AbstractID: 8966 Title: EUD based margin selection: a simulation including a target, critical organ, and set-up uncertainties In radiotherapy treatment planning, estimates of organ motion and set-up uncertainties are used to determine the margin around the CTV, thus defining the PTV and ultimately the field size. Prescribed (or isocenter) dose is not usually regarded as a variable for individual patients. Using the Pinnacle3 Treatment Planning System and a simple phantom containing a target and an organ at risk (OAR) we have examined the relationships between Equivalent Uniform Doses (EUD) and Dose Volume Histograms (DVH) and geometric uncertainties, margins, isocenter doses and CTV-OAR separations. 20 courses of 35 fractions each were simulated using a Monte Carlo approach for each combination of variables. In the direction perpendicular to the plane containing the two pairs of parallel opposed beams, for example, with random and systematic uncertainties of 2 and 4mm respectively and 0mm CTV-OAR separation, increasing the isocenter dose by about 3.5Gy on a 6mm-margin plan resulted in the statistically equivalent EUD value to that with a 10mm-margin, for the CTV, while the OAR EUD is dropped by 1Gy. In the plane containing the beams’ central axes, the sensitivity of CTV EUD to margin reduction was lower although the gain in reduced OAR EUD remained significant. These observations, however, were not immediately apparent from the DVH presentation. Assuming the validity of the EUD concept, it may be advantageous to regard the isocenter dose as well as the margin to be variables during the planning process.