Nuclear interactions of the projectile carbon ion in biological soft tissue for cancer treatment purpose are studied. Elastic interaction of carbon ion with carbon, oxygen and nitrogen nuclei existing in soft tissue leads to beam divergence especially in Bragg peak region, where the carbon ion is slowing down. Monte Carlo simulation shows the amount of carbon ion beam divergence in soft tissue. For carbon ion beam with 2.4 GeV energy and 2mm diameter, at 85mm penetration depth the beam spreads out to 4mm diameter. Non-elastic interactions are modeled as well. Such interactions are important due to secondary radiation produced in patient’s body. The product particles include positrons and neutrons, being important in therapeutic dose verification with PET imaging and extra dose in the hospital ambient, respectively. Computer code ALICE produces reaction cross sections that might be used to roughly estimate the neutron and positron yield. Computer code ALICE was used to assess the cross section and yield of products from carbon nuclei interaction with soft tissue.