Treatment plan evaluation using dose-volume histogram (DVH) and spatial dose-volume histogram (zDVH)

Objective: The dose-volume histogram (DVH) has been accepted as a tool for treatment-plan evaluation. However, DVH lacks spatial information. A new co ncept, the z-dependent dose-volume histogram (zDVH), is presented as a supp lement to the DVH in three-dimensional (3D) treatment planning to provide t he spatial variation, as well as the size and magnitude of the different do se regions within a region of interest. Materials and Methods: Three-dimensional dose calculations were carried out with various plans for three disease sites: lung, breast, and prostate. DV Hs were calculated for the entire volume. A zDVH is defined as a differenti al dose-volume histogram with respect to a computed tomographic (CT) slice position. In this study, zDVNs were calculated for each CT slice in the tre atment field. DVHs and zDVHs were compared. Results: In the irradiation of lung, DVH calculation indicated that the tre atment plan satisfied the dose-volume constraint placed on the lung and zDV H of the lung revealed that a sizable fraction of the lung centered about t he central axis (CAX) received a significant dose, a situation that warrant ed a modification of the treatment plan due to the removal of one lung. In the irradiation of breast with tangential fields, the DVH showed that about 7% of the breast volume received at least 110% of the prescribed dose (PD) and about 11% of the breast received less than 98% PD. However, the zDVHs of the breast volume in each of seven planes showed the existence of high-d ose regions of 34% and 15%, respectively, of the volume in the two caudal-m ost planes and cold spots of about 40% in the two cephalic planes. In the t reatment planning of prostate, DVHs showed that about 15% of the bladder an d 40% of the rectum received 102% PD, whereas about 30% of the bladder and 50% of the rectum received the full dose. Taking into account the hollow st ructure of both the bladder and the rectum, the dose-surface histograms (DS H) showed larger hot-spot volume, about 37% of the bladder wall and 43% of the rectal wall. The zDVHs of the bladder revealed that the hot-spot region was superior to the central axis. The zDVHs of the rectum showed that the high-dose region was an 8-cm segment mostly superior to the central axis. T he serial array-like of the rectum warrants a closer attention with regard to the complication probability of the organ. Conclusions: Although DVH provides an averaged dose-volume information, zDV H provides differential dose-volume information with respect to the CT slic e position, zDVH is a 2D analog of a 3D DVH and, in some situations, more s uperior. It provides additional information on plan evaluation that otherwi se could not be appreciated. The zDVH may be used along with DVH for plan e valuation and for the correlation of radiation outcome. (C) 1999 Elsevier S cience Inc.