Calculating dose from a 2.5 MV imaging beam using a commercial treatment planning system

Abstract

Patient dose from 2.5 MV images on the TrueBeam linear accelerator is not easily quantified, primarily because this beam energy is not normally modeled by commercial treatment planning systems. In this work we present the feasibility of using the Eclipse® treatment planning system to model this beam. The Acuros XB and the AAA dose calculation algorithms were tested. Profiles, PDDs, and output factors were measured for the 2.5 MV unflattened imaging beam and used for beam modeling. The algorithms were subsequently verified using MPPG 5.a guidelines. Calculated doses with both algorithms agreed with the measurement data to within the following criteria recommended for conventional therapeutic MV beams: 2% local dose‐difference in the high‐dose region, 3% global difference in the low‐dose region, 3 mm distance to agreement in the penumbra, and a gamma pass rate of >95% for 3%/3 mm criteria. Acuros was able to accurately calculate dose through cork and bone‐equivalent heterogeneities. AAA was able to accurately calculate dose through the bone‐equivalent heterogeneity but did not pass within the recommended criteria for the cork heterogeneity. For the 2.5 MV imaging beam, both the AAA and Acuros algorithms provide calculated doses that agree with measured results well within the 20% criteria for imaging beams recommended by AAPM TG‐180.

Treatment planning with a 2.5 MV photon beam for radiation therapy

“…The 2.5 MV beam has a peak and mean energies of around 120 and 500 keV, respectively, 17,19 compared to 1 and 1.5 MeV, 41 respectively, for a 6 MV beam. The considerable amount of low-energy photons in the 2.5 MV beam can lead to a dose enhancement if used in combination with gold nanoparticles (GNPs).…”

“…The study also demonstrated a sharp dose fall-off for carbon target percentage depth dose (PDD) measured in water. Other studies [18][19][20] have been performed for characterizing the physical properties of the lowenergy photons such as PDDs, dose profiles, and energy spectrum for 2.5 MV from a TrueBeam machine. Some of them 19,20 evaluated the dose to patient of the 2.5 MV imaging photons through commissioning and modeling of the beam.…”

“…Other studies [18][19][20] have been performed for characterizing the physical properties of the lowenergy photons such as PDDs, dose profiles, and energy spectrum for 2.5 MV from a TrueBeam machine. Some of them 19,20 evaluated the dose to patient of the 2.5 MV imaging photons through commissioning and modeling of the beam. The dose-volume histograms (DVHs) of the imaging fields demonstrated that the maximum doses of 2.5 MV beam to organs at risk (OARs) can be reduced to about half of 6 MV photons.…”

“…Furthermore, as the effectiveness of high-Z nanoparticles in reducing the cell survival fraction for the kV-range photons, due to the photoelectric and Auger electrons effect, has been demonstrated previously, 3,4 the 2.5 MV beam may be considered as a suitable beam for this application. The 2.5 MV spectrum contains a greater proportion of low-energy x-rays peaking at around 120 keV, 17,19 and the dose enhancement with 2.5 MV photons with the presence of gold nanoparticles (GNPs) has been indicated recently in an in vitro study, 1 while the dose enhancement for 6 MV energy is very small. 26,27 Additionally, a MC study 28 showed that the dose enhancement due to GNPs for a 2.5 MV low atomic number target is twofold of 6.5 MV beam.…”