DICOM IN RADIOTHERAPY

First, what is DICOM?

In the 1980's, it became clear to radiologists and the manufacturers of medical imaging equipment that the tremendous growth in image acquisition systems, display workstations, archiving systems, and hospital-radiology information systems made vital the connectivity and interoperability of all pieces of equipment. In order to simplify and improve equipment connectivity, medical professionals joined forces with medical equipment manufacturers in an international effort to develop DICOM, the Digital Imaging and Communications in Medicine Standard. When DICOM is built into a medical imaging device, it can be directly connected to another DICOM-compatible device, eliminating the need for a custom interface. DICOM defines the interface.

DICOM was first developed to address connectivity and inter-operability problems in radiology, but today there are sections of the DICOM standard which define objects for many other modalities, including ultrasound, X-ray, and now radiotherapy. The DICOM standard currently provides the following capabilities:

How and why was DICOM RT developed?

During the RSNA meeting of 1994, a meeting was held at which a clear need was expressed for standardization of the way radiotherapy data (such as external beam and brachytherapy treatment plans, doses, and images) are transferred from one piece of equipment to another. The importance of such a standard was clear. The cost of developing custom interfaces, especially in radiotherapy departments where multi-vendor installations are common, is high, an expense which must be passed on to the user. Not only are such developments costly, they can be technically difficult, slowing down the progress of integration of the radiotherapy department, and are also safety critical. Although a standard such as DICOM does not eliminate these issues, it can facilitate development of safe, reliable inter-operability.

As a result of the RSNA meeting, an ad-hoc Working Group, later to become Working Group 7 (Radiotherapy Objects) was formed under the auspices of NEMA (National Electrical Manufacturers' Association). Participating members of this group include many manufacturers of radiotherapy equipment, some academics, and also members involved with the IEC.

In 1997, four DICOM RT objects were ratified: RT Structure Set, RT Plan, RT Dose, and RT Image. Currently, three additional objects, for recording treatment delivery data, are close to being standardized.

What can DICOM RT provide?

In order to understand what DICOM RT can and cannot provide it is important for radiotherapy professionals to distinguish between DICOM connectivity and application interoperability. DICOM connectivity refers to the DICOM message exchange standard responsible for establishing connections and exchanging properly structured messages so that an information object sent from one node will be completely received by the receiving node. In other words, the successful transfer of information: the successful "plug and exchange" between two pieces of equipment.

Beyond connectivity lies application interoperability - the ability to process and manipulate information objects. DICOM RT plays a crucial role in enabling such interoperability, but sometimes "plug and play" at this level requires more than the standardized definition and coding of information provided by DICOM. Specification and testing of the clinical application capabilities and data flow needs to be performed by the healthcare facility to ensure effective integration of the various DICOM applications. For example, transfer of IMRT (intensity-modulated) data from an IMRT-capable treatment planning system requires a record and verify or treatment system capable of managing such dynamic treatments. As we will see later, DICOM requires implementers to explicitly specify these application-specific information needs in a DICOM Conformance Statement which will provide the basis for achieving such application interoperability.

What are the DICOM RT objects and what do they do?

There are four DICOM RT objects that have been implemented today:

In addition, three Treatment Record objects are in the final stages of standardization. These objects will be used to record actual treatment session parameters (rather than planned ones).

A DICOM RT example

To illustrate how DICOM and DICOM RT objects can be used during an external beam patient treatment, here is a list of possible treatment steps, and their associated DICOM objects:

  1. The patient is scanned on a CT scanner, producing a DICOM CT image study. Other DICOM image modalities, such as MR, could also be produced.
  2. A virtual simulation application queries the scanner using DICOM, retrieves the images, and performs a virtual simulation. An RT Structure Set object is produced, containing identified structures such as the tumor and critical organs. An associated RT Plan is also created, containing beam geometry information. Digitally-reconstructed radiographs (DRRs) may also be created as RT Image objects.
  3. A treatment planning system (TPS) then reads the CT Images, RT Structure Set, and RT Plan. It adds beam modifiers, modifies the beam geometries where necessary, and also calculates dosimetry data for the plan. A new RT Plan object is created, and RT Image DRRs may also be produced.
  4. A record and verify system then obtains the completed RT Plan object, and uses the data contained within it to initialize a linac treatment. Alternatively, the linac itself could make use of the object directly. An EPID can create RT Image verification images, or compare acquired images with DRRs created by the above steps.
  5. Periodically during the course of treatment, the linac or record and verify system creates Treatment Record objects, generally one for each treatment session.

The above sequence illustrates just one scenario. In reality there is a wide variety of different utilizations possible, and DICOM RT objects have been designed with this flexibility in mind.

The DICOM Conformance Statement

Connectivity between two pieces of equipment can be evaluated ahead of time by the use of the equipment's DICOM Conformance Statements. However, for RT applications, it is not usually possible to determine inter-operability – this must be done by extensive testing. Nevertheless, conformance statements provide a foundation to determine connectivity and assess the potential inter-operability of two products, and in some cases identify potential problems without ever having physically connected them.

It is not enough for a vendor to simply claim conformance to DICOM, and the statement "This product is DICOM" has even less meaning in the radiotherapy domain, in which inter-operability is a very complex issue. Radiotherapy professionals should insist upon a conformance statement for any device that claims to be DICOM or DICOM RT conformant. Many manufacturers make their conformance statements available on the Internet.

How are the manufacturers doing today?

Developing and testing product inter-operability is a time consuming process. This is particularly so in radiotherapy, where the complexity of the objects far exceeds those found in nearly all other modalities. After over a year of behind-the-scenes effort, many manufacturers are now selling or demonstrating versions of their products with DICOM RT capability.

In addition to vendors demonstrating DICOM CT or MR compatibility, the following manufacturers indicated as of 25 July 1998 that they would be showing some kind of DICOM RT capability at AAPM '98:

Elekta Nucletron
GE Medical Systems Picker International
Helax ROCS
IMPAC Medical Systems Siemens Medical Systems
Merge Technologies SSGI
Multidata Theratronics
NOMOS  

There will be approximately nine implementations of RT Structure Set, seven implementations of RT Plan, and nine implementations of RT Image.

Further information

For information on DICOM resources, visit Merge Technologies' web site, www.merge.com/DICOM.

For radiotherapy-specific information on DICOM, visit AAPM's web page at www.aapm.org.

For information regarding NEMA, or how to order the DICOM standard (including the RT objects), visit the NEMA web site at www.nema.org/medical/dicom. Alternatively, contact Dave Snavely, Industry Manager, NEMA, 1300 North 17th Street, Suite 1847, Rosslyn, VA 22209 USA. Phone: (703) 841-3285; fax (703) 841-3385, dav_snavely@nema.org.