In vivo validation of 3D transperineal ultrasound estimates of prostate motion during radiotherapy

Alexander Grimwood1, Helen McNair2, Tuathan O’Shea2, Jeffrey C Bamber1, Alison Tree2, Emma Harris1, 1Radiotherapy and Imaging, Institute of Cancer Research, 2Radiotherapy and Imaging, Royal Marsden Hospital, London


Ultrasound is increasingly used for image guidance in radiotherapy (RT)1,2. Good soft-tissue contrast, excellent spatial resolution and non-ionising nature make it an attractive modality for estimating prostate motion during treatment.

The Elekta Clarity® ultrasound system is designed for RT guidance applications. Here we present an in-vivo validation of the system for monitoring 3D prostate motion during RT fractions.


Imaging data from 17 patients across 80 fractions were analysed as follows.

Sequences of 3D ultrasound images were acquired during radiotherapy delivery. Simultaneously acquired X-ray portal image sequences were contrast enhanced and reviewed by three observers to manually identify the positions of three intra-prostatic fiducial markers. The Clarity® system’s prostate position monitoring data were compared to marker positions in the portal images.

Three observers rated the ultrasound image sequences into four image quality levels. A sequence from each level was selected. Prostate motion was estimated using an in-house automated echo-tracking algorithm3, and by manually tracking anatomical features within the prostate. These estimates were compared to Clarity®.


Clarity measurements of prostate position exhibited a 1 mm mean geometric error and 95% uncertainty of 3 mm relative to portal images. Alternative motion estimation methods produced estimates similar to Clarity, independent of image quality.


Clarity accurately estimates prostate motion regardless of image quality, demonstrating its potential usefulness in motion compensation strategies during radiotherapy delivery.


1.       Fontanarosa D, et. al., 2015, Review of ultrasound image guidance in external beam radiotherapy: I. Treatment planning and inter-fraction motion management, Phys. Med. Biol. 60 R77–114

2.      O’Shea T, et. al., 2016, Review of ultrasound image guidance in external beam radiotherapy part II: intra-fraction motion management and novel applications, Phys. Med. Biol. 61 R90–137

3.      O’Shea T, et. al., 2015, Temporal regularization of ultrasound-based liver motion estimation for image-guided radiation therapy, Med. Phys. 43 455-46

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