F Van den Heuvel*, I Jacobs, T Harthoorn, I De Marco, B Mous - Van Der Wegen, V Coen, Zuidwest Radiotherapeutisch Instituut, Vlissingen, NL

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  F Van den Heuvel

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PO-GePV-T-216 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: The migration from hypofractionated (HF) (20 fractions) prostate treatments, to ultra-hypo fractionated (UHF) treatments using only 5 fractions, have been studied in terms of equivalence in terms of effectiveness and normal tissue complications with favorable results One remaining issue is whether the current margins ensuring adequate coverage are still valid.Indeed, the margins used with a higher number of fractions were retained, but the statistical power of the various clinical trials was likely to be insufficient to detect any inadequacies therein.

Methods: We selected 10 patients treated to the prostate only (i.e. no seminal vesicles, lymph nodes) for an audit study. Plans were generated for treatments consisting of 3Gy x 20 fractions (HF) as well as 7.25Gy x 5 fractions (UHF), both using recommended constraints for normal tissue and a standard margin of 5 mm. For each patient in the study, 20 systematic errors are generated, and doses are recalculated. The number of shifts from this position (i.e. sampling the random error) is determined by the number of fractions. For each treatment instance (i.e. patient with given systematic error and fractionation) the Effective Uniform Dose (EUD) for relevant structures is calculated. Here we report on the Bladder, Rectum (α/β = 3.0) and the Prostate ( α/β = 1.2).

Results: The EUD’s obtained for the prostate are higher in the UHF case compared to HF (ca 20%). This compensates for a larger variation in EUD values due to shifts (i.e EUD values are still at the same level as in the HF case). For the organs at risk like the rectum, we see that both approaches are indistinguishable.

Conclusion: The current clinical margins exhibit equivalent NTCP and do not affect the effectiveness of the treatment. The only downside of a larger fraction could be the impact of the intra-fractional movement.

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