Technical Note: Sensitive volume effects on ion chamber responses in longitudinal magnetic fields.

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Computational Physics

Background:

• Ion chamber dose response simulations are crucial for accurate radiation therapy.
• Previous studies explored magnetic field effects on ion chambers in transverse fields.
• The impact of sensitive volume variations in longitudinal magnetic fields remains less understood.

Purpose of the Study:

• To investigate the effect of ion chamber sensitive volume on dose response in longitudinal magnetic fields.
• To determine if sensitive volume changes influence Monte Carlo simulation accuracy under these conditions.
• To extend previous findings from transverse to longitudinal magnetic field orientations.

Main Methods:

• Utilized the EGSnrc/egs_chamber application for Monte Carlo simulations.
• Modeled Exradin A19, A28, and A1SL ion chambers in uniform longitudinal magnetic fields.
• Varied the ion chamber sensitive volume by excluding air volumes (0.2–3 mm) near the stem and normalized dose response to 0 T.

Main Results:

• When aligned parallel to radiation and magnetic fields, all chambers showed dose responses within statistical limits across sensitive volume variations.
• In perpendicular alignment, A28 and A1SL chambers exhibited negligible effects.
• The A19 chamber showed a 0.5% under-response with a 3 mm sensitive volume reduction in perpendicular alignment.

Conclusions:

• Sensitive volume changes do not affect ion chamber dose response in longitudinal magnetic fields when the chamber axis is parallel to the fields.
• Perpendicular alignment shows sensitive volume-dependent dose response changes only in the A19 chamber for ≥1 mm insensitive volume.
• Monte Carlo correction factors for similar chambers under comparable conditions can assume complete charge collection.