61. Dose-area-product determination with large planes-parallel ionization chambers: New approach to reference dosimetry in active scanning protontherapy

Abstract

Purpose Reference dosimetry in pencil beam scanning (PBS) proton therapy is not still well established. Recently a few experiences dealing with the use of large area planes-parallel ionization chambers (LPPICs) in determining integrated-dose-to-water (Dose-Area-Product, DAPw) were reported [1] . Aim of this work was to determine DAP w,Q (equal to M Q ∗ N DAPw,Q0 ∗ K Q,Q0 ) with a LPPIC using a cross-calibrated chamber factor (NDAPw) obtained directly under a proton beam and not via Co 60 calibration. Methods Absorbed point dose-to-water (Dw) determined with a small detector in the middle of a broad homogeneous field obtained as the superposition of proton pencil beams regularly-spaced by Δ x and Δ y is equivalent to a determination of DAPw using a LPPIC and a single pencil beam at the same depth. Thus, as a valid approximation, DAPw = Dw*Δx*Δy. A LPPIC (Bragg peak chamber PTW-37040, radius = 4,1 cm) was cross-calibrated in water vs. a reference Markus type IC in the entrance region of a 16 × 16 cm2 homogeneous 190 MeV scanned proton pseudomonoenergetic layer with a spot spacing of 2 mm. Then, determinations of DAPw for single pencil beams (energy from 70 to 226 MeV) at 2 cm depth in water were performed. The values of DAPw scored with LLPIC were compared to those obtained with a PPC05 (Iba-Dosimetry) in the middle of 10 × 10 cm2 unmodulated layers of the same energies at the same depth. Results Chamber response homogeneity has been assessed with good results. The cross-calibrated N DAPw was equal to 1,512E08 Gy/C. K Q,Q0 was set to 1 as the same beam quality was used both for cross-calibration and measurements. Figure and table show the results of the comparison in terms of DAPw between the LLPIC and PPC05. Download : Download high-res image (441KB) Download : Download full-size image Conclusions Reference dosimetry based on DAPw and LPPICs is feasible. DAPw values got with Bragg peak chamber in single pencil beams and with PPC05 in large scanned layers are in good agreement. The determination of N DAPw via cross-calibration in proton beams allows to get rid of chamber specific K Q,Q0 factors reducing uncertainties.