(Saturday, 4/4/2020) [Mountain Time (GMT-6)]
Purpose: To compute and explain small-field dose distributions in heterogeneous media.
Methods: A cylindrical lung-equivalent (? = 0.21 g/cc) slab phantom (outer radius 15 cm, height 30 cm) consisted of 3 cm water, then 7 cm lung-equivalent material then 20 cm water; the bone-equivalent slab phantom consisted of 3 cm water then 2 cm bone-equivalent material, then 25 cm water. Additionally, a homogeneous (water) phantom of the same dimensions was modelled. The absorbed dose, D, was derived from Monte-Carlo code DOSRZnrc for field sizes, FSs, of 0.25 × 0.25 to 5 × 5 cm² (6 MV) and 0.25 × 0.25 to 16 × 16 cm² (15 MV) defined at 100 cm SSD in heterogeneous and homogeneous phantoms at various depths along the beam axis, using full linac geometry phase-space files. Dose-perturbation factors, DPFs, bone/lung-to-water, were derived. The dose to collision kerma ratio, D/Kcol, at the centre of the bone/ lung slabs was also computed vs FS at 6 MV and 15 MV.
Results: Dose enhancement in bone was obtained for <1×1 cm². At FSs broad enough (=2 × 2 cm²) for quasi-CPE, dose downstream from the proximal water-bone interface is reduced, due to lower mass energy-absorption coefficient in bone than water. A large reduction in dose within lung was found – this is due to its low density, hence longer electron ranges compared to water. Lateral electronic equilibrium is achieved in bone for FS =1 × 1 cm² at 6 MV and =5 × 5 cm² at 15 MV and in lung for =5 × 5 cm² at 6 MV and =16 × 16 cm² at 15 MV. At 0.25 × 0.25 cm², DPFs are 1.287 and 0.327 for bone and lung respectively for the 15 MV beam.
Conclusion: Small, non-equilibrium megavoltage photon fields pose major challenges for treatment-planning algorithms.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by a research grant from Varian Medical Systems, Palo Alto, CA.
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