A precise and rapid analytical algorithm has been developed for calculating
proton energy deposition in media. The results covering the radiotherapy e
nergy range (50-250 MeV) from this algorithm and data from Monte Carlo code
are given. Comparison between them shows that 1) The Nonelastic nuclear in
teraction have severe influence on proton energy deposition, especially whe
n the initial energy of proton beams is high. 2) The algorithm is 100 times
faster than Monte Carlo methods. It satisfies the requirement of precision
and speed. The algorithm shows its great potentiality in proton radiothera
py.