Experimental determination and verification of the parameters used in a proton pencil beam algorithm

We present an experimental procedure for the determination and the verifica tion under practical conditions of physical and computational parameters us ed in our proton pencil beam algorithm. The calculation of the dose deliver ed by a single pencil beam relies on a measured spread-out Bragg peak, and the description of its radial spread at depth features simple specific para meters accounting individually for the influence of the beam line as a whol e, the beam energy modulation, the compensator, and the patient medium. For determining the experimental values of the physical parameters related to proton scattering, we utilized a simple relation between Gaussian radial sp reads and the width of lateral penumbras. The contribution from the beam li ne has been extracted from lateral penumbra measurements in air: a linear v ariation with the distance collimator-point has been observed. Analytically predicted radial spreads within the patient were in good agreement with ex perimental values in water under various reference conditions. Results indi cated no significant influence of the beam energy modulation. Using measure ments in presence of Plexiglas slabs, a simple assumption on the effective source of scattering due to the compensator has been stated, leading to acc urate radial spread calculations. Dose measurements in presence of complexl y shaped compensators have been used to assess the performances of the algo rithm supplied with the adequate physical parameters. One of these compensa tors has also been used, together with a reference configuration, for inves tigating a set of computational parameters decreasing the calculation time while maintaining a high level of accuracy. Faster dose computations have b een performed for algorithm evaluation in the presence of geometrical and p atient compensators, and have shown good agreement with the measured dose d istributions. (C) 2001 American Association of Physicists in Medicine.