Compact Rotational Ultrasound Probe Holder for Brachytherapy
Authors: Jelínek Filip , Kronreif Gernot , van de Wardt Cor 
Published: October 2017

Image-guided adaptive brachytherapy (IGABT) is a promising method for precise targeting and treatment of cervical and prostate cancer where one can adapt the radiation delivery according to tumor spread and organs at risk (OAR). The currently developing procedural modality incorporating optical tracking and transrectal ultrasound (TRUS) for superior soft tissue contrast, later combined with CT data, will likely develop into a readily affordable option, as the use of MRI will become unnecessary [1]. This is especially relevant for developing countries (mainly in SE Africa and S America) where the MRI scanners are scarce and the occurrence specifically of cervical cancer is the highest worldwide [2].

In brachytherapy, the procedure requires either the use of an active source (cervix) or implantation of radioactive seeds (prostate) in an accurate way as to enable sufficient safety margin with respect to the OAR. The sources irradiate the tissue of interest through gynecological (GYN) applicators (inserted internally) as well as hollow needles guided by either these applicators or prostate templates (positioned externally). Due to the localized use of several devices, an uninterrupted verification of source or needle placement can only be provided if the TRUS probe scans the volume of interest without much movement, likely leading to physical interference. Hence, the longitudinal ultrasound (US) array is used to obtain a 3D-US volume using a rotational sweep.

The use of a TRUS probe for an automatic US image reconstruction requires the probe to be housed in a motorized unit, a so-called stepper or a probe holder, which should ideally be integrated seamlessly into the overall procedure without impeding the established workflow or imposing any unnecessary space and time restrictions within the operating room (OR). Seamless integration should also mean minimal assembly steps required by the OR staff, whether nurses or surgeons themselves, as well as the possibility to use the TRUS probe in a natural unrestricted way. More specifically, when housed in the stepper, the probe’s geometry and weight should not be exaggerated as to allow enough haptic feedback for the surgeon when guiding the probe through the bodily cavities or probing for any unnatural or pathological tissue.

Examples of conventional brachytherapy steppers include Mikrostepper MST 200 (GfM, Riedstadt-Leeheim, Germany), Transperineal Stepper (D&K Technologies, Barum, Germany), AccuCARE™ Classic Stepper (Civco, Coralville, IA, USA), STP 110 Precision™ Stepper (Best NOMOS, Pittsburgh, PA, USA), or OncoSelect Stepper equipped with EndoCavity Rotational Mover (Elekta, Veenendaal, NL). Each of these devices provides precise rotational and translational fine-tuning, motorized image acquisition as well as high positioning rigidity. Yet, in most cases, this is at the expense of their oversized dimensions, large mass often requiring a counterweight, limited range of motion or cumbersome handling. As a result, their overall usability is compromised, especially in applications requiring better haptic experience, such as an initial free-hand scan during GYN brachytherapy. Hence, the aim of this paper is to present a novel, compact and versatile solution compensating for the aforementioned limitations and enabling seamless integration.