RT Dose¶

The RT Dose IOD was introduced 1996 with Supplement 11 together with RT Image, RT Structure Set and RT Plan as the first RT-specific DICOM objects.

The focus for this Radiotherapy Dose IOD (RT Dose IOD) is to address the requirements for transfer of dose distributions calculated by radiotherapy treatment planning systems. These distributions may be represented as 2D or 3D grids, as isodose curves, or as named or unnamed dose points scattered throughout the volume. This IOD may also contain dose-volume histogram data, single or multi-frame overlays, audio annotations, and application-defined lookup tables. [...] [From DICOM Supplement 11]

Besides the RT Series module, the RT Dose IOD contains the following new (introduced with Supp 11) modules:

• RT Dose Module
• RT DVH (RT Dose Volume Histograms) Module
• Structure Set Module
• ROI Contour Module
• RT Dose ROI Module

Besides these new RT modules, for some dose definitions (see below) the RT Dose IOD re-uses some common modules normally used for image objects, e. g. Image Pixel and the Multi-Frame Module.

It was decided to put RT Dose information into a seperate type of object (RT Dose) instead of integrating RT Dose information into the RT Plan object. This was done due for the following reasons [DICOM Supplement 11]:

• To allow for the multiplicity of possible dose calculations using beam models for the same basic plan
• To avoid undesirable transmission of large amounts of data with the treatment plan
• To accomodate the fact that CT Simulation and other "beam geometry" generating devices which use the RT Plan IOD do not have or require access to this data, either for transmission or storage.

Defining Doses¶

As stated in the introduction, there are different possibilities how doses can be defined, mainly by using the new modules introduced with the RT Dose IOD. The different possibilities are described below. All those possibilities are not mutually exclusive from each other - they can co-exist in a single RT Dose object instance (e. g. there can be 3D grid dose grids as well as iso dose curves).

2D and 3D grids¶

2D or 3D (set of 2D) grids can be defined using the RT Dose module in conjunction with the common modules used for image objects, ie the General Image, Image Pixel and Multi-Frame module are "re-used" for that.

The RT Dose module is used to convey 2D or 3D radiation dose data generated from treatment planning systems or similar devices. The attributes defined within the module support dose for a single radiation beam (potentially comprised of multiple segments, as delivered in a dynamic treatment) or a group of beams comprising either a fraction group or a complete treatment plan (potentially the sum of multiple fraction groups). [DICOM Supplement 11]

The module is comprised of the following "functional" parts:

• "Overriding" of Image Pixel Module attributes (e. g. Photometric Interpretation)
• General Dose information
• Referenced RT Plan Sequence
• Grid Frame Offset Sector

“Overriding” of Image Pixel Module attributes (e. g. Photometric Interpretation)¶

In re-using the Image Pixel module attributes, a dose grid is defined like an image. The RT Dose Module places some restrictions on the "image" attributes, e. g. Photometric Intepretation is always MONOCHROME2 with Samples Per Pixel=1; Bits Allocated and Bits Stored are 16 or 32, which means that RT Dose allows for defining images (in this case doses) with color (dose) depth of 32 Bit. One pixel in the Pixel Data element then represents a dose at a specific point.

General Dose Information¶

This part of the module contains general information about the dose specified. With attribute Dose Units the physical unit of the doses of all dose point in Pixel Data is defined. This unit can either be GY (for Gray units) or RELATIVE, which means that the dose is relative to an implicit reference point. In case of GY one can specify in the attribute Dose Grid Scaling a scaling factor for converting dose data in Pixel Data element into GY.

Another important attribute is Dose Summation Type which describes what kind of dose summation is specified with this dose, Defined Terms are PLAN, FRACTION, BEAM or BRACHY. Thus, the value denotes whether the dose summation is for an entire RT Plan, a single fraction group and so on.

Referenced RT Plan Sequence¶

If Dose Summation Type is one of the defined terms (PLAN, FRACTION, BEAM or BRACHY) then the Referenced RT Plan Sequence is used to reference the corresponding RT Plan object instance or the corresponding subparts (e. g. the fraction group in case of FRACTION). For PLAN, the sequence just references the very plan via its instance UID, for FRACTION there is an additional reference to the very fraction referenced inside the plan via the fraction number, for BEAM there is both plus a reference to the very beam inside the fraction. For BRACHY the referencing is the same as for fraction, but instead referencing a fraction group inside a plan there is a reference to the corresponding Brachy Application Setup in the plan.

