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

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 d(ref) = 0.6R(50)-0.1 cm, where R-50 is the depth of 50% depth dose. In order to calibrate electron beams at d(ref) with a Farmer type cylindrical ionization chamber, the values of the perturbation correction factors P-wall and P-fl at d(ref) 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 d(ref) as a function of R-50 of clinical electron beams (6 less than or equal to nominal energy E less than or equal to 22 MeV). Assuming that P-wall for the PTW chamber is unity in electron beams, the measured P-fl 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 d(ref) requires the knowledge of R-50. A relation between I-50 and R-50 is given in the IAEA Protocol [TRS No. 277 (IAEA, Vienna, 1987), pp. 1-98] for broad beams at SSD=100 cm. It has been shown experimentally that the equation R-50 = 1.029XI(50)-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 R-50 from I-50, where I-50 is the depth of 50% ionization. The largest difference between the measured value of R-50 and that calculated by using the above equation has been found to be about 1 mm at 22 MeV. (C) 1997 American Association of Physicists in Medicine.