To investigate the amount that radiation-induced secondary cancer would be 
reduced by using proton beam therapy (PBT) in place of intensity-modulated X-ray 
therapy (IMXT) in pediatric patients, we analyzed lifetime attributable risk 
(LAR) as an in silico surrogate marker of the secondary cancer after these 
treatments. From 242 pediatric patients with cancers who were treated with PBT, 
26 patients were selected by random sampling after stratification into four 
categories: (i) brain, head and neck, (ii) thoracic, (iii) abdominal, and (iv) 
whole craniospinal (WCNS) irradiation. IMXT was replanned using the same 
computed tomography and region of interest. Using the dose-volume histograms 
(DVHs) of PBT and IMXT, the LARs of Schneider et al. were calculated for the 
same patient. All the published dose-response models were tested for the organs 
at risk. Calculation of the LARs of PBT and IMXT based on the DVHs was feasible 
for all patients. The means ± standard deviations of the cumulative LAR 
difference between PBT and IMXT for the four categories were (i) 1.02 ± 0.52% 
(n = 7, P = 0.0021), (ii) 23.3 ± 17.2% (n = 8, P = 0.0065), (iii) 16.6 ± 19.9% 
(n = 8, P = 0.0497) and (iv) 50.0 ± 21.1% (n = 3, P = 0.0274), respectively (one 
tailed t-test). The numbers needed to treat (NNT) were (i) 98.0, (ii) 4.3, (iii) 
6.0 and (iv) 2.0 for WCNS, respectively. In pediatric patients who had undergone 
PBT, the LAR of PBT was significantly lower than the LAR of IMXT estimated by in 
silico modeling. Although a validation study is required, it is suggested that 
the LAR would be useful as an in silico surrogate marker of secondary cancer 
induced by different radiotherapy techniques.