ESTRO 2021 Abstract Book

S1295

ESTRO 2021

deforms the prostate. On average, we found decrements in PTV coverage (V95%) from (97.3±2.2)% to (87.0±10.5)% and near-maximum dose (D98%) from (37.9±0.3)Gy to (35.0±3.0)Gy when comparing delivered dose to the planned dose. However, the average differences in CTV's V95% and D98% are negligible: from (92.8±2.8)% to (92.7±0.6)% and from (36.9±0.7)Gy] to (36.8±0.9)Gy, respectively. It should be noted, though, that we have one catastrophic patient for whom V95% of the CTV is below 80% and is thus being followed up.

Conclusion Our results show the importance of accounting for bladder changes in the definition of the margins from CTV to PTV in prostate SBRT, where random day-to-day variations are less likely to be compensated throughout the treatment. In our case, the margins applied (5mm isotropic expansion except in the posterior direction, where it is reduced to 3mm), seem to be sufficient to guarantee good coverage despite large bladder volume variations. The presence of one catastrophic case also shows how beneficial it would be to have an adaptive radiotherapy system in these type of treatments.