AbstractID: 9094 Title: IMRT plan optimization using Pseudo Volume Constraint (PVC)
technique
One of the most significant challenges in inverse treatment planning is to control the shape of dose
distribution by judiciously modifying the dose optimization objective functions. It is not unusual for an
IMRT plan to have hot spots outside the target volume. We introduce a technique called PVC (Pseudo
Volume Constraint) that improves the dose conformity and homogeneity in target volume and removes
the possibility of a hot spot in the surrounding normal tissue. In this technique, we define a pseudo
volume (PV) that surrounds the target volume. This volume is assigned a dose limit that is a certain
percentage of the prescribed dose. For example, in a case of cavernous sinus meningioma, an optimal
conformal and homogeneous plan was obtained by defining a PV of 13 mm shell that is placed 2mm
vector distance from the target volume. This volume was constrained to 80% of the prescribed dose.
Further limiting the dose to PV improved the conformity but at the price of dose inhomogeneity. This
methodology has allowed us to develop unambiguous class solutions for optimal plans in all disease
sites. Our plan evaluation is based on a new technique in which we calculate the average distance
between the prescription isodose (100%) and 80% isodose surface. The distance between target and
the prescription isodose surface is also considered for plan evaluation. This method is more logical than
the ratio of prescribed isodose volume to target volume (PITV) index, which is very sensitive to the
absolute target volume.