Purpose: This study investigated the effect of dose enhancement and beam attenuation on the depth dose, when gold nanoparticles (GNP) were added to the soft tissue, using the flattening filter-free (FFF) photon beams in GNP-enhanced radiotherapy.
Methods: A cubic water phantom irradiated with the 10 MV FF and FFF photon beams (field size=10×10 cm²) was used. GNPs with concentrations ranging from 3 to 40 mg/mL were added to water. The depth doses of the phantom were determined using Monte Carlo simulation under the macroscopic approach. The depth dose enhancement ratio (DDER), defined as the ratio of the dose with GNP addition at a depth to the dose without GNP addition at the same depth, was calculated for each GNP concentration.
Results: When GNPs were added, the DDERs were found larger than one in the depth range of 4-6 and 4-8 cm for the 10 MV FF and FFF photon beams, respectively. However, when the depth was larger than 6 (FF beam) and 8 cm (FFF beam), the enhancement ratios were less than one. This shows that the presence of GNPs did not increase the dose beyond 6 and 8 cm, and it is because the GNP addition not only enhances the dose, but attenuates the beam along the depth. Comparing the FF and FFF photon beams with GNP concentration increased from 3 to 40 mg/mL, the DDERs were found increased from 1 to 1.004 (FF beam) and 1 to 1.015 (FFF beam). This is because the FFF beam contained more low-energy photons than the FF.
Conclusion: When GNPs were added to the phantom, the dose enhancement decreased with depths deeper than 6 and 8 cm for the FF and FFF photon beams, respectively. In addition, the dose enhancement was found decreasing at a higher rate when the GNP concentration increased.