The importance of leaf-positional analysis in dynamic delivery of IMRT

Abstract

The analysis of dynamic MLC positions for the delivered IMRT plans is crucial in that it may capture dose delivery problems otherwise difficult to observe and quantify in the conventional dosimetric measurements with a film or with ionization chamber. In some IMRT systems delivery of IMRT fields starts with a maximum MLC opening (roughly the shape of the target in the BEV) and then proceeds to the actual dynamic MLC sub-fields. No irradiation is required in going from the initial segment (maximum opening) to the next one and theoretically, no dose should be delivered in that initial moment. However, due to a finite sampling time of the MLC controller, finite speed of the MLC and a finite leaf tolerance there may be some dose delivered between the first and the second segment. The amount of the excess dose is higher for larger dose rates and smaller number of the total monitor units per IMRT field. The magnitude of dose errors could be in order of a few percent. Effects similar to the maximum MLC opening may occur in other situations as well. For instance, when leaves are forced to move over large distances in a short time. Confounding this are dose errors due to the uncertainty in MLC transmission. The analysis of the actual leaf positions recorded in the dynamic MLC log file is helpful in differentiating between the two types of errors and in determining the optimal dynamic MLC delivery parameters.