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= 0.6 R 50− 0.1   cm , where R 50is the depth of 50% depth dose. In order to calibrate electron beams at d refwith a Farmer type cylindrical ionization chamber, the values of the perturbation correction factors P walland P flat d refare required. Using a parallel plate Holt chamber as a reference chamber, the product P wallP flhas been determined for a 6.1‐mm‐diameter PTW cylindrical ionization chamber at d refas a function of R 50of clinical electron beams ( 6 ⩽ nominal energy E ⩽ 22   MeV ). Assuming that P wallfor the PTW chamber is unity in electron beams, the measured P flvalues 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 refrequires the knowledge of R 50 . A relation between I 50and R 50is 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= 1.029 × I 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 50from I 50 , where I 50is the depth of 50% ionization. The largest difference between the measured value of R 50and that calculated by using the above equation has been found to be about 1 mm at 22 MeV.