ON THE USE OF UNSHIELDED CABLES IN IONIZATION-CHAMBER DOSIMETRY FOR TOTAL-SKIN ELECTRON THERAPY

The dosimetry of total-skin electron therapy (TSET) usually requires i onization chamber measurements in a large electron beam (up to 120 cm x 200 cm). Exposing the chamber's electric cable, its connector and pa rt of the extension cable to the large electron beam will introduce un wanted electronic signals that may lead to inaccurate dosimetry result s. While the best strategy to minimize the cable-induced electronic si gnal is to shield the cables and its connector from the primary electr ons, as has been recommended by the AAPM Task Group Report 23 on TSET, cables without additional shielding are often used in TSET dosimetry measurements for logistic reasons, for example when an automatic scann ing dosimetry is used. This paper systematically investigates the cons equences and the acceptability of using an unshielded cable in ionizat ion chamber dosimetry in a large TSET electron beam. In this paper, we separate cable-induced signals into two types. The type-I signal incl udes all charges induced which do not change sign upon switching the c hamber polarity, and type II includes all those that do. The type-I si gnal is easily cancelled by the polarity averaging method. The type-II cable-induced signal is independent of the depth of the chamber in a phantom and its magnitude relative to the true signal determines the a cceptability of a cable for use under unshielded conditions. Three dif ferent cables were evaluated in two different TSET beams in this inves tigation. For dosimetry near the depth of maximum buildup, the cable-i nduced dosimetry error was found to be less than 0.2% when the two-pol arity averaging technique was applied. At greater depths, the relative dosimetry error was found to increase at a rate approximately equal t o the inverse of the electron depth dose. Since the application of the two-polarity averaging technique requires a constant-irradiation cond ition, it was demonstrated that an additional error of up to 4% could be introduced if the unshielded cable's spatial configuration were alt ered during the two-polarity measurements. This suggests that automati c scanning systems with unshielded cables should not be used in TSET i onization chamber dosimetry. However, the data did show that an unshie lded cable may be used in TSET ionization chamber dosimetry if the siz e of cable-induced error in a given TSET beam is pre-evaluated and the measurement is carefully conducted. When such an evaluation has not b een performed, additional shielding should be applied to the cable bei ng used, making measurements at multiple points difficult.