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

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.

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_CBCT1) were recalculated on CBCT2 (ART_CBCT2) and compared to treatment plans on CBCT1 after image guidance (IGRT_CBCT1) for relevant DVH metrics: V_40Gy(CTV), V_37Gy(bladder), V_36Gy(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.

oART improved median V_40Gy(CTV) from 86% in IGRT_CBCT1 to 94% in ART_CBCT2. Inter-fractional prostate swelling ( $r_{Tx,Vol\left(prostate\right)}=0.98,p=0.005)$ was responsible for CTV deviations. Bladder filling ( $r_{\Delta T,Vol\left(bladder\right)}=0.34,p=0.002$ ) and rectal gas migration during the median adaptation time $\Delta T=24.0min$ increased V_37Gy(bladder) from 4.9 cm³ in ART_CBCT1 to 6.5 cm³ in ART_CBCT2 and V_36Gy(rectum) from 0.5 cm³ to 0.6 cm³ and led to 10 constraint violations, each.

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.