Risk projection methods allow for timely assessment of the potential magnitude 
of radiation-related cancer risks following low-dose radiation exposures. The 
estimation of such risks directly through observational studies would generally 
require infeasibly large studies and long-term follow-up to achieve reasonable 
statistical power. We developed an online radiation risk assessment tool 
(RadRAT) which can be used to estimate the lifetime risk of radiation-related 
cancer with uncertainty intervals following a user-specified exposure history 
(https://irep.nci.nih.gov/radrat). The uncertainty intervals constitute a key 
component of the program because of the various assumptions that are involved in 
such calculations. The risk models used in RadRAT are broadly based on those 
developed by the BEIR VII committee for estimating lifetime risk following 
low-dose radiation exposure of the US population for eleven site-specific 
cancers. We developed new risk models for seven additional cancer sites, oral, 
oesophagus, gallbladder, pancreas, rectum, kidney and brain/central nervous 
system (CNS) cancers, using data from Japanese atomic bomb survivors. The 
lifetime risk estimates are slightly higher for RadRAT than for BEIR VII across 
all exposure ages mostly because the weighting of the excess relative risk and 
excess absolute risk models was conducted on an arithmetic rather than a 
logarithmic scale. The calculator can be used to estimate lifetime cancer risk 
from both uniform and non-uniform doses that are acute or chronic. It is most 
appropriate for low-LET radiation doses < 1 Gy, and for individuals with 
life-expectancy and cancer rates similar to the general population in the US.