A transparent gel pad bolus enabling direct skin recognition in surface-guided radiation therapy

Objectives To evaluate the feasibility of repurposing a transparent ultrasound gel pad as a radiotherapy bolus, we focused on dosimetric performance and compatibility with surface-guided radiation therapy (SGRT). Methods A gel pad bolus composed of tamarind seed gum and polyhydric alcohols was compared with a conventional gel bolus and a thermoplastic bolus. Percent depth dose was measured for 6 MV and 10 MV photon beams and 6 MeV and 9 MeV electron beams. Straight-line optical transmittance was assessed over 380–830 nm. Surface recognition accuracy was quantified using the AlignRT SGRT system on a breast phantom. Results The gel pad produced a measurable bolus effect for both photon and electron beams, although superficial dose enhancement was lower than that of conventional materials (85.5% vs 90.3–91.5% at 6 MV). Deep doses beyond dmax agreed within 1% for photon beams. At 730 nm, the gel pad bolus demonstrated high transmittance (92.3%), compared with the gel (19.8%) and HM (36.6%) boluses. The SGRT recognition rate with the gel pad bolus (89.10%) was comparable to the no-bolus condition (92.93%, p = 0.13), whereas conventional boluses significantly reduced recognition (p < 0.001). Conclusions The transparent gel pad bolus enabled direct SGRT-based recognition of the underlying skin surface while maintaining clinically relevant bolus functionality. Advances in knowledge Optical transparency enables SGRT monitoring through a bolus, highlighting optical compatibility as an important design consideration.