Room: ePoster Forums
Purpose: This study compared the variation of bone dose in an air cavity of the head-and-neck site between the flattening-filter-free (FFF) and flattening filter (FF) photon beams. Monte Carlo simulation was used to determine the photon energy spectrum and relative depth dose of the bone associated with the mucosa in the cavity.
Methods: A heterogeneous phantom mimicking an air cavity in the head-and-neck region was used. The phantom contained mucosa, air and bone, with the mucosa thickness varying from 0.5â€“3 mm. The phantom was irradiated by the 6MV FFF, 6MV FF, 10MV FFF and 10MV FF photon beams with field size equal to 10Ã—10 cmÂ². The photon energy distribution at the mucosa and mean relative bone dose for the photon beams were determined using the EGSnrc Monte Carlo code.
Results: For the photon beam energy distribution, the FFF beams (6 and 10MV) had more low-energy photons than the FF beams with the same beam energy. Moreover, the photon beam energy distribution was found independent on the change of mucosa thickness. For the mean bone dose under the mucosa in the air cavity, both the 6 and 10MV FFF photon beams had lower bone dose than the FF beams. The differences of mean bone dose between the FFF and FF beams at the upper bone was 0.5%-1%, while for the lower bone was 1%-1.5%. This lower bone dose from the FFF beam was due to the lower depth dose of the beam at the same depth compared to the FF beam for the same beam energy.
Conclusion: By comparing the mean bone dose in an air cavity between the FFF and FF photon beams, it is concluded that lower bone dose was found using the FFF beam, even it contained more low-energy photon than the FF beam.