(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To investigate dosimetric effects of Biozorb markers for proton accelerated partial breast irradiation.
Methods: Two Low-Profile (LP) and four Spiral (SP) markers were selected. Each marker contained 6 titanium clips (clip diameter×width in mm: LP=3×1 and SP=6×1). LP marker sizes (width×length in cm) were 2×3 and 3×3, and SP sizes (diameter×length in cm) were 2×2, 2×3, 3×4, and 4×4. The markers were inserted at the bottom of 5-cm thick breast phantoms (solidifying powder). The phantoms were placed on 8-cm thick of solid water for CT scans. To simulate dose perturbation, a 8×8-cm2 proton beam of 103.23 MeV (range: 8.07 g/cm²) was delivered to the phantoms. For SP2×2 and SP2×3, doses were measured at five depths with a PPC05 chamber with and without clips to determine mass density of clips on CT images in RayStation. EDR2 films were placed 0 and 2 cm below the phantoms and compared to RayStation calculations with and without overriding the density of clips.
Results: Clinically used CT slice thickness of 2.5 mm exaggerated clip sizes and HU values. The chamber measurements revealed the clip density of 1.6 g/cc to produce an acceptable range perturbation in RayStation. Film measurements revealed hot and cold regions surrounding the clips while RayStation (2×2×2-mm³ grid size) only predicted higher dose due to range pullback. Dose differences between film and RayStation at 2-cm depth were slightly greater than those at 0-cm depth. Contouring the clips close to physical size and overriding clip density with 1.6 g/cc (clip artifact with 1.05 g/cc) lowered dose differences (film vs. TPS) by ~2.0%. Dose perturbation by SP was greater than LP at 2-cm depth.
Conclusion: Markers perturbed dose by ~10% more than RayStation could predict. Overriding proper density improved the RayStation dose prediction. The perturbation depended on marker compactness, clip size, and clip orientation.
Keywords
Protons, Breast, Macular Degeneration
Taxonomy
TH- External Beam- Particle/high LET therapy: Proton therapy – quality assurance
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