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Investigation of the Dosimetric Equivalent Thickness of Multiple Layers of Mesh Bolus as An Alternative to Tissue-Equivalent Bolus

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K Yan

K Yan1*, C Able2 , A Pennington1 , (1) Florida Cancer Specialists, Tampa, FL, (2) Florida Cancer Specialist, New Port Richey, FL


SU-I-GPD-T-489 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: This study aims to investigate equivalent thickness (ET) of multiple layers of mesh bolus (MLMB) as an alternative to tissue-equivalent bolus. Brass mesh bolus (MB) has the advantage of better conformity on irregular surfaces. Clinical use of MB requires predictability of scatter characteristics and depth equivalency to tissue.

Methods: To measure the depth dose characteristics, a Farmer chamber was placed at 1cm depth – solid water (SW) (6MV, 100SAD, 10x10 and 100mu). MLMB were placed on top of the phantom and compared to SW of different buildup thicknesses (0.2 to 1cm). Also, the surface dose was evaluated using parallel plate chamber and optically stimulated luminescent dosimeters (OSLD). The surface measurements compared doses beneath MB, synthetic gel (Superflab) bolus, and treatment planning system (TPS) calculated doses. Finally, a breast tangent field was delivered to an anthropomorphic phantom using MLMB. OSLD doses at different locations were compared to TPS doses.

Results: The depth dose differences between MB and SW range from -0.94% to -2% when using the manufacture recommended 2mmET, and from -0.01% to -1.56% when using 1mmET. When comparing surface dose, the difference ranges from -12.57% to -14.09% for 2mmET and from 0.5% to 9.4% for 1mmET. OSLD measurements showed that the difference is -0.27% between 5 layers of MB and 5mm Superflab, and 1.96% between 5 layers of MB and 5mm water buildup in TPS. TPS analysis of the doses for the anthropomorphic phantom indicates ET is close to 1mm/layer when using MLMB.

Conclusion: Previous studies report a single layer of MB equals 2mm or 3mm water ET. However, results of this study suggests when using MLMB, the ET is 1mm/layer for 6MV photons. The TPS can model MB, but caution should be taken when using TPS to predict surface dose with fewer layers (less than 3) of MB.

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