EXPERIMENTAL-DETERMINATION OF FLUENCE CORRECTION FACTORS AT DEPTHS BEYOND D(MAX) FOR A FARMER TYPE CYLINDRICAL IONIZATION-CHAMBER IN CLINICAL ELECTRON-BEAMS
Ms. Huq et al., EXPERIMENTAL-DETERMINATION OF FLUENCE CORRECTION FACTORS AT DEPTHS BEYOND D(MAX) FOR A FARMER TYPE CYLINDRICAL IONIZATION-CHAMBER IN CLINICAL ELECTRON-BEAMS, Medical physics, 24(10), 1997, pp. 1609-1613
Recently, it has been recommended that electron beam calibrations be p
erformed at a new reference depth [Burns et al., Med. Phys. 23, 383 (1
996)] 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 Fa
rmer type cylindrical ionization chamber, the values of the perturbati
on correction factors P-wall and P-fl at d(ref) are required. Using a
parallel plate Holt chamber as a reference chamber, the product PwallP
fl has been determined for a 6.1-mm-diameter PTW cylindrical ionizatio
n 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 in
creased. These results are in close agreement with recently reported c
alculated 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 [TR
S No. 277 (IAEA, Vienna, 1987), pp. 1-98] for broad beams at SSD=100 c
m. It has been shown experimentally that the equation R-50 = 1.029XI(5
0)-0.063 cm, derived by Ding et al. [Med. Phys. 22, 489 (1995)] from M
onte Carlo simulations of realistic clinical electron beams, can be us
ed 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.