Possibility of using cylindrical ionization chambers for percent depth-dose measurements in clinical electron beams

This study investigated the possibility of using cylindrical ionization chambers for percent depth-dose (PDD) measurements in high-energy clinical electron beams.

The cavity correction factor, P(cav), for cylindrical chambers with various diameters was calculated as a function of depth from the surface to R50, in the energy range of 6-18 MeV electrons with the EGSnrc C(++) -based user-code CAVITY. The results were compared with those for IBA NACP-02 and PTW Roos parallel-plate ionization chambers. The effective point of measurement (EPOM) for the cylindrical chamber and the parallel-plate chamber was positioned according to the IAEA TRS-398 code of practice. The overall correction factor, P(Q), and the percent depth-ionization (PDI) curve for a PTW30013 Farmer-type chamber were also compared with those of NACP-02 and Roos chambers.

The P(cav) values at depths between the surface and R50 for cylindrical chambers were all lower than those with parallel-plate chambers. However, the variation in depth for cylindrical chambers equal to or less than 4 mm in diameter was equivalent to or smaller than that for parallel-plate chambers. The P(Q) values for the PTW30013 chamber mainly depended on P(cav), and for parallel-plate chambers depended on the wall correction factor, P(waII), rather than P(cav). P(Q) at depths from the surface to R50 for the PTW30013 chamber was consequently a lower value than that with parallel-plate chambers. However, the variation in depth was equivalent to that of parallel-plate chambers at electron energies equal to or greater than 9 MeV. The shift to match calculated PDI curves for the PTW30013 chamber and water (perturbation free) varied from 0.65 to 0 mm between 6 and 18 MeV beams. Similarly, the shifts for NACP-02 and Roos chambers were 0.5-0.6 mm and 0.2-0.3 mm, respectively, and were nearly independent of electron energy.

Calculated PDI curves for PTW30013, NACP-02, and Roos chambers agreed well with that of water by using the optimal EPOM. Therefore, the possibility of using cylindrical ionization chambers can be expected for PDD measurements in clinical electron beams.