Electron beam quality k(Q,Q0) factors for various ionization chambers: a Monte Carlo investigation with PENELOPE.

In this work we calculate the beam quality correction factor k(Q,Q0) for various plane-parallel ionization chambers. A set of Monte Carlo calculations using the code PENELOPE/PENEASY have been carried out to calculate the overall correction factor f(c,Q) for eight electron beams corresponding to a Varian Clinac 2100 C/D, with nominal energies ranging between 6 MeV and 22 MeV, for a (60)Co beam, that has been used as the reference quality Q0 and also for eight monoenergetic electron beams reproducing the quality index R50 of the Clinac beams. Two field sizes, 10 × 10 cm(2) and 20 × 20 cm(2) have been considered. The k(Q,Q0) factors have been calculated as the ratio between f(c,Q) and f(c,Q0). Values for the Exradin A10, A11, A11TW, P11, P11TW, T11 and T11TW ionization chambers, manufactured by Standard Imaging, as well as for the NACP-02 have been obtained. The results found with the Clinac beams for the two field sizes analyzed show differences below 0.6%, even in the case of the higher energy electron beams. The k(Q,Q0) values obtained with the Clinac beams are 1% larger than those found with the monoenergetic beams for the higher energies, above 12 MeV. This difference can be ascribed to secondary photons produced in the linac head and the air path towards the phantom. Contrary to what was quoted in a previous work (Sempau et al 2004 Phys. Med. Biol. 49 4427-44), the beam quality correction factors obtained with the complete Clinac geometries and with the monoenergetic beams differ significantly for energies above 12 MeV. Material differences existing between chambers that have the same geometry produce non-negligible modifications in the value of these correction factors.