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  4. Customized Bolus Is More Effective Than Standard Bolus to Mitigate the Increase in Exit Dose During MRI-Guided Radiation Therapy

Customized Bolus Is More Effective Than Standard Bolus to Mitigate the Increase in Exit Dose During MRI-Guided Radiation Therapy

S David1*, H Lee2 , G Ibbott3 , (1) ,,,(2) UT MD Anderson Cancer Center, Houston, TX, (3) UT MD Anderson Cancer Center, Houston, TX

Presentations

(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To compare customized boluses with Superflab bolus to reduce the increased skin dose at the beam exit side due to the electron return effect for MR-image guided radiation therapy (MR-IGRT) systems.

Methods: Three silicone phantoms mimicking simple anthropomorphic geometries were molded in the shapes of a hemisphere, flat cube, and adult hand. The phantoms were designed to incorporate 2x2 cm2 EBT-3 film pieces on the beam exit surface (the non-flat surface). Custom 1cm-thick 3D printed polylactic acid (PLA) boluses and 1cm-thick eXaSkin boluses were created from CT images and molded directly onto each phantom, respectively, for comparison with 1cm-thick Superflab bolus. PLA and eXaSkin boluses were also tested in combination with ultrasound gel (UG) to remove any small air gaps. Each phantom-bolus combination was irradiated with a preclinical 7 MV MR-linac and a 6 MV linac with gantry angle 180° and 10x10 cm2 fields. Each film was read out using an Epson flatbed scanner prior to irradiation and 24 hours post-irradiation; the optical densities were converted to a relative dose for comparison.

Results: For irradiations in the MR-linac, surface doses using the Superflab, PLA, eXaSkin, PLA+UG, eXaSkin+UG on the hemisphere/cube/hand phantom were decreased by 20%/26%/28%, 7.2%/0.4%/20%, 10.9%/2.5%/26%, 21.2%/26%/23%, and 7.8%/29%/33%, respectively, relative to the “bare� scenario with no bolus added.In contrast, irradiations in the linac (no magnetic field) resulted in minimal differences with the addition of the different bolus materials. The surface doses on the hemisphere/cube/hand phantom changed -0.2%/0.3%/0.2%,-2%/0%/0%, 0.3%/-0.4%/-1.3%, 0.3%/0%/-2.3%, and -1.7%/-0.5%/-0.8%, respectively.

Conclusion: Our findings suggest that the Superflab and PLA+UG bolus materials reduced the skin dose at the beam exit surface in the MR-linac the most, indicating that the ERE for the beam exit was moved to the bolus surface from the phantom surface for irradiations in a strong magnetic field.

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