Calculation of pion dose distributions in water

Techniques for calculating negative pion beam depth and off-axis dose distributions in a water phantom have been developed at the Clinton P. Anderson Meson Physics Facility in Los Alamos. The superposition of the unmodulated depth-dose curve produced modulated depth-dose curves. The addition of the collimator neutron dose, which has been shown to depend on field size, to the modulated depth-dose curve yields the collimated depth-dose distributions. Off-axis dose distributions under a collimator are produced by calculating the distortion of the uncollimated beam caused by multiple Coulomb scattering and beam phase space. Several comparisons of calculated and measured distributions are shown with agreement of normally +/- 3% of peak dose of +/- 3 mm for a particular dose contour. These distribution are them modified by computerized tomographic data to give patient isodose distributions.