Fig. 4.

Measurements of normalized absorbed dose (D) versus lateral position (y), revealing perturbations caused by 6.4-mm, 9.5-mm, and 15.9-mm diameter stainless steel spheres in a proton beam with a mean residual range of 161 mm. As in Fig. 3, the air gaps presented are (a) 20 mm, (b) 150 mm, (c) 300 mm, and (d) 630 mm. The same general features are observed as in Fig. 3, though differences are apparent between the curves resulting from the old and new multiple Coulomb scattering algorithms. In general, the results from the new algorithm more accurately represent the measurements and are an improvement where the old algorithm underestimates dose perturbations.