Publication:

Experimental characterization of ionization chamber stem effects in low-density media for flattened and flattening-filter-free photon beams

Background Accurate dosimetry in low-density media is essential for stereotactic body radiation therapy (SBRT) of lung tumors, particularly with small radiation fields and flattening filter-free (FFF) photon beams. This study experimentally evaluated ionization chamber stem effects under low-density conditions.

Methods This experimental dosimetric study was conducted from March 2025 to June 2025 at the Department of Radiation Oncology, Sutter Gould Medical Foundation, Modesto, CA, USA. A PTW TN30013 Farmer-type ionization chamber was evaluated for 6 MV, 6 MV FFF, 10 MV and 10 MV FFF photon beams in a low-density Styrofoam phantom (0.04–0.05 g/cm³) and in-air conditions for field sizes ranging from 2×2 cm² to 20×20 cm² and depths from 1.5 to 20 cm.

Results Within the phantom, ionization readings for FFF beams were 5–20% lower than those for flattened beams at shallow depths due to reduced scatter. In-air measurements for FFF beams were up to 30% higher than flattened beams for small field sizes. Stem effects remained below 0.6% across all tested conditions. Collimator rotation dependence was negligible ( 0.05). Significant interactions between beam type, setup and depth were observed (p<0.001).

Conclusions The PTW TN30013 chamber demonstrated stable and reliable performance in low-density media relevant to lung SBRT. Stem-related perturbations were clinically negligible, although beam-specific correction factors remain important for FFF small-field dosimetry.