Interstitial brachytherapy requires up to 20 straight needles to surround and irradiate deep-seated tumors, but may offer sub-optimal radiation dosage in cases of advanced cancers. A steerable stylet can be used to guide the needle within the tissue, improving procedure accuracy and reducing the number of needles required for each operation. This work introduces the design of a novel tendon-assisted magnetically steered (TAMS) robotic stylet to steer commercially available brachytherapy needles. The dual-actuation modality (magnetic and tendon-driven) allows for increased bending compliance while retaining axial rigidity at extremely small diameters (OD: 1.4 mm), key properties for steering hollow needles from within their lumen. We also develop a two-tube Cosserat rod model that estimates the behavior of the TAMS robot and needle assembly under actuation from tendons, external magnetic fields, and finally combined magnet+tendon forces. We validate our model in free space and demonstrate the capability of the TAMS robot and dual-actuation modalities to steer brachytherapy needles to high curvatures inside phantom tissue.

Cervical cancer accounts for a significant portion of the global cancer burden among women. Interstitial brachytherapy (ISBT) is a standard procedure for treating cervical cancer; it involves placing a radioactive source through a straight hollow needle within or in close proximity to the tumor and surrounding tissue. However, the use of straight needles limits surgical planning to a linear needle path. We present the OncoReach stylet, a handheld, tendon-driven steerable stylet designed for compatibility with standard ISBT 15- and 13-gauge needles. Building upon our prior work, we evaluated design parameters like needle gauge, spherical joint count and spherical joint placement, including an asymmetric disk design to identify a configuration that maximizes bending compliance while retaining axial stiffness. Free space experiments quantified tip deflection across configurations, and a two-tube Cosserat rod model accurately predicted the centerline shape of the needle for most trials. The best performing configuration was integrated into a reusable handheld prototype that enables manual actuation. A patient-derived, multi-composite phantom model of the uterus and pelvis was developed to conduct a pilot study of the OncoReach steerable stylet with one expert user. Results showed the ability to steer from less-invasive, medial entry points to reach the lateral-most targets, underscoring the significance of steerable stylets.