uses this data to repeat the registrations and compare to the actual clinical registrations. Agreement within 5 mm was considered acceptable for credentialing. Deviations of greater than 10 mm were confirmed with the submitting institution. Results: Images of 130 fractions from the initial and repeat submissions from 42 institutions between June 2013 to present were reviewed. The median agreement between clinical registrations and study reviewer was 3 mm, with 95% of all fractions within 5 mm. A sub-analysis investigated differences between use of low contrast soft tissue (nZ63 images) or highcontrast surrogates (nZ67) (e.g. implanted fiducial markers, surgical clips, metallic stents) for registration. Soft tissue registrations agreed within 3 mm in 50% of fractions and 9 mm in 95%, while high-contrast surrogate registrations agreed within 3 mm in 50% and 5 mm in 95%. Soft tissue registrations exceeded 10 mm in 3% of fractions, while no high-contrast surrogate registrations exceeded 5 mm. Conclusion: The RTOG 1112 credentialing experience suggests most institutions perform liver IGRT with sufficient accuracy to deliver SBRT safely. Both soft tissue and high-contrast surrogates appear adequate for consistent registration in most instances; however, some mis-registrations were observed when using soft-tissue registration targets. The use of highcontrast surrogates appears to reduce the small risk of substantial geographic miss due to mis-registration in liver IGRT.

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