Related Experiment Video
Updated: Oct 24, 2026
Dynamic Lung Tumor Tracking for Stereotactic Ablative Body Radiation Therapy
Published on: June 7, 2015
A transparent gel pad bolus enabling direct skin recognition in surface-guided radiation therapy
Shunta Hirano1,2, Kenji Matsumoto1, Masakazu Otsuka3
1Department of Medical Physics, Graduate School of Medical Sciences, Kindai University, 1-14-1, Miharadai, Minami-ku, Sakai-shi, Osaka, 590-0197, Japan.
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.
