Experimental determination of beam quality factors, kQ, for two types of Farmer chamber in a 10 MV photon and a 175 MeV proton beam.
Absorbed doses determined with a sealed water calorimeter operated at 4 degrees C are compared with the results obtained using ionization chambers and the IAEA TRS-398 code of practice in a 10 MV photon beam (TPR(20,10) = 0.734) and a 175 MeV proton beam (at a depth corresponding to the residual range, R(res) = 14.7 cm). Three NE 2571 and two FC65-G ionization chambers were calibrated in terms of absorbed-dose-to-water in (60)Co at the Swedish secondary standard dosimetry laboratory, directly traceable to the BIPM. In the photon beam quality, calorimetry was found to agree with ionometry within 0.3%, confirming the k(Q) values tabulated in TRS-398. In contrast, a 1.8% deviation was found in the proton beam at 6 g cm(-2) depth, suggesting that the TRS-398 tabulated k(Q) values for these two ionization chamber types are too high. Assuming no perturbation effect in the proton beam for the ionization chambers, a value for (w(air)/e)(Q) of 33.6 J C(-1) +/- 1.7% (k = 1) can be derived from these measurements. An analytical evaluation of the effect from non-elastic nuclear interactions in the ionization chamber wall indicates a perturbation effect of 0.6%. Including this estimated result in the proton beam would increase the determined (w(air)/e)(Q) value by the same amount.
Medin
Joakim
J
Department of Medical Radiation Physics, Lund University, Malmö University Hospital, SE-205 02 Malmö, Sweden. Joakim.Medin@med.lu.se
Ross
Carl K
CK
Klassen
Norman V
NV
Palmans
Hugo
H
Grusell
Erik
E
Grindborg
Jan-Erik
JE
D002138:Calibration; D002151:Calorimetry; D003037:Cobalt Radioisotopes; D004583:Electrons; D007477:Ions; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D017785:Photons; D011522:Protons; D011786:Quality Control; D011874:Radiometry; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy / Q000379:methods*; D014867:Water
D003037:Cobalt Radioisotopes; D007477:Ions; D011522:Protons; D014867:Water