Re-Imagining Radiotherapy Bolus As a Soft Robot: A Preliminary Investigation
Abstract
Purpose
Robotic bolus has potential to enhance surface dosimetry by reducing air gaps through adaptive surface contouring and providing immobilization for treatment regions. This study evaluated filler materials for a jamming-style robotic bolus as an initial proof-of-concept. Jamming was selected as a mechanism because it enables passive shape conformity and stiffness without complex actuation. The work supports development of a patient-agnostic, reusable, homogeneous, non-adhesive, robotic bolus using simple pneumatic control.
Methods
Prototype devices were fabricated with hydrated silicone (SmoothOn EcoFlex-30), degassed and cured in 3D printed molds, consisting of an inner 6cm x 6cm grid region with optional outer petals. The grid region was designed to maintain uniform thickness over a treatment region under deflection and take advantage of granular jamming for stabilization and shape conformity to the surface. Jamming materials and mixtures of round glass microbeads (100-170 mesh), coarse sugar, and ground coffee were evaluated for radiological homogeneity using a 6MV beam normalized to an open field. EPID measurements were normalized to an open field and analyzed within central ROIs.
Results
A soft robotic prototyping method was established using curable silicone. EPID measurements demonstrated that none of the fillers matched silicone in 6MV transmission, although glass beads showed a small improvement (1.3%) over air. A mixture of 60/40 weight percent sugar and glass beads had the lowest max/min and avg/min intensity ratios in the ROI (1.017 and 1.009, respectively). Evacuation enabled temporary, non-adhesive fixation, suggesting potential for reproducibility and patient comfort.
Conclusion
A fabrication process and candidate filler materials were established for jammed soft robotic bolus designs, using radiological homogeneity as a design constraint. Mixed sugar-glass bead fillers demonstrated improved uniformity relative to air. Temporary, non-adhesive fixation could be achieved via mild suction, with further research supporting appropriate design of hybrid systems.