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The electromagnetic tracking (EMT) system for intra-op US based HDR brachytherapy is a new technology for tracking and automatic reconstruction of catheters. The aim of this work is to summarise the challenges pose by the technology and how to tackle them

The Uronav Therapy system from Philips Disease Management Solutions was commissioned and integrated in our clinical practice (Fig.1). The calibration of the EMT to the US image is a crucial step, and to obtain good results, a list of conditions should be respected. The calibration validation must be done at the OR with the same clinical set-up and in a US compatible prostate phantom. The clinical use of that system also comes with special requirements starting with the patient set-up, equipment used around the system, etc. Finally, organ delineation and catheter’s tracking also come with challenges. Various solutions were explored and are presented

A critical step to ensure accurate results is the registration of the EMT reference frame to the US. This needs to be done in accordance to TG-128 (salty water - 43g/L). No metal part should be within the EM field. A small metal rod hidden in our water container hinge introduced an error up to 1.8mm. This further includes any support with metallic parts used to hold the calibration phantom. Another aspect is the clinical environment. Metallic parts on the OR table can create EM disturbances and introduce errors (Fig.2). In our set-up, the distance between the EM generator and the table stand needs to be more than 70cm to avoid disturbance, which translate in error of 2mm on tip positions and height of reconstructed catheters. Cell phones, metal clamps and so on shouldn’t be placed close to the EM field, as they introduce error relative to the calibration up to 2mm. However, stirrups as well as surgical or vasectomy clips did not show any effect. In this trial, no patient with pacemaker, neurostimulator, implanted insulin pump, hip or knee prosthesis was allowed in order to avoid disturbance. Finally, the stylet used for EM tracking and automatic reconstruction is not rigid enough to allow for an easy tissue perforation, but stiffer than the source cable, sometimes introducing catheter motion during retraction with an AP shift of the tip up to 3mm. Thus, it remains important to visualize the reconstruction with sagittal live US imaging.

EM tracking offers the possibility of fast and accurate solution for catheter guidance and reconstruction in US-guided prostate HDR. Pointers were given beyond the vendor provided guidance to avoid potential pitfalls and ensure that the stated accuracy is indeed reached