.The introduction of small fields leads to improved coverage, but the physics of these fields arenot fully understood. So, when used in patient treatment, it resulted in unaccounted radiationexposure due to inaccurate commissioning and inaccurate absolute dose calibration at these fieldsizes. The errors were due to incorrect detectors used for data collection, and incorrectapplication of factors when performing absolute dose calibration.This report investigated the characteristics of these small fields using different detectors whilstvarying the SSD and the incident photon beam energy. The measurements included beamprofiles, percentage depth dose (PDD) curves as well as the relative output factors (ROF).Materials and MethodsThe photon energies, 6 MV, 10 MV and 15 MV were delivered using the Synergy LINAC, which isequipped with Agility multileaf collimators (MLCs). The detectors that were investigated werethe CC01 ion chamber, EFD-3G diode, PTW60019 microdiamond, EBT2 radiochromic film and theEDR2 radiographic film. Measurements were carried out using water as a medium for the CC01ion chamber, EFD-3G diode and the PTW60019. Films were placed in between water equivalentRW3 phantom slabs. These measurements were carried out at 90 cm, 95 cm, 100 cm and 110 cmsource 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. The10×10 cm² field size was included as a reference field.Results and DiscussionThe results showed that the beam profiles were insignificantly different at the various SSDs forthe detectors. The EBT2 film showed the sharpest penumbra, with the EDR2 and the CC01showing broad penumbrae, but the difference was negligible.The PDD measurements showed that the difference between the detectors after Depth ofmaximum 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 werebeing analyzed. The PDD measurements indicated that the setup used for the films was notadequate 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 wasshown that there is a change in the manner in which this factor changes with field size. Below acertain field size, 2 cm for the 6 MV and 10 MV and 3 cm for the 15 MV, the Dmax would startshifting 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 hadlateral electronic equilibrium (LEE). This relation broke down as the field sizes decreased due tothe onset of lateral electronic disequilibrium (LED). The high-density detector, PTW60019 gavethe highest ROF for the different energies, with the less dense CC01 giving the lowest ROFs. Thisshowed that the density of the detector had an effect on the output factor measured.ConclusionThe fields were characterized with the different detectors, barring the artefacts experienced withfilm measurements in some instances, these detectors can be used safely for the small fields. TheROFs can be measured at longer SSDs as they showed little variation due to increased SSDs.

[发布机构] University of the Free State