Sensitive volume effects on Monte Carlo calculated ion chamber response in magnetic fields

Abstract

Purpose: The development of magnetic resonance-guided radiation therapy (MRgRT) necessitates accurate Monte Carlo (MC) models of ion chambers for computing ion chamber corrections to compensate for the presence of the magnetic field. This study evaluates the sensitivity of the ion chamber dose response in a magnetic field on the collection volume used in the MC simulation. Methods: The EGSnrc system's egs_chamber application is used with a recently developed and validated magnetic field transport code. The calculated dose to the sensitive volume of the chamber per unit incident photon fluence, normalized to that at 0 T, is evaluated as a function of magnetic field for the PTW 30013, PTW 31006, PTW 31010, Exradin A12S, and Exradin A1SL chambers. The sensitive region is varied by excluding the volume corresponding to either 0, 0.5, or 1 mm of distance away from the stem. The photon field, magnetic field, and ion chamber are all oriented perpendicular to each other as in the majority of published experimental works. Results: The calculations for a Co-60 source demonstrate that variations from the 0 mm simulations are on the order of several percent with a maximum deviation, occurring at 0.5 T, of 1.75 AE 0.03% and 3.39 AE 0.06% for the 0.5 mm or 1 mm simulations, respectively, for a 0.057 cm 3 A1SL chamber. Larger volume chambers showed smaller, but still non-negligible, variations. Simulations of the A1SL chamber with a 7 MV photon source, corresponding to the Elekta MR-linac machine, demonstrate that the effect is slightly reduced but still persists with a maximum deviation of 1.97 AE 0.08% for the 1 mm reduction. Conclusions: Usually, the geometric sensitive volume of the ion chamber is used in MC calculation as a substitute for the potentially unknown, smaller, true collection volume (governed by the complex electric field distribution inside the chamber). The calculations in this study demonstrate that even a small variation in simulated volume can lead to fairly large variations in the MC calculated ion chamber response in a magnetic field. This is an important effect that must be addressed to ensure proper calibration of MRgRT machines using MC ion chamber correction factors. This effect may play a role, even where there is no magnetic field, in small-field dosimetry when volume averaging effect are important.

Monte Carlo study of ionization chamber magnetic field correction factors as a function of angle and beam quality

“…On the whole, the 60 Co simulations demonstrate a larger variation on k as the volume is reduced because at the lower energy, compared to the 7 MV beam, secondary electrons having a tighter radius of curvature in the magnetic field. Additional analysis of the impact of the sensitive volume on the dose‐to‐air in the sensitive volume at 90° and 270° can be found in our earlier paper …”

“…The details on the chambers used for the calculations in Section 2.A. are found in Fig. , and additional information is in previous studies . The chambers are the Exradin A1SL, A12S, and A19, the PTW 31006 and 31010, and the NE 2571.…”

“…Previously, the magnetic field transport code used in this work has been benchmarked via the Fano cavity test and comparison to experimental work . Experimental variables such as beam orientation and the unknown sensitive volume have been noted to cause discrepancies between measurements and Monte Carlo models, particularly at a magnetic field strengths higher than 1 T .…”

“…As mentioned above, when a magnetic field is present the true size of the ion chamber's collection volume plays an important role in the simulated dose‐to‐air . This is due to the charge collection electric field of the chamber having a fringing effect near the stem and guard electrode, and a portion of “dead" air is formed from which the charge released is not collected .…”

“…The dependence of both %dd(10) x and TPR on magnetic fields for several beam qualities are explored in this work. Furthermore, the effect of the true sensitive collection volume of the ion chamber has been found to impact the MC determined chamber response, that is, dose to the chamber's air cavity, when a magnetic field is present . Here the variation in k due to changes in the sensitive volume are looked at, and a resolution of the issue is sought through finding an optimal orientation with respect to the magnetic field.…”