Grid Frame Offset Vector¶

This attribute must be included for multi-frame dose data (ie. Multi-frame Module and corresponding Pixel Data element are present). The content is a list of z coordinates in mm which indicate the plane coordinates of the dose data.

Isodose Curves and Points¶

For defining isodose curves and points, the Structure Set, ROI Contour and RT Dose ROI Modules are used. As defined in the RT Dose IOD's Module Table, either all three modules will be used (defining Isodose Curves and Points) or none of them.

The usage mechanism of the modules for defining those isodose curves and points is as follows:

1. Using the Structure Set Module a set of ROIs (Region of Interests) can be selected and given a name, a description and so on. Each ROI is uniquely identified within a Structure Set with a ROI Number. For each ROI also the Frame of Reference is defined and a number of images can be referenced used in defining the Structure Set. The Structure Set (which is the container for the ROIs) defined in this module is additionally described with label, a name, date of last modification date and time.
2. In the ROI Contour Module each ROI is referenced using the ROI Number. In the Contour Image Sequence one or more images containing the contour are referenced. Each contour is defined using a geometric figure (POINT, OPEN_PLANAR, OPEN_NONPLANAR, CLOSED_PLANAR) and the Contour Data which lists the points (x,y,z) belonging to the contour.
3. Last but not least in the (short) RT Dose ROI Module, which consists of the RT Dose ROI Sequence. For each ROI (referenced by ROI Number) defined in the Structure Set Module a Dose Value can be specified as well as the Dose Units (either GY=Gray or RELATIVE for doses relative to implicit reference value).

Conclusion: The Structure Set module defines some ROIs (and some referenced images used to derive them). Those ROIs are then filled with live with Contours in the ROI Contour module, which defines points/planes for each ROI. In the RT Dose ROI Module each ROI (and therefore each point/plane defined in the ROI Contour Module) is given a dose value and so completes the definition of iso dose points/curves.

TODO: It is not clear to me what exactly the referenced images in the Structure Set describe: Are these images (typically CT or MR) called Structure Set itself or is that only a reference to the images that were used to extract the ROIs from. The formulation in the standard regarding the referenced images is "images...used in defining the Structure Set (typically CT or MR images)". I tend to the second intepretation, also because the ROI generation algorithm is expressed for each ROI in the Structure Set Module (defined terms: AUTOMATIC=calculated ROK, SEMIAUTOMATIC and MANUAL).*

Dose-Volume Histograms (DVH)¶

In order to describe dose-volume histograms the RT DVH Module must be used. The RT DVH module provides fior the inclusion of differential or cumulative dose volume histogram data. The data contained within this module may supplement dose data in the RT Dose and/or RT Dose ROI modules, or it may be present in the absence of other dose data. [Supplement 11]. A short article about DVH can be found on rt-image.com (Link ist down).

The RT DVH Module is made of the following structure:

1. Referenced Structure Set Sequence: References exactly one Structure Set, which may contain one or more ROIs.
2. DVH Normalization: Info about Normalization Point and dose value of that point if Dose Units (defined later) are RELATIVE and not in GLY
3. DVH Sequence: Contains one or more histograms (one per item). First ROIs from the Structure Set are referenced, which were used to calculate the DVH and whether they were excluded or included into the histogram summation. Also it is stated whether the histogram is DIFFERENTIAL or CUMULATIVE (see above linked article at rt-image.com). Some other information like Dose Units, Dose Scaling, number of histogram bins follow, as well as the DVH Data attribute itself which contains the DVH data.

Further discussion¶

bbennet in comp.protocols.dicom, 2008-03-14:

An RT-Dose represents a calculated dose. It references an RT-Plan, which, in turn, references an RT-Structure Set, which, in turn, references a set of images (usually CT's). Calculation of Dose is based primarily on the RT-Plan, but may involve use of data in the RT-Structure Set, and even the CT's. It also involves models of Radiation Source which are outside of DICOM.