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Influence of Bone Tissue On the Dose Distribution Characteristics for a Low-Energy Low-Dose-Rate Brachytherapy Implant

M Rivard

Mark J Rivard*, Tufts University, Boston, MA


SU-I-GPD-T-47 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Conventional brachytherapy treatment planning systems calculate dose-to-water for clinical applications based on TG-43 reference conditions, but sometimes implants are located in the vicinity of high-Z tissues such as bone. The radiological differences between water and bone are dramatic for low-energy photon-emitting radionuclides such as ¹²⁵I and ¹⁰³Pd. The dosimetric influence of bone in comparison to water was evaluated for several geometries specific to the CivaSheet ¹⁰³Pd brachytherapy device.

Methods: The geometry of an individual CivaSheet element (i.e., CivaDot) was simulated with the MCNP6 radiation transport code. To evaluate various implant placements relative to surrounding tissue, doses for CivaDot distances 0≤d≤0.5cm along the central axis from bone were examined in 0.1cm high/wide voxels with comparisons to the all-water reference geometry.

Results: Along the CivaDot central axis when in contact (d=0) with bone, dose-to-bone was 6.3x higher than dose-to-water at the surface (d=0), becoming equivalent due to photon attenuation by bone at d=0.3cm, and 3.5x lower at d=0.5cm. Offset 0.6cm laterally to approximate the mean rectilinear spacing (0.8cm x 0.8 cm) between CivaDots, dose-to-bone was 4.5x higher than dose-to-water at the surface, becoming equivalent due to bone attenuation at d=0.05cm, and 13x lower at d=0.5cm. For the geometry with the CivaDot at d=0.5cm, dose-to-bone was 3.3x higher than dose-to-water at the surface, becoming equivalent due to bone attenuation at d=0.12cm, and 15x lower at d=0.5cm. This trend was similar to when no intervening material was present and for 0≤d≤0.5cm. When correcting dose distributions to account for arbitrary implant placement, the results became equivalent within the simulation uncertainties (2% for d<0.5cm). Consequently a distance-dependent scaling correction to the water dose distribution could accurately replicate the bone dose distribution.

Conclusion: Using this technique for correcting TPS-derived dose permits accurate depiction of ¹⁰³Pd dose distributions in the vicinity of high-Z tissues such as bone.

Funding Support, Disclosures, and Conflict of Interest: Research support for this project is provided by CivaTech Oncology, Inc. (Durham, NC).

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