K(Q) FACTORS FOR IONIZATION-CHAMBER DOSIMETRY IN CLINICAL PROTON-BEAMS

We discuss a formalism for clinical proton beam dosimetry based on the use of ionization chamber absorbed dose-to-water calibration and beam quality correction factors. A quantity k(Q), the beam quality correct ion factor, is defined which corrects the absorbed dose-to-water calib ration factor N-D,N-w, in a reference beam of quality Q(0) to that in a user's beam of quality Q(1). This study of proton beam quality corre ction factors used Co-60 (k(Q gamma)) and proton (k(Qp)) reference bea ms. The k(Q gamma) factors were measured using combined water calorime try and ionometry for PTW and Capintec-Farmer-type ionization chambers , and were computed from standard dosimetry protocols. Agreement betwe en measured and calculated k(Q gamma), values for both chambers was fo und within 1.2% in the plateau region for a monoenergetic 250-MeV beam and within 1.8% at the spread-out Bragg peak for a 155-MeV range-modu lated beam. Comparison of absorbed doses to water determined in the ra nge modulated 155-MeV beam was performed with the PTW chamber using th ree calibration methods: N-gas calibration (AAPM Report 16), N-D,N-w,N -gamma calibration in a Co-60 beam in conjunction with a k(Q gamma) fa ctor, and N-D,N-w,N-p calibration in a proton beam in conjunction with a k(Qp), factor. Absorbed doses to water obtained with the three meth ods agreed within 2% when ionization chamber dosimetry data were analy zed using the proton W-value for air from the AAPM Report 16 and the I CRU 49 proton stopping powers. The use of the proton-calibrated refere nce ionization chamber, in conjunction with the beam quality correctio n factor k(Qp), significantly reduced the systematic uncertainty of th e absorbed dose determination. (C) 1996 American Association of Physic ists in Medicine.