Experimental determination of fluence correction factors at depths beyond dmax for a Farmer type cylindrical ionization chamber in clinical electron beams

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Abstract
Recently, it has been recommended that electron beam calibrations be performed at a new reference depth [Burns et al., Med. Phys. 23, 383 (1996)] given by dref = 0.6R50-0.1 cm, where R50 is the depth of 50% depth dose. In order to calibrate electron beams at dref with a Farmer type cylindrical ionization chamber, the values of the perturbation correction factors Pwall and Pfl at dref are required. Using a parallel plate Holt chamber as a reference chamber, the product PwallPfl has been determined for a 6.1-mm-diameter PTW cylindrical ionization chamber at dref as a function of R50 of clinical electron beams (6 < or = nominal energy E < or = 22 MeV). Assuming that Pwall for the PTW chamber is unity in electron beams, the measured Pfl values ranged from 0.96 to 0.98 as the energy is increased. These results are in close agreement with recently reported calculated values. Determination of dref requires the knowledge of R50. A relation between I50 and R50 is given in the IAEA Protocol [TRS No. 277 (IAEA, Vieńna, 1987), pp. 1-98] for broad beams at SSD = 100 cm. It has been shown experimentally that the equation R50 = 1.029 x I50-0.063 cm, derived by Ding et al. [Med. Phys. 22, 489 (1995)] from Monte Carlo simulations of realistic clinical electron beams, can be used satisfactorily to obtain R50 from I50, where I50 is the depth of 50% ionization. The largest difference between the measured value of R50 and that calculated by using the above equation has been found to be about 1 mm at 22 MeV.