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

Purpose: It has been shown that ion chamber dose response Monte Carlo simulations, in transverse magnetic fields, are susceptible to small changes in the scored sensitive volume. Changes in sensitive volumes have been investigated as a surrogate for modeling the true collection volume governed by the electric field. This work has not been fully extended to longitudinal fields. This study investigates the effect of the sensitive volume of ion chambers on magnetic field dose response within longitudinal magnetic fields.

Methods: The egs_chamber application within EGSnrc was used to model the Exradin A19, A28, and A1SL ion chambers within a uniform longitudinal magnetic field. The sensitive volume of the ion chambers was varied by removing portions of the air volume (from 0.2 to 3 mm) closest to the chamber stem from the dose scoring region; the dose to the scoring volume as a function of magnetic fields strength was normalized to 0 T in all cases.

Results: With the chamber long axis oriented parallel to the radiation and magnetic fields, for every magnetic field strength, all investigated chambers' dose response remained inside statistical variations for all investigated chamber sensitive volumes. When the chamber long axis is oriented perpendicular to the radiation and magnetic fields, the A28 and A1SL show negligible sensitive volume effects, while the A19 under responded by 0.5% for the largest removed volume of 3 mm.

Conclusions: When the ion chambers' long axis is parallel to the radiation and magnetic fields, the magnetic field dose response is unaffected by small changes in sensitive volume. When oriented perpendicularly, only the A19 exhibits sensitive volume based magnetic field dose response changes, and only with ≥ 1 mm of assumed insensitive volume. We can therefore safely assume complete charge collection in Monte Carlo calculated correction factors involving similar chambers in similar conditions.