What is the clinical significance of R50 in lung SBRT?
Our dosimetrists rarely meet the RTOG constraint using a non-coplanar technique, especially for very small lesions. I haven't been able to find a correlate in the literature between R50 and radiation pneumonitis. How important is this value? Especially for small lesions?
2 Answers
Mednet MemberInvited Expert
Radiation Oncology · Mayo Clinic
Answered on
Small lesions are the hardest to plan and meet the constraints like the R50%. You are right that those parameters have not been associated with symptomatic pneumonitis. The only reliable predictor in retrospective studies has been increased PTV size. The R50, conformality index, and dose spillage va...
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Radiation Oncology · Washington University School of Medicine
Answered on
I agree with @Dr. First Last, and would like to add my own 2 cents!
R50 is indeed a metric derived from the intracranial SRS world, where we usually refer to it as the gradient or gradient index. There are subtle differences in how to calculate an R50 or gradient as it relates to your denominator, w...
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