Room: AAPM ePoster Library
Purpose: evaluate the impact of smoothing on flattened photon and electron beam characteristics, including flatness and symmetry, as well as the radial profile correction factor (Prp) defined in TG-51 addendum.
Methods: sets of clinical photon (6, 15, and 18 MV) and electron (6, 9, 12, 16, and 20 MeV) beam profiles acquired on matched Varian linear accelerators were retrospectively analyzed. A CC04 field chamber and CC01 or CC04 reference chamber were used during beam scanning. The impact of smoothing on beam flatness, symmetry as well as Prp was evaluated by applying an envelope filter with the kernel sizes at 2, 5, and 10 mm. Beam flatness and symmetry were measured using the Varian formulas and Prp from the inline beam profile for a 0.6-cc farmer-type chamber. The measured beam characteristics were compared against those obtained without smoothing.
Results: beam profiles were found noisy due to the limited sensitivity of CC04 and CC01 chambers, and an envelope filter with increasing kernel size improved reliability of these profiles. A sharp decrease in beam flatness and symmetry was induced by applying a 2-mm envelope filter, and stabilized with the kernel sizes at 5 and 10 mm. The maximum differences in beam flatness induced by smoothing were between 0.34±0.13% and 0.88±0.34%. More dramatic decreases in beam symmetry were observed, ranging from 0.79±0.24% to 1.38±0.49%. Although the Prp values for photon beams before and after profile smoothing were comparable, its coefficient of variation dropped significantly from 0.21% to 0.05% for 6 MV beam, and from 0.38% to 0.06% for 15 and 18 MV beams, respectively.
Conclusion: profile smoothing may cause large changes in photon and electron beam flatness and symmetry, therefore, consistent data post-processing is recommended. The precision of measured photon beam Prp can benefit from appropriate profile smoothing.