BACKGROUND: To compare proton beam therapy (PBT) and intensity-modulated 
radiation therapy (IMRT) with conformal radiation therapy (CRT) in terms of 
their organ doses and ability to cause secondary cancer in normal organs.
METHODS: Five patients (median age, 4 years; range, 2-11 years) who underwent 
PBT for retroperitoneal neuroblastoma were selected for treatment planning 
simulation. Four patients had stage 4 tumors and one had stage 2A tumor, 
according to the International Neuroblastoma Staging System. Two patients 
received 36 Gy, two received 21.6 Gy, and one received 41.4 Gy of radiation. The 
volume structures of these patients were used for simulations of CRT and IMRT 
treatment. Dose-volume analyses of liver, stomach, colon, small intestine, 
pancreas, and bone were performed for the simulations. Secondary cancer risks in 
these organs were calculated using the organ equivalent dose (OED) model, which 
took into account the rates of cell killing, repopulation, and the neutron dose 
from the treatment machine.
RESULTS: In all evaluated organs, the mean dose in PBT was 20-80% of that in 
CRT. IMRT also showed lower mean doses than CRT for two organs (20% and 65%), 
but higher mean doses for the other four organs (110-120%). The risk of 
secondary cancer in PBT was 24-83% of that in CRT for five organs, but 121% of 
that in CRT for pancreas. The risk of secondary cancer in IMRT was equal to or 
higher than CRT for four organs (range 100-124%).
CONCLUSION: Low radiation doses in normal organs are more frequently observed in 
PBT than in IMRT. Assessments of secondary cancer risk showed that PBT reduces 
the risk of secondary cancer in most organs, whereas IMRT is associated with a 
higher risk than CRT.