Current ICRU definitions of volumes: limitations and future directions.

Volume definitions used in radiation treatment planning of 3-dimensional conformal radiation therapy (3DCRT) and intensity modulated radiation therapy (IMRT) play and important role in advancing these treatment modalities. IMRT is now recognized to be more sensitive to geometric uncertainties than 3DCRT and conventional radiation therapy because of its characteristic sharper dose gradients around target volumes and organs at risk. This article reviews the current state of the art in specifying volumes for 3DCRT and IMRT and discusses some of the limitations and potential pitfalls with the current International Commission on Radiation Units and Measurements (ICRU) Reports 50 and 62 recommendations, particularly in regard to issues of organ motion when irradiated by a time-dependent irradiation modality such as dynamic multileaf collimation IMRT or other methods that have a time component in beam delivery.