D and V notation in radiation therapy planning

One of the commonly used notation conventions in radiation therapy planning is the D and V notation (e.g. D90 and V120). In this short post I will explain them.

The D and V notation are conventions that are designed to designate volumes and doses that correspond to features and goals in a treatment plan (doses and volumes).

DXX designates the minimum absorbed dose received by XX% of the total volume of a structure of interest.

Example: D90 of the prostate = 65 Gy.
This means that 90% of the prostate volume receives at least 65 Gy. This is determined from the cummulative DVH (explained in this post) and can be read off the cDVH. The D90 can be visualized in a TPS by setting the minimum display dose to 65 Gy.

Note: Instead of percent of the volume DXX could be specified in absolute volume, such as cm³.

D50 of the breast volume is determined on the cDVH to be equal to 307 cGy.

VXX designates the volume that receives XX Gy.

Example: V120 of the prostate
This would be the volume of the prostate that receives 120 Gy from the current plan. This value is also determined from the cDVH. Your graphical TPS will display this as a 3D volume or an isodose contour.

Note: Instead of gray the dose in VXX could be specified as percent dose (typically normalized to DRx).

V8 (or V57%) of the breast volume is determined from the cDVH to be equal to 147.8 cm³.

The V and D notations are simply arbitrary conventions, which are easy to confuse. Hopefully this post will serve as a reference to remind you what the notations mean.

I hope this helps and let me know if you have questions or corrections.

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.