Dose-volume histogram basics

A dose-volume histogram (DVH) is a mathematical tool to assess the appropriateness of a given radiation therapy plan. It can be used to assess whether a plan meets desired constraints for a voulme of interest, within certain limitations. DVH’s are widely used and understanding how they work is a basic skill for treatment plan assessment. In this post I’ll discuss some DVH basics.

A typical cumulative dose-volume histogram (cDVH).

A DVH is nothing more than a histogram, but it is important to understand where the data comes from and how the DVH is representing the data. Modern treatment plans are created based on 3D image sets created using CT, MRI, etc. These data sets consist of voxels (the 3D equivalent of pixels). A volume of interest, e.g. a PTV, consists of a subset of these voxels. The basic data in a DVH is generated by binning the dose values from each voxel in the volume. (Interpolation may be necessary if the bound of the volume intersects a voxel.) This binned dose frequency data comprises a differential dose-volume histogram, or dDVH, which I will discuss in more detail in a future post. The dDVH looks like a common histogram and gives you an idea of how many voxels receive a certain dose, e.g. the dDVH might show that 85% of the PTV voxels received 98% - 102% of the prescribed dose and 46% received exactly 100% of the prescribed dose.

The more familiar form of DVH is the cumulative dose-volume histogram, or cDVH. This DVH is calculated by summing the dDVH starting at the dose of interest, D, up to the max dose, D_max (Eq. 1).

Eq. 1

The cDVH displays the percent/number of voxels in a volume which receive at least a dose D, i.e. the cDVH of a volume irradiated perfectly uniformly to 100 cGy will show that 100% of the voxels received at least 30 cGy, 50 cGy, 80 cGy, etc, but 0% received 105 cGy. Thus for an ideal treatment plan, the cDVH’s of the target volumes will have a rectangular, step-down function appearance and the cDVH’s of critical volumes will drop immediately to zero.

In the real world treatment plans are not ideal (I know, it’s sad). Instead acceptable dose constraints are set for targets and critical structures. DVH’s can be used to determine if these constraints are mets. One caveat is that standard DVH’s do not directly provide spatial information about the dose distribution. One less than ideal method is to create sub-volumes, but creating useful/meaningful sub-volumes is a non-trivial exercise.

Top image from Vorwerk et al. Radiation Oncology 2008 3:31, doi:10.1186/1748-717X-3-31, used under CC License terms.