The use of 0.5r_cav as an effective point of measurement for cylindrical chambers may result in a systematic shift of electron percentage depth doses

Abstract

Electron dosimetry can be performed using cylindrical chambers, plane-parallel chambers, and diode detectors. The finite volume of these detectors results in a displacement effect which is taken into account using an effective point of measurement (EPOM). Dosimetry protocols have recommended a shift of 0.5 r cav for cylindrical chambers; however, various studies have shown that the optimal shift may deviate from this recommended value. This study investigated the effect that the selection of EPOM shift for cylindrical chamber has on percentage depth dose (PDD) curves. Depth dose curves were measured in a water phantom for electron beams with energies ranging from 6 to 18 MeV. The detectors investigated were of three different types: diodes (Diode-E PTW 60017 and SFD IBA), cylindrical (Semiflex PTW 31010, PinPoint PTW 31015, and A12 Exradin), and parallel plate ionization chambers (Advanced Markus PTW 34045 and Markus PTW 23343). Depth dose curves measured with Diode-E and Advanced Markus agreed within 0.2 mm at R 50 except for 18 MeV and extremely large field size. The PDDs measured with the Semiflex chamber and Exradin A12 were about 1.1 mm (with respect to the Advanced Markus chamber) shallower than those measured with the other detectors using a 0.5 r cav shift. The difference between the PDDs decreased when a Pinpoint chamber, with a smaller cavity radius, was used. Agreement improved at lower energies, with the use of previously published EPOM corrections (0.3 r cav ). Therefore, the use of 0.5 r cav as an EPOM may result in a systematic shift of the therapeutic portion of the PDD (distances < R 90 ). Our results suggest that a 0.1 r cav shift is more appropriate for one chamber model (Semiflex PTW 31010). K E Y W O R D S dosimetry protocols, electrons, EPOM, PDD, radius of cavity (r cav )

Dosimetric accuracy of a cross-calibration coefficient for plane-parallel ionization chamber obtained in low-energy electron beams using various cylindrical dosimeters

“…10-12 Monte Carlo simulations have shown that, depending on the method of EPOM determination, 0.5rcyl shift is correct or is too large, resulting in a systematic error 13,14 of ionization chamber readings. The smaller is the size of a thimble chamber, the smaller should be ∆z the shift of EPOM 10, 14 . However, for plane-parallel chambers, the lower was the electron beam energy, the larger EPOM shift was observed, as reported in 13, 15 .…”