Results of a multicenter intensity modulated radiation therapy treatment planning comparison study for a sample prostate cancer case

Abstract

To determine the influence of different medical physicists, photon energies, treatment planning systems and treatment machines on the resulting external beam radiotherapy dose distribution for a sample prostate cancer case. A pre-contoured computed tomography (CT) dataset containing planning target volume 1 (PTV1) prostate and seminal vesicles (single dose [SD] 1.8\u202fGy, total dose [TD] 59.4\u202fGy), PTV2 prostate (simultaneously integrated boost [SIB], SD 2.0\u202fGy, TD 66\u202fGy), PTV3 prostate and seminal vesicles approach (SD 1.8\u202fGy, TD 73.8\u202fGy/80.4\u202fGy SIB) as well as organs at risk (OAR: rectum, bladder, femoral heads, bowel, anus) was offered to the members of the task group IMRT (intensity-modulated radiation therapy) of the German Society for Medical Physics. The purpose was to calculate one combined treatment plan (TP) for PTV1 and PTV2, as well as a separate one for PTV3. Dose volume histograms (DVH), different dose values, conformity index (CI), homogeneity index (HI), gradient index (GI) and a new “better than average score” were used to analyse the dose distributions. Altogether 44 institutions took part in this study and submitted acceptable dose distributions for the PTVs. However, there were statistically significant differences, especially for the doses administered to the OAR, such as rectum, bladder and femoral heads. Differences between the treatment plans were not easily detectable by visual inspection of the isodose distribution. Dose maxima may occur outside the PTV. Even though scoring indices are already published, the new “better than average score” was needed to identify a plan that minimises dose to all OAR simultaneously. Different medical physicists or dosimetrists, photon energies, treatment planning systems, and treatment machines have an impact on the resulting dose distribution. However, the differences only become apparent when comparing DVH, analysing dose values, comparing CI, HI, GI, as well as reviewing the dose distribution in every single plane. A new score was introduced to identify treatment plans that simultaneously deliver a low dose to all OAR. Such inter- and intra-institutional comparison studies are needed to explore different treatment planning strategies; however, there is still no automatic solution for an “optimal” treatment plan.