Advanced HyperSight™ imaging for patients with adaptive SBRT of prostate cancer: a longitudinal analysis of tissue demarcation

Cone-beam computed tomography (CBCT)-based adaptive radiotherapy (ART) at the Ethos® linear accelerator (eLinac) allows for daily anatomical and dosimetric adjustments and relies on robust image quality. This study evaluated the longitudinal image quality of the novel HyperSightTM-CBCT (hCBCT) compared to planning CT (pCT), using phantoms and data of prostate cancer patients undergoing adaptive stereotactic body radiotherapy (SBRT). Building on this, the longitudinal contour sharpness of the organs in fractional hCBCT and their usability for ART workflow across fractions was evaluated. Between December 2023 and May 2024, 26 prostate cancer patients receiving ART at the eLinac with hCBCT technology were enrolled. Phantom studies assessed pCT and hCBCT image quality. Patient based analyses of all 156 imaging scans (pCT and each of the fractional hCBCT) analyzed, longitudinally examined firstly image quality, secondly contour sharpness of prostate, seminal vesicles, and rectal wall, and thirdly confidence to delineate the structures for the ART workflow. Time required for the ART based parameters were recorded. Quantitative metrics included CT number changes in the fat adjacent to the prostate and seminal vesicles. Friedman’s test with Bonferroni correction, Spearman and Intraclass Correlation Coefficient (ICC) were used for statistics. hCBCT scans showed robust image quality parameters in the phantom and patient based analysis across fractions. Inter-observer agreement was moderate, with lower rating score for resident compared to the experienced radiation oncologist (p < 0.001). Patient based analysis showed no significant differences of the contour sharpness of the prostate and seminal vesicles between pCT and initial hCBCT scan, but contour sharpness ratings declined across treatment fractions. Confidence for the delineation of prostate and seminal vesicles during ART was significantly decreased at later fractions (each padj<0.05) and this correlated significantly with longer assessment times (padj≤0.05). The CT attenuation of the fat tissue adjacent to the prostate and seminal vesicles significantly increased across the fractions (padj<0.05). High-quality imaging for adaptive SBRT in prostate cancer is provided by hCBCT, which offers equivalent tissue visualization compared to pCT. Fraction-dependent decreases in contour sharpness can be detected using longitudinal hCBCT imaging. These decreases are likely related to treatment-induced tissue changes and may impact ART workflow. The rating of the observed effects may potentially be influenced by the observer’s experience.