The purpose of this work was to compare the risk of developing a second cancer 
after craniospinal irradiation using photon versus proton radiotherapy by means 
of simulation studies designed to account for the effects of neutron exposures. 
Craniospinal irradiation of a male phantom was calculated for 
passively-scattered and scanned-beam proton treatment units. Organ doses were 
estimated from treatment plans; for the proton treatments, the amount of stray 
radiation was calculated separately using the Monte Carlo method. The organ 
doses were converted to risk of cancer incidence using a standard formalism 
developed for radiation protection purposes. The total lifetime risk of second 
cancer due exclusively to stray radiation was 1.5% for the passively scattered 
treatment versus 0.8% for the scanned proton beam treatment. Taking into account 
the therapeutic and stray radiation fields, the risk of second cancer from 
intensity-modulated radiation therapy and conventional radiotherapy photon 
treatments were 7 and 12 times higher than the risk associated with scanned-beam 
proton therapy, respectively, and 6 and 11 times higher than with passively 
scattered proton therapy, respectively. Simulations revealed that both passively 
scattered and scanned-beam proton therapies confer significantly lower risks of 
second cancers than 6 MV conventional and intensity-modulated photon therapies.