Yulin Song¹, Hao-Yun Hsu¹*, Wang-Chia Ko², Paul Tamas⁴, Miao Zhang⁵, Boris A. Mueller⁶, David Guttmann⁷, Lior Z. Braunstein⁸, (1) Department of Medical Physics, Memorial Sloan Kettering Cancer Center, NY, USA (2) Department of Biostatistics, MSPH, Columbia University, NY, USA (3) Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, NY, USA

PO-GePV-M-256 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: Standard light-opaque bolus is suboptimal for radiotherapy (RT) patient setup because it obscures surface skin marks. Clearsight bolus is a transparent polymer gel bolus designed for enhanced patient setup with surface visibility. We evaluated the clinical characteristics of Clearsight bolus to determine its suitability for breast SGRT applications.

Methods: Room-light transparency was evaluated by placing bolus (30 cm x 30 cm x 3 mm) on an AlignRT calibration plate. The visibility, deformation, and sharpness of the black blobs were examined. Radiation attenuation was measured by placing bolus on a 15-cm stack of solid water, with ion chamber at d5 for 6x, 6xFFF, 15x, d3 for 6E, 9E, and d4 for 12E, 16E, and 20E. The readings were compared with those obtained from an equivalent thickness of solid water. Bolus-to-skin conformity was evaluated by placing bolus over surfaces with various curvatures and examining the air gap between the two. Effects on 3D optical surface imaging tracking were assessed by placing bolus on various surfaces and measuring 6D shifts using AlignRT Advance.

Results: Clearsight bolus had good room-light transparency. The black blobs and crosshair on the calibration board were easily visible through the bolus. The mean attenuation ratio between bolus and solid water was 1.0010±0.0006 for photon energies and 1.0499±0.0682 for electron energies. Bolus-to-skin conformity was good for flat and cylindrical surfaces, but inadequate for hemispherical surfaces. Clearsight bolus was not entirely transparent to the structured light from AlignRT Advance: it induced a modest translational shift (range -1.0 to 1.5mm) and rotational shift (range -1.5⁰ to 1.5⁰) depending on surface topography.

Conclusion: Clearsight bolus is suitable for breast SGRT applications. When using AlignRT Advance, capturing a new reference surface is necessary after bolus placement and, for some patients, larger than usual bolus may be required.

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