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 (ARTCBCT1) were recalculated on CBCT2 (ARTCBCT2) and compared to treatment plans on CBCT1 after image guidance (IGRTCBCT1) for relevant DVH metrics: V40Gy(CTV), V37Gy(bladder), V36Gy(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 V40Gy(CTV) from 86% in IGRTCBCT1 to 94% in ARTCBCT2. Inter-fractional prostate swelling ( r T x , V o l ( p r o s t a t e ) = 0.98 , p = 0.005 ) was responsible for CTV deviations. Bladder filling ( r Δ T , V o l ( b l a d d e r ) = 0.34 , p = 0.002 ) and rectal gas migration during the median adaptation time Δ T = 24.0 m i n increased V37Gy(bladder) from 4.9 cm3 in ARTCBCT1 to 6.5 cm3 in ARTCBCT2 and V36Gy(rectum) from 0.5 cm3 to 0.6 cm3 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.