Evaluating intra-fraction motion and its dosimetric impact in ethos online adaptive radiotherapy for prostate cancer.

PURPOSE: This study evaluated the geometric and dosimetric uncertainties during online adaptive radiotherapy (ART) for prostate cancer.

METHODS

Sequential cone beam computed tomography (CBCT) scans from 52 sessions involving 13 patients were analyzed. An ART plan was generated using CBCT1, followed by a verification scan (CBCT2) acquired 13.1 ± 3.0 minutes before treatment delivery. New contours (prostate, seminal vesicles, bladder, rectum, and bowel) were delineated on CBCT2 and transferred to CBCT1 for dose distribution analysis. Three dimensional contour variations were quantified, new planning target volume (PTV) margins were calculated, and their dosimetric benefits were assessed.

RESULTS

No significant differences were observed in the high-dose volumes (V60 Gy and V57 Gy) of the bladder and rectum (P > 0.05). PTV margins were 2.6 mm, 2.4 mm, and 2.6 mm in the lateral, vertical, and longitudinal directions for the prostate, and 3.9 mm, 3.8 mm, and 4.3 mm for the seminal vesicles, respectively. These margin reductions led to a 37% reduction in the dose to the surrounding critical organs, while maintaining consistent target coverage.

CONCLUSION

This study supports symmetric PTV margins of 3 mm for the prostate and 4.5 mm for the seminal vesicles in online ART, thereby contributing to the development of optimized treatment strategies for prostate cancer.