No-fly zone.
Ever since the notorious 2012 Penn case report, rad oncs everywhere have been terrified of performing stereotactic radiation therapy (SBRT) on “central” (<2 cm from bronchus) lung tumors. Give too high a dose and it may cause a fatal toxicity, give too low a dose and it may cause a fatal failure. But at least the latter can be chalked up to Mother Nature, right? This review of several recent phase 1/2 trials on the subject succinctly covers what we’ve learned since then. Including slightly limiting the biologically effective dose to drop the grade 5 toxicity by an order of magnitude, from the no-fly zone of 10% to the fly-carefully zone of 1%. One problem lies in the vast expanse among definitions of “stereotactic” techniques, both in terms of prescribed doses and allowed hot spots. Another takeaway? Distance matters. Tumors ≤2 cm from bronchus aren’t the same as those ≤1 cm from bronchus. Which brings up the other concept muddying the waters: “acceptable toxicity”—a term that standardly carries a threshold well into the double-digits when used by oncologic surgeons. TBL: For central lung tumors, a good compromise between tumor control and treatment toxicity (even as defined by a rad onc) appears to be 7.5 Gy x 8. | Palma, Int J Radiat Oncol Biol Phys 2019
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ReplyDeleteI gently disagree with the conclusion that 60/8 appears to be best for ultracentral tumors. Recall that the BED for 60/8 is 105, while the BED for 50/5 is 100.
ReplyDeleteFirstly, the VUMC Tekatli RTO paper from 2015 for 60/8 within 2 cm of PBT demonstrated 3-5% possibly/likely grade 5 toxicity (https://doi.org/10.1016/j.radonc.2015.09.028).
Another paper from Israel in 2018 demonstrated *60*/5 was delivered safely to tumors < 2 cm from the PBT, but they prioritized OAR over PTV (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5918762/).
More importantly, Wang's IJROBP 2018 paper demonstrated 3 patients with fatal pulmonary hemorrhage had max point doses to proximal bronchial tree of >= 50 Gy, suggesting limiting PBT to 105% dmax per RTOG 0813 is not appropriate (https://doi.org/10.1016/j.ijrobp.2018.06.118).
The importance of revised PBT constraints to RTOG 0813 was further demonstrated by Manyam from Cleveland Clinic in IJROBP 2018, which suggests PBT D0.33cc < 46.5 Gy should be added to constraints for 50/5. An easier way to remember this might be by limiting the PBT to 95% of Rx, as opposed to the 105% of Rx. Maybe we should also be considering 45/5 for ultracentral patients where this 95% dose to PBT endpoint is not easily achievable (https://doi.org/10.1016/j.ijrobp.2018.06.117).
TL;DR: Verdict is still out as to which regimen is best. For 50/5 ultracentral, limit the PBT to 95% Dmax instead of the recommended 105% Dmax on 0813. The real question might be prioritizing OAR over PTV in order to achieve this tight constraint in central lesions, and less so fractionation, especially considering the BED is actually higher for 60/8 as compared to 50/5.