INTRODUCTION: The potential of proton therapy to improve the sparing of the 
healthy tissue has been demonstrated in several studies. However, even small 
doses delivered to the organs at risk (OAR) may induce long-term detriments 
after radiotherapy. In this study, we investigated the possibility to reduce the 
risk of radiation-induced secondary cancers with intensity modulated proton 
therapy (IMPT), when used for radiosurgery of liver metastases.
MATERIAL AND METHODS: Ten patients, previously treated for liver metastases with 
photon-beam based stereotactic body radiation therapy (SBRT) were 
retrospectively planned for radiosurgery with IMPT. A treatment plan comparison 
was then performed in terms of calculated risk of radiation-induced secondary 
cancer. The risks were estimated using two distinct models (Dasu et al., 2005; 
Schneider et al., 2005, 2009). The plans were compared pairwise with a two-sided 
Wilcoxon signed-rank test with a significance level of 0.05.
RESULTS: Reduced risks for induction of fatal and other types of cancers were 
estimated for the IMPT plans (p<0.05) with the Dasu et al.
MODEL: Using the Schneider et al. model, lower risks for carcinoma-induction 
with IMPT were estimated for the skin, lungs, healthy part of the liver, 
esophagus and the remaining part of the body (p<0.05). The risk of observing 
sarcomas in the bone was also reduced with IMPT (p<0.05).
CONCLUSION: The findings of this study indicate that the risks of 
radiation-induced secondary cancers after radiosurgery of liver metastases may 
be reduced, if IMPT is used instead of photon-beam based SBRT.