Monitoring intra- and inter-fraction motion using transperineal ultrasound (TPUS), for patients receiving...

Daily image guidance is essential in the delivery of hypofractionated radiotherapy to the prostate, given the use of smaller planning target volumes and the potential for pelvic anatomical motion. This study compared the accuracy of transperineal ultrasound (TPUS) with cone-beam computed tomography (CBCT) using fiducial markers and evaluated intra-fractional prostate motion in patients undergoing stereotactic ablative radiotherapy (SABR).

A total of 120 image registration translations were analysed in patients treated with five fractions of definitive SABR. Agreement between TPUS and CBCT was assessed using Bland-Altman analysis and intra-class correlation coefficients. TPUS was also used for intra-fraction monitoring, with motion alerts set at a 3 mm tolerance to ensure treatment accuracy.

Modality agreement within 3 mm was observed in 88.3 %, 79.4 %, and 86.7 % of cases in the lateral, longitudinal, and vertical directions, respectively. Mean differences (±SD) between TPUS and CBCT were -0.04 ± 3.2 mm (lateral), 0.13 ± 2.8 mm (longitudinal), and -0.2 ± 3.9 mm (vertical). Prostate displacement beyond 3 mm during beam-on time occurred in 40 % of treatment fractions.

TPUS provides real-time intra-fractional imaging, which is particularly valuable in ultra-hypofractionated protocols such as prostate SABR. The findings demonstrate good correlation between TPUS and CBCT, supporting the comparable accuracy of these modalities.

Real-time monitoring and correction of prostate motion can improve the accuracy of SABR, maintaining 3 mm target coverage while minimising radiation exposure to adjacent organs at risk, especially in the posterior direction, where motion is most frequent.