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Dosimetric Implications for Post-Mastectomy Radiation Treatment (PMRT) with AlloX2 Tissue Expander

J Pan1*, M Lafreniere2, J Yang3, E Hirata4, M Sharma5, (1) UCSF Medical Center, Millbrae, CA, (2) Department of Radiation Oncology at UCSF, San Francisco, CA, (3) University Of California, San Francisco, ,,(4) University of California San Francisco, San Francisco, CA, (5) University of California San Francisco, Albany, CA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The purpose of this study was to investigate the dosimetric implications of using a unique breast tissue expander, AlloX2 (Sientra Inc) for PMRT.

Methods: AlloX2 breast tissue expander has two high density ports for injection and draining, perturbing the radiation. A retrospective study was done on three patients with AlloX2 tissue expanders. The initial CT scans were performed on Siemens CT with Iterative Metal Artifact Reduction (iMAR) and planned in RayStation treatment planning system (TPS) using Varian TrueBeam 6MV photon VMAT or IMRT technique. Three models were created for metallic ports. Model 1 was based on visual inspection on CT images with uniform density override (titanium), Model 2 was based on vendor specified values of dimensions and material (magnet and titanium) and Model 3 had no density override. Planned target (PTV_Eval) coverage was compared between three models in RayStation TPS. An in-house phantom with AlloX2 was also irradiated with nanoDot Optically Stimulated Luminescence (OSLD) for dose comparison.

Results: The PTV_Eval prescription coverage varied between 0.6% - 2.9% among three models. OSLD measurements in the phantom showed that Model 1 had better agreement with measurements at shallow depth (1cm) but overestimated dose (<1%) , whereas Model 3 had better agreement with measurements at deep depth (2.5cm) but underestimated dose (<5%). OSLD measurements away from metallic port edge showed minimum discrepancies (< 2.2%) for all models. Interestingly, Model 2 which had the most accurate geometry and composition of AlloX2 had the largest discrepancy (~8%) from OSLD measurements at 2-3cm depth.

Conclusion: PMRT with AlloX2 presents significant clinical challenge. With our three models, we found that the dose calculation accuracy is limited with the Collapsed Cone (CC) dose algorithm used in RayStation TPS. Further studies will be conducted to find the best approach for PMRT with AlloX2.

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Keywords

Treatment Planning, Breast, Modeling

Taxonomy

TH- External Beam- Photons: General (most aspects)

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