Po‐Thur Eve General‐11: Data for Intensity Modulated Radiotherapy Planning with Pinnacle for a 6MV Siemens Primus Beam

Abstract

Background and purpose: Intensity‐modulated radiation therapy (IMRT) with a Siemens Primus linear accelerator and the Pinnacle V7.6c (IMRT V2.0) treatment planning system requires additional measurements and quality assurance (QA). This study presents the small field data for a 6MV beam and results of the additional QA for small monitor unit (MU) fields and the multi‐leaf collimator (MLC).Materials and methods: Beam measurements included relative output factors, MLC transmission and MLC interleaf transmission, percentage depth doses, and dose profiles. Beam modeling in Pinnacle is discussed. QA measurements included MU linearity and stability, MLC leaf position and abutment, and small MU field symmetry and flatness.Results and discussion: For field sizes 2×2cm2 and larger, ROFs for the two linacs were well within 0.6% of one another. MLC transmission and MLC interleaf transmission agreed with published values. The penumbral width was larger for the upper Y jaws compared to the lower MLC collimator by 1.4mm for the 2×2cm2 field. The Pinnacle model parameters that were adjusted to improve the fit to measured data were C2, K and ECD10×10, as well as the source size. MU linearity was adjusted to reduce the linearity error introduced for small MU IMRT segments. MLC leaf abutment test results are consistent with results at other centres with the same MLC.Conclusions: MU stability and MLC leaf position calibration have an important influence over the accuracy of the delivered dose for small MU segments. Field flatness in the inplane direction will also degrade dose accuracy for small MU segments.