ESTRO 2024 - Abstract Book

S4058

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

The use of an 8 mm PTV margin resulted in excellent target coverage, with prostate bed CTV V95 ≥ 99% in all treatment fractions, and only 6/199 fractions (3%) failed to meet the CTV V95 ≥ 99% of the vesicle bed CTV (range 98% to 100%). However, bladder V60 <25% was not achieved in 5% (10/199) of fractions, and rectum V60 <5% was not achieved in 33% (65/199). Reducing the PTV margin to 6 mm resulted in prostate bed CTV V95 =100% in 92% (183/199) fractions and vesicle bed CTV V95 =100% in 86% (171/199). Furthermore, all fractions met the bladder V60 <25% dose constraint, and 97% (194/199) of fractions met the rectum V60 <5% dose constraint. With the use of PTV margins <8 mm, a correlation between OARs volume change and CTV coverage was observed, as CTV coverage decreased with larger bladder and rectum volume change (Pearson’s r= -0.484, p<0.001, and Pearson’s r = 0.189, p= 0.011, respectively).

Conclusion:

Moderately hypofractionated sEBRT yielded comparable OAR volume changes compared to conventionally fractionated sEBRT. We observed large bladder volume changes in both groups. Nevertheless, the moderately hypofractionated sEBRT within the PERYTON-trial is effectively administered, with excellent CTV coverage and acceptable OARs dose constraints using our currently employed PTV margin of 8 mm. Moreover, employment of a smaller PTV margin (of 6 mm) for the prostate bed CTV ensured sufficient coverage and will be considered for clinical implementation in our image guided sEBRT.

Keywords: Salvage radiotherapy, prostate bed, CBCT