Effects of collimator backscatter in an Elekta linac by Monte Carlo simulation
The effects of radiation backscattered from the secondary collimators into the monitor chamber in an Elekta linac (producing 6 and 10 MV photon beams) are investigated using BEAMnrc Monte Carlo simulations. The degree and effects of this backscattered radiation are assessed by evaluating the changes to the calculated dose in the monitor chamber, and by determining a correction factor for those changes. Additionally, the fluence and energy characteristics of particles entering the monitor chamber from the downstream direction are evaluated by examining BEAMnrc phase-space data. It is shown that the proportion of particles backscattered into the monitor chamber is small (<0.35 %), for all field sizes studied. However, when the backscatter plate is removed from the model linac, these backscattered particles generate a noticeable increase in dose to the monitor chamber (up to ≈2.4 % for the 6 MV beam and up to 4.4 % for the 10 MV beam). With its backscatter plate in place, the Elekta linac (operating at 6 and 10 MV) is subject to negligible variation of monitor chamber dose with field size.
At these energies, output variations in photon beams produced by the clinical Elekta linear accelerator can be attributed to head scatter alone. Corrections for field-size-dependence of monitor chamber dose are not necessary when running Monte Carlo simulations of the Elekta linac operating at 6 and 10 MV.
Key wordslinear accelerator Monte Carlo dose calculation radiation therapy scatter
- 6.Paelinck, L., de Wagter, C., van Esch, A., Duthoy, W., Depuydt, T. and de Neve, W.,Comparison of build-up dose between Elekta and Varian linear accelerators for highenergy photon beams using radiochromic film and clinical implications for IMRT head and neck treatments, Phys. Med. Biol. 50:413–428, 2005.CrossRefPubMedGoogle Scholar
- 13.Rogers, D. W. O., Walters, B. and Kawrakow, I.BEAMnrc Users Manual, Ionising Radiation Standards, National Research Council of Canada, Ottawa, 2004.Google Scholar
- 14.Kairn, T., Cassidy, D., Sandford, P. M. and Fielding, A. L.,Radiotherapy treatment verification using radiological thickness measured with an amorphous silicon electronic portal imaging device: Monte Carlo simulation and experiment, Phys. Med. Biol. 53(14):3903–3919, 2008.CrossRefPubMedGoogle Scholar