Monte-Carlo-computed dose, kerma and fluence distributions in heterogeneous slab geometries irradiated by small megavoltage photon fields*

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 is the elucidation of the fundamental physics of small fields through the computation of absorbed dose, kerma and fluence distributions in heterogeneous media using the Monte-Carlo (MC) method. Absorbed dose and kerma were computed using the DOSRZnrc 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 demonstrates 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 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 differ significantly from that in water. Beam penumbrae relative to water are narrower in the bone inhomogeneity and broader in the lung inhomogeneity for all field sizes.