Review of ultrasound image guidance in external beam radiotherapy part II: intra-fraction motion management and novel applications

Abstract

Imaging has become an essential tool in modern radiotherapy (RT), being used to plan dose delivery prior to treatment and verify target position before and during treatment. Ultrasound (US) imaging is cost-effective in providing excellent contrast at high resolution for depicting soft tissue targets apart from those shielded by the lungs or cranium. As a result, it is increasingly used in RT setup verification for the measurement of inter-fraction motion, the subject of Part I of this review (Fontanarosa et al 2015 Phys. Med. Biol. 60 R77-114). The combination of rapid imaging and zero ionising radiation dose makes US highly suitable for estimating intra-fraction motion. The current paper (Part II of the review) covers this topic. The basic technology for US motion estimation, and its current clinical application to the prostate, is described here, along with recent developments in robust motion-estimation algorithms, and three dimensional (3D) imaging. Together, these are likely to drive an increase in the number of future clinical studies and the range of cancer sites in which US motion management is applied. Also reviewed are selections of existing and proposed novel applications of US imaging to RT. These are driven by exciting developments in structural, functional and molecular US imaging and analytical techniques such as backscatter tissue analysis, elastography, photoacoustography, contrast-specific imaging, dynamic contrast analysis, microvascular and super-resolution imaging, and targeted microbubbles. Such techniques show promise for predicting and measuring the outcome of RT, quantifying normal tissue toxicity, improving tumour definition and defining a biological target volume that describes radiation sensitive regions of the tumour. US offers easy, low cost and efficient integration of these techniques into the RT workflow. US contrast technology also has potential to be used actively to assist RT by manipulating the tumour cell environment and by improving the delivery of radiosensitising agents. Finally, US imaging offers various ways to measure dose in 3D. If technical problems can be overcome, these hold potential for wide-dissemination of cost-effective pre-treatment dose verification and in vivo dose monitoring methods. It is concluded that US imaging could eventually contribute to all aspects of the RT workflow.

Initial results for patient setup verification using transperineal ultrasound and cone beam CT in external beam radiation therapy of prostate cancer

“…Colvill et al studied the dosimetric impact of tracking and gating to account for intrafraction motion during prostate radiotherapy [22]. O’Shea et al reviewed the applicability of intrafraction monitoring with US for prostate and other treatment sites [23]. Until further results on the practicability and accuracy of TPUS are available, certainly the combination of setup verification by CBCT and intrafraction target organ and OAR motion monitoring by TPUS imaging can use the benefits of both imaging modalities.…”

Dosimetric Implications of Residual Tracking Errors During Robotic SBRT of Liver Metastases

“…using ultrasound [40] to remove correlation errors altogether), and the online integration of the 4D EDC during CyberKnife treatment.…”

Monte Carlo investigation of the dosimetric effect of the Autoscan ultrasound probe for guidance in radiotherapy

“…Whilst the advantages of using real-time US to guide the delivery of RT are clear, 8 very little is still known about the effect of the US probe on the radiation dose delivered to the patient. For example Hsu et al (2005) pointed out that work remains to be done on the effect of the probe on surface dose when examining the feasibility of utilizing transabdominal US for real-time monitoring of target motion in RT.…”