Abstract:
Set-up errors are the inherent features of the radiation treatment process. Coverage of target volume is a direct function of set-up margins, which should be optimized to prevent unintended irradiation of adjacent normal tissues. The aim of this study was to evaluate three dimensional set-up errors and propose optimum margins for target volume coverage in prostate cancer radiotherapy. 1620 Portal images were obtained for an average of 30 fractions per patient for 54 prostate cancer patients. Electronic Portal Image (EPI) displacements obtained with iViewGTTM imaging software along the three major axes (X, Y, Z) were analysed. Mean displacements, population systematic and random errors and
Three-dimensional vectors of displacements were calculated using StatPlus 2009 Professional 5.8.0 and Microsoft Excel (2013). Planning target margins were calculated using three different published margin recipes; the International Commission on Radiation Units and measurements (ICRU) 62, Stroom and van Herk. The random errors (σ) along vertical, lateral and longitudinal were 3.19 mm, 4.13 mm and 3.83 mm respectively. Similarly, the systematic errors (Σ) were 1.81 mm, 2.83 mm and 2.51 mm along vertical, lateral and longitudinal axes respectively. The population mean (Mpop) displacement in vertical, lateral and longitudinal axes were -0.49 mm, 0.05 mm and -0.40 mm respectively and standard deviation in vertical, lateral and longitudinal axes were 3.75 mm, 5.20 mm and 4.76 mm respectively for the prostate cancer patients. Using ICRU report 62, the clinical target volume to planning target volume margins (CTV-PTV) were 3.67 mm, 5.00 mm and 4.58 mm along vertical, lateral and longitudinal axes respectively. The corresponding values were 5.85 mm, 8.54 mm and 7.69 mm with Stroom’s formula whiles 3.75 mm, 6.96 mm and 5.95 mm were with van Herk’s formula. The results suggest that there is a significant difference within the three major axes. Hence, the need for local departmental planning target margin protocol was recommended.
