Room: AAPM ePoster Library
Experimental determination of dosimetric parameters in small fields is challenging and exhibit a high degree of uncertainty. The present study analyzes the impact of the ion chamber orientation in the determination of dosimetric parameters.
A TrueBeam STx equipped with HDMLC and photon energies of 6 and 10 MV was used. A PTW 31016 3D PinPoint ion chamber perpendicular and parallel orientation to the beam axis was used to measured dosimetric parameter using a 3D Blue Phantom 2 (IBA). A 10x10 cm² field was used to compare the beam quality (D20/D10) using both ion chamber orientations. For MLC field sizes ranging from 0.5?0.5 cm² to 3?3 cm² (primary jaws from 0.8x0.8 cm² to 22x30 cm²) the scatter factors (following the TRS483 recommendations) at SSD=100 cm and d=10 cm was measured and compared. PDD and profiles were measured and compared for the square MLC field sizes. Perpendicular orientation was taken as reference measurement.
For a 10x10 cm² and 6MV (10MV) D20/D10 had a variation of 0.2% (0%). Also, no variation was found for 3x3 cm² for both energies. For 0.5?0.5 cm² D20/D10 differences were 2.0% (0.8%) for 6MV (10 MV). Scatter factors were lower for perpendicular orientation compared to parallel orientation, for both energies. For 0.5?0.5 cm² differences were 5.0% (7.7%) for 6 MV (10 MV). No difference was found for field sizes larger or equal to 2x2 cm². No difference was found in terms of FWHM and penumbra width.
Differences on scatter factors and beam quality of ionization chamber orientations in small fields were associated to volume averaging effect. TRS483 do not specify ion chamber orientation for scatter factors and PDD measurements, but careful on dosimetric parameters acquisition relative to ionization chamber orientation should be consider.