A cone-beam computed tomography based workflow for online adaptive ultra-hypofractionated radiotherapy of prostate cancer.

[BACKGROUND AND PURPOSE] Cone beam computed tomography (CBCT)-based approaches for online adaptive radiation therapy (oART) have recently become clinically available for ultra-hypofractionated prostate stereotactic body radiation therapy (SBRT). This work assessed the changes of relevant dose-volume-histogram (DVH) parameters and their robustness against morphologic variations during adaptation.

[MATERIALS AND METHODS] Sixteen prostate cancer patients were treated with oART in an SBRT regimen (40 Gy in 5 treatment sessions (Tx) to the clinical target volume (CTV), PACE-B constraints). Two CBCTs were acquired daily: CBCT1 for adaptive planning and CBCT2 after adaptation for position verification. Adapted plans optimized on CBCT1 (ART) were recalculated on CBCT2 (ART) and compared to treatment plans on CBCT1 after image guidance (IGRT) for relevant DVH metrics: V(CTV), V(bladder), V(rectum). Spearman's rank coefficients r with p-values (5% significance level) were determined to analyze correlations between adaptation time (ΔT) and bladder filling as well as Tx and median prostate volume.

[RESULTS] oART improved median V(CTV) from 86% in IGRT to 94% in ART. Inter-fractional prostate swelling ( was responsible for CTV deviations. Bladder filling ( ) and rectal gas migration during the median adaptation time increased V(bladder) from 4.9 cm in ART to 6.5 cm in ART and V(rectum) from 0.5 cm to 0.6 cm and led to 10 constraint violations, each.

[CONCLUSION] Compared to IGRT, daily oART substantially improved CTV coverage. Besides inter-fractional prostate swelling, constraint violations originated from seminal vesicles motion, rectal gas or bladder filling during adaptation. Treatment adaptation times should therefore be minimized whenever possible.