ESTRO 2024 - Abstract Book

S4057

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

The currently used PTV margin of 8 mm was evaluated in 10 patients treated with hypofractionated sEBRT. The prostate bed CTV and vesicle bed CTV were delineated on daily CBCTs. To evaluate interfractional dosimetric changes, all CBCT contours (CTVs and OARs) were rigidly transferred to the planning-CT after image registration, and the CTV coverage and dose constraints of the bladder and rectum were evaluated. A CTV coverage constraint of V95 ≥ 99% in >90% of all fractions was a prerequisite. Additionally, the correlation between OARs volume and CTV coverage was assessed. For OAR dose constraint evaluation, compliance to bladder V60 <25% and rectum V60 <5% were analysed. Furthermore, we simulated smaller PTV margins of 2, 4 and 6 mm using a voxel-wise minimum robustness approach and all dosimetric parameters of the CBCT contours were evaluated.

Results:

A total of 577 pre-treatment CBCT images were identified, of which 38 were excluded for analyses due to imaging artefacts. This resulted in 340 pre-treatment CBCTs of 10 patients receiving 35 x 2 Gy and 199 CBCTs of 10 patients receiving 20 x 3 Gy. The median percent change in bladder and rectum volumes were -33.25% (IQR -48.73% to -3.86%) and -7.32% (IQR - 29.78% to +15.63%), respectively. OAR volume changes were not correlated with the number of treatment fractions and were not significantly different between the two treatment schedules. (Figure 1)

Figure 1 – Interfractional volume change of bladder (A) and rectum (B) during conventional (35 x 2 Gy) versus hypofractionated (20 x 3 Gy) sEBRT

In the hypofractionated group, the CTV prostate bed volume on CBCT showed limited changes compared to the planning-CT, with a median volume change of -3.12% (IQR -10.71% to 0.00%). The vesicle bed CTV showed a median volume change of -3.08% (IQR -14.29% to 0.00%). (Figure 2)

Figure 2 – Example of all interfractional CTV prostate bed and CTV vesicle bed volumes in one patient receiving sEBRT 20x3Gy with a PTV margin of 8 mm (red contour)