[Influence of daily set-up errors on dose distribution during pelvis radiotherapy]

An external beam radiotherapy (EBRT) using megavoltage beam of linear accelerator is usually the treatment of choice in cancer patients. The goal of EBRT is to deliver the prescribed dose to the target volume, with as low as possible dose to the surrounding healthy tissue. A large number of procedures and different professions involved in radiotherapy, uncertainty of equipment and daily patient set-up errors can cause a difference between the planned and delivered dose.We investigated a part of this difference caused by measuring daily patient set-up errors for 35 patients. These set-up errors were simulated on five patients, using 3D treatment planning software XiO. The simulation investigated differences in dose distributions between the planned and shifted geometry. Additionally, we investigated the influence of the error on treatment plan selection by analysing changes in dose volume histograms, planning target volume conformity index (CIPTV), and homogeneity index (HI).Simulations showed that patient daily set-up errors can cause significant differences between the planned and actual dose distributions. Moreover, for some patients, those errors could affect the choice of treatment plan since CIPTV fell under 97%. Surprisingly, HI was not as sensitive to set-up errors as CIPTV. Our results have confirmed the need to minimise daily set-up errors through quality assurance programmes.