SU‐GG‐T‐411: Monte‐Carlo‐Based Cavity Correction Factors for Ion Chambers in Clinical Electron Beams

Abstract

Purpose: To calculate the cavity correction factor, Pcav at various depths for cylindrical chambers and plane‐parallel chambers, NACP‐02 and ROOS, in high‐energy electron beams by means of the EGSnrc/Cavity code. Method and Materials: The chamber cavity was simulated in detail by the EGSnrc/Cavity code. Pcav was calculated at various depths which include a reference depth, dref, and a half‐value depth, R50, for 6, 9, 15, and 18 MeV electron beams. The effective point of the calculation was at a point shifted 0.5r upstream from the cavity center for cylindrical chambers and was the front face of the cavity for plane‐parallel chambers. Pcav for the cylindrical chamber cavities were calculated with combinations of the diameters of 2, 4, and 6 mm and the lengths of 5, 10, and 20 mm. The results were compared with the values recommended by TRS‐398 and other published data. Results: Pcav values increased as a function of a depth. Especially, the Pcav values at dref for cylindrical chambers were 4–10% higher than those at R50 in 6 MeV. Pcav values were lower with increasing the cavity diameter and decreasing the cavity length. This is because the lateral scattered electrons entering into the chamber cavity from the surrounding water increase. The effect is significant in lower‐energy beams. Similarly, the Pcav values are 0.993 at dref and 1.030 at R50 for NACP‐02 and 1.001 and 1.030 for ROOS in 6 MeV. Pcav for NACP‐02 varies depending on depths (electron mean energy). The variation in Pcav for ROOS values is smaller than NACP‐02 because its guard‐ring is larger. Conclusion: The calculated Pcav value was sufficiently different from TRS‐398 data. The Pcav values vary depending on electron mean energy and the chamber cavity size. For plan‐parallel chambers, the sufficient guard‐ring width needs to decrease the cavity correction.