Dose Tolerance for Stereotactic Body Radiation Therapy.

Normal tissue complication probability (NTCP) results were detailed in the July 2001 issue of Seminars in Radiation Oncology for conventionally fractionated radiation therapy. After 7 years, an extensive collection of stereotactic ablative body radiotherapy (SABR) or stereotactic body radiation therapy (SBRT) dose-tolerance limits was presented in the October 2008 issue of Seminars in Radiation Oncology, but estimates of risk were not yet available.We now have sufficient data to combine the 2: NTCP for SBRT. Physicians need a single parameter to make a clinical decision for each critical structure in the treatment plan. Ideally, this parameter would be directly associated with the expected outcome like NTCP. If every patient 's tumor control probability (TCP) was 99% or higher, and if every patient 's NTCP was 1% or lower, we would not need surrogate metrics like the conformality index, tumor coverage, and dosetolerance limits. Note that in 3 consecutive sentences, we went from “need” to “ideally” to “if,” and in reality TCP and NTCP are often still uncertain, and are rarely as good as 99% and 1%, so we usually are highly dependent on the surrogate metrics of plan quality. In this issue of Seminars, we focus on both clinical practice and rigorous statistics, spending as little time in the middle as possible. Maximum likelihood parameter fitting and other statistical methods are required to obtain reliable estimates of risk, but the focus of this work is on the clinical utility. Dose-tolerance limits are the stable bridge between clinical practice and rigorous estimation theory. Even as estimates of NTCP are continually updated with every new publication containing toxicity estimates, the same dose-tolerance limits can usually still be maintained, as long as the new estimate of NTCP is not dramatically different than the current expectations. This strategy of a constant dose limit is more stable than attempting to maintain a fixed NTCP limit, as estimates of NTCP continue to evolve in each new study. The selection criteria for data in this issue of Seminars are the complete opposite of the criteria for the American Association of Physicists in Medicine (AAPM) SBRT Working Group (WGSBRT). The WGSBRT manuscripts are namedHigh Dose per Fraction, Hypofractionated Treatment