Room: Exhibit Hall
Purpose: This study investigated the dependences of mucosal dose and photon energy spectrum on the flattened and unflattened photon beams with different energies using Monte Carlo simulation.
Methods: A heterogeneous mucosal phantoms containing water (mucosa and soft tissue), air and bone, with mucosa thicknesses varying from 0.5â€“3 mm were irradiated by the 6 and 10 MV unflattened and flattened photon beams (field size = 10Ã—10 cmÂ²) generated by a Varian TrueBeam linear accelerator. The photon energy spectrum at the mucosa and depth doses at the mucosa and bones were calculated using the EGSnrc Monte Carlo code.
Results: From the photon energy spectra of the unflattened beams, the 6 MV beam was found to contain more particle fluence than the 10 MV in the low-energy range between 0 and 100 keV. However, the mucosal doses for the unflattened photon beams were calculated to be lower than the flattened beams for both the 6 and 10 MV. Although both mucosal and skin structure involve an air-soft tissue interface, the skin dose enhancement generally observed in an unflattened photon beam irradiation was not found in the mucosa. Moreover, the particle fluence of the 6 and 10 MV unflattened photon beams was found to increase with an increase of the mucosal thickness. This agreed with the relative depth doses that increased in the ranges of 3.4%-3.6% (6 MV) and 2.5%-3.0% (10 MV) at the upper mucosa, and 5.1%â€“5.7% (6 MV) and 2.9%â€“3.2% (10 MV) at the lower mucosa, with their thickness increasing from 0.5â€“3 mm when the flattening filter was removed from the beam path.
Conclusion: Mucosal dose was found lower for the unflattened photon beam compared to the flattened, and the 6 MV unflattened beam has a greater mucosal dose dependence on its thickness than the 10 MV.