Author: Wesley S. Culberson, Larry A. DeWerd, Ahtesham Ullah Khan, Zachary James Welchel 👨🔬
Affiliation: Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison, University of Wisconsin-Madison 🌍
Purpose: Flattening filter free (FFF) linac beams exhibit a centrally peaked beam profile due to the forward-directed bremsstrahlung production from MeV electrons, leading to volume averaging effects in ionization chamber measurements. This study investigates the radial profile correction factor (Prp) for Farmer-type chambers under varying photon beam energy, symmetry, and source-to-surface distances (SSDs).
Methods: Monte Carlo (MC) calculated beam profiles were generated utilizing a linac beam model developed in TOPAS. Profiles were generated for 4MV-12MV photon beams at SSDs of 80 cm and 100 cm. To evaluate the effect of beam symmetry on Prp, the angle radial current was adjusted on a Varian TrueBeam to produce varying inline dose profiles, which were measured with a PTW microDiamond detector and DoseView 3D water tank. Profiles were fit to polynomials, and Prp was determined through integration of each polynomial over a length of 2 cm, which represented the active length of a Farmer-type chamber.
Results: Simulated profiles demonstrated greater spatial variation near the central axis at 80 cm SSD, leading to higher Prp values compared to 100 cm SSD. Prp increased with beam energy, ranging from 1.0011-1.0042 (dd%(10): 61.0%-72.7%) at 100 cm SSD and 1.0020-1.0061 (dd%(10): 57.3%-69.4%) at 80 cm SSD. Beam symmetry variations within ±3% had minimal impact on Prp, with mean corrections of 1.0020 and 1.0038 for 6FFF and 10FFF respectively.
Conclusion: Calculated Prp values demonstrated a strong dependence on beam quality, and a relationship between Prp and dd%(10) was determined which reduces the need for manual calculations based on beam profiles. Prp was determined to be sensitive to SSD which is consistent with the behavior of kvol reported in IAEA TRS-483 and TRS-398 protocols. Additionally, Prp exhibited minimal variation across the evaluated beam symmetries, suggesting Prp changes can be disregarded when beam symmetry remains within clinically acceptable limits.