The influence of Monte Carlo source parameters on detector design and dose perturbation in small field dosimetry

Technical Note: Small field dose correction factors for radiochromic film in lung phantoms

PURPOSE

Radiochromic film has been established as a detector that can be used without the need for perturbation correction factors for small field dosimetry in water. However, perturbation factors in low density media such as lung have yet to be published. This study calculated the factors required to account for the perturbation of radiochromic film when used for small field dosimetry in lung equivalent material.

METHOD

Monte Carlo simulations were used to calculate dose to Gafchromic EBT3 film when placed inside a lung phantom. The beam simulated had a nominal energy of 6 MV and the field sizes simulated ranged from 10 × 10 mm² to 30 × 30 mm² . The lung density simulated was varied between 0.2 and 0.3 g/cm³ . Each simulation was repeated with the film replaced by lung material (the same as the surrounding medium), and the required correction factors for film dosimetry in lung ( D M e d , Q D D e t , Q ) were calculated by dividing the dose in lung by the dose in film.

RESULTS

For field sizes 30 × 30 mm² and larger, no correction factors were required. At a 20 × 20 mm² field size, small corrections were required, but were within the approximate accuracy of film dosimetry (~2%). For a 10 × 10 mm² field size, significant correction factors need to be applied (0.935 for lung density of 0.20 g/cm³ to 0.963 for lung density of 0.30 g/cm³ ). The values lower than one mean that the film is over-responding. At the "upstream" lung-water interface the correction factors were close to unity; while at the downstream interface the corrections required were marginally smaller to those at the center of lung. One centimeter or more away from the interfaces, the correction factor did not vary as a function distance from the interface (in the beam direction). Away from the central axis (perpendicular to the beam direction), the correction factors increased slightly (away from unity) as a function of off-axis distance, before abruptly changing direction at the penumbra, with the film actually under-responding by ~10% outside the field edges.

CONCLUSION

Accurate dosimetry of very small fields (15 × 15 mm² or smaller) using radiochromic film requires correction factors for the perturbation of the film on the surrounding lung material. This correction factor was as high as 6.5% for a 10 × 10 mm² field size and a density of 0.2 g/cm³ . This will increase if either the density or the field size decrease further. This correction factor does not vary as a function of depth in lung once charged particle equilibrium is established.