| DICOM PS3.3 2025b - Information Object Definitions |
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The control point sequence for RT Ion Beams is defined using the same rule set as in the RT Beams Module (see Section C.8.8.14.5). Specifically, the following rules apply:
All parameters that change at any control point of a given beam shall be specified explicitly at all control points (including those preceding the change).
All parameters of an irradiation segment (i.e., with Values of the Cumulative Meterset Weight (300A,0134) different at the beginning and at the end of the segment) shall therefore be specified in 2 separate control points denoting the beginning and at the end of this segment. Each irradiation segment is therefore represented by 2 control points.
Parameters changing during the segment shall be represented by their different values at those control points. Parameters that do not change during the segment shall be represented with equal values at both control points (unless they are constant for all control points of the beam). For example, a beam delivery involving two independent irradiation segments will require 4 control points. Control Points 0 and 1 define the first irradiation segment. Between control points 1 and 2, no radiation is given (Meterset is constant), but other parameters may change. Finally, the second irradiation segment occurs between control points 2 and 3.
This definition allows unambiguous and explicit determination of those parameters changing while irradiation is occurring, as opposed to those parameters that change between irradiation segments. No assumptions are made about the behavior of machine parameters between specified control points, and communicating devices shall agree on this behavior outside the Standard.
Some examples of ion scanning beam specification using control points are as follows:
For each segment, the positions in the Scan Spot Position Map (300A,0394) are identical for the corresponding pair of control points.
1st segment is described by Control Points 0 and 1 pair.
For Control Points 0, Scan Spot Meterset Weights (300A,0396) Values add up to Meterset difference between Control Points 0 and 1.
For Control Points 1, Scan Spot Meterset Weights (300A,0396) Values are 0.0, because the Cumulative Meterset Weight (300A,0134) difference between Control Point 1 and 2 is 0.0.
The following example illustrates the case of a scanning beam with 3 segments for 3 fixed gantry angles, each with a single beam energy level, and Final Cumulative Meterset Weight (300A,010E) of 90. As the Gantry Angle (300A,011E) Value remains unchanged between consecutive pairs of control points with changing Cumulative Meterset Weight (300A,0134), such a scanning beam shall have Beam Type (300A,00C4) Value equal to STATIC.
1st segment is described by Control Points 0 and 1.
For Control Points 0, Scan Spot Meterset Weights (300A,0396) Values add up to Meterset difference between Control Points 0 and 1. Rotation direction is NONE because there is no rotation of gantry between Control Points 0 and 1.
For Control Points 1, Scan Spot Meterset Weights (300A,0396) Values are 0.0, because the Cumulative Meterset Weight (300A,0134) difference between Control Point 1 and 2 is 0.0. Rotation direction is specified because the gantry rotates between Control Points 1 and 2 while beam energy is modified.
The following two examples illustrate the case of a DYNAMIC scanning beam with 3 segments with continuous rotation of the gantry, each with a single beam energy level, and Final Cumulative Meterset Weight (300A,010E) of 90. As the Gantry Angle (300A,011E) Value changes between consecutive pairs of control points with changing Cumulative Meterset Weight (300A,0134), such a scanning beam shall have Beam Type (300A,00C4) Value equal to DYNAMIC.
In this first example, both the energy and the gantry angle change between segments leading to changing gantry angles at successive Control Points.
1st segment is described by Control Points 0 and 1.
For Control Points 0, Scan Spot Meterset Weights (300A,0396) Values add up to Meterset difference between Control Points 0 and 1. Rotation direction is defined because the gantry rotates between Control Points 0 and 1.
For Control Points 1, Scan Spot Meterset Weights (300A,0396) Values are 0.0, because the Cumulative Meterset Weight (300A,0134) difference between Control Point 1 and 2 is 0.0. Rotation direction is not defined because the gantry continuously rotates in the same direction between Control Points 1 and 2 while beam energy is modified.
2nd segment is described by Control Points 2 and 3.
3rd segment is described by Control Points 4 and 5.
In this second example, only the energy changes between segments.
1st segment is described by Control Points 0 and 1.
For Control Points 0, Scan Spot Meterset Weights (300A,0396) Values add up to Meterset difference between Control Points 0 and 1. Rotation direction is defined because the gantry rotates between Control Points 0 and 1.
For Control Points 1, Scan Spot Meterset Weights (300A,0396) Values are 0.0, because the Cumulative Meterset Weight (300A,0134) difference between Control Point 1 and 2 is 0.0. Rotation direction is NONE because the gantry angle does not change between control point 1 and 2.
Arc delivery could also be performed with a rotation of the patient support instead of a rotation of the gantry. In such a case, Gantry Rotation Direction (300A,011F) and Gantry Angle (300A,0112) should be replaced by Patient Support Rotation Direction (300A,0123) and Patient Support Angