Citation: Physics in medicine and biology. 2020 June. Online ahead of print
Author: Sudhir Kumar, Alan E Nahum, Indrin J Chetty
Abstract: Small-field dosimetry is central to the planning and delivery of radiotherapy to patients with cancer. Small-field dosimetry is beset by complex issues, such as loss of charged-particle equilibrium (CPE), source occlusion and electron scattering effects in low-density tissues. The purpose of the present research was to elucidate the fundamental physics of small fields through the computation of absorbed dose, kerma and fluence distributions in heterogeneous media using the Monte-Carlo method. Absorbed dose and kerma were computed using the DOSRZnrc Monte-Carlo (MC) user-code for beams with square field sizes ranging from 0.25 × 0.25 to 7× 7 cm2 (for 6 MV 'full linac' geometry) and 0.25 × 0.25 to 16 × 16 cm2 (for 15 MV 'full linac' geometry). In the bone inhomogeneity the dose increases (vs. homogeneous water) for field sizes < 1 × 1 cm2 at 6 MV and ≤ 3 × 3 cm2 at 15 MV and decreases (vs. homogeneous water) for field sizes ≥ 3 × 3 cm2 at 6 MV and ≥ 5 × 5 cm2 at 15 MV. In the lung inhomogeneity there is negligible decrease in dose compared to in uniform water for field sizes > 5 × 5 cm2 at 6 MV and ≥ 16 × 16 cm2 at 15 MV, consistent with the Fano theorem. The near-unity value of the absorbed-dose to collision-kerma ratio, D/Kcol, at the centre of the bone and lung slabs in the heterogeneous phantom demonstrated that CPE is achieved in bone for field sizes > 1 × 1 cm2 at 6 MV and > 5 × 5 cm2 at 15 MV; CPE is achieved in lung at field sizes > 5 × 5 cm2 at 6 MV and ≥ 16 × 16 cm2 at 15 MV. Electron-fluence perturbation factors for the 0.25 × 0.25 cm2 field were 1.231 and 1.403 for bone-to-water and 0.454 and 0.333 for lung-to-water were at 6 and 15 MV respectively. For field sizes large enough for quasi-CPE, the MC-derived dose-perturbation factors, lung-to-water, were close to unity; electron-fluence perturbation factors, lung-to-water, were ~1.0, consistent with the 'Fano' theorem. At 15 MV in the lung inhomogeneity the magnitude and also the 'shape' of the primary electron-fluence spectrum differed significantly from that in water. Beam penumbrae relative to water were narrower in the bone inhomogeneity and broader in the lung inhomogeneity for all field sizes.
Keywords: Fano theorem; Monte Carlo; absorbed dose; electron-fluence perturbation; kerma; non-equilibrium photon fields.
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