RT Workflow¶
When it is decided to have a radiotherapy treatment for a patient, the anatomical region to be treated must be selected. There are different methods to do that. First, the region can be selected by a physician just relying on it's experience and estimation.
Making a plan: Selecting Target Volume, Dose¶
However, traditionally DICOM CT series are used for selecting the regions of interest which should be treated. These images are not RT objects (not one of the specific IOD objects for RT) but can be CT images (CT Image Storage SOP Class or Enhanced CT Image Storage). Looking and analyzing those images at a Planning System, the responsible doctor/physician manually or (semi-)automatically marks those regions on the CT images which should be treated. The result is a so-called RT Structure Set (specific DICOM object), which specifies anatomical regions which should be treated.
Also, the physician at the Planning System defines which dose should be applied to the selected regions (target volume); that dose must not be homogenous throughout the volume but can (and probably? does often) differ. This dose that should be used for a specific treatment session is represented in DICOM by a RT Dose object. The Planning System for selection of the anatomical region to be treated and the System to plan the dose distribution can be different (Geometric Planner / Dose Planner in IHE).
Defining the dose and the anatomical region to be treated is not sufficient. There must be an overall plan which combines both (or in this case, references both) and that specifies exactly how often and when exactly (dates) a treatment should be applied. Most of the times a treatment plan is fractionated, i.e. not applied once but the plan exists out of more than one appointment for the patient (e. g. two times a week over 10 weeks) where e. g. the dose applied can differ at each of the appointments. Such a plan is represented in DICOM by an RT Plan object.
The images resulting from the simulation test run can be stored in DICOM using the RT Image object.
These four object types (RT Structure Set, RT Dose, RT Plan, RT Image) were the first to be introduced to DICOM in 1996 to support the data resulting from that basic planning and simulation workflow. However, the workflow management is not adressed by the services introduced with that Supplement 11 in 1996.
Simulation / Verification¶
The correctness of plan, dose and anatomical region of the treatment is crucial. If there are any errors then the patient could get too much dose, healthy tissue could be treated etc. That's why all parameters of the planned treatment are simulated (or "verified") before actually applying them to the patient. A device allowing such simulations is called a Simulator. Such a simulator exactly replays all treatment actions defined in the treatment plan without applying the "real, hard" treatment dose. The simulator can be a medical device of it's own but there are also devices that are being used for the treatment itself but also have equipment on board to be used for only verification.
Usually verification is done by adjusting the patient on the simulator to be sure that the plan created - based on existing CT images for example - will work at the treatment device. An important issue that is tested during simulation is whether the target volumes planned from the CT images and the anatomic regions actually treated later are the same (which is not self-evident because of different devices used and different patient positionings).
The simulator actually uses (usually?) a conic radiation source to take a simple radiograph of the target area instead of having/using a strong source of ionizing radiation which will be used for later treatment. So the simulator (if it is a dedicated device) might look very similar to the treatment device or both devices are combined in one machine.
Treatment¶
The treatment can exist out of one or more sessions, where the patient is placed on the treatment device, which emits ionizing radiation to the target volume as specified in the RT plan.
Workflow Variants¶
There are different variants of the workflow described above which introduce some further concepts.
Conventional External Beam Radiotherapy¶
From Wikipedia:
Conventional external beam radiotherapy (2DXRT) is delivered via two-dimensional beams using linear accelerator machines. 2DXRT mainly consists of a single beam of radiation delivered to the patient from several directions: often front or back, and both sides. Conventional refers to the way the treatment is planned or simulated on a specially calibrated diagnostic x-ray machine known as a simulator because it recreates the linear accelerator actions (or sometimes by eye (Michael: which means without real simulation but only looking at the patient on the treatment device from different angles?!), and to the usually well-established arrangements of the radiation beams to achieve a desired plan. This technique is well established and is generally quick and reliable. The worry is that some high-dose treatments may be limited by the radiation toxicity capacity of healthy tissues which lay close to the target tumor volume.
Virtual simulation¶
To overcome the problem of high dose applications damaging healthy tissue the planning/simulation has advanced from 2-dimensional to a 3-dimensional approach. With the help of advanced CT/MR scanners and planning software, [to be continued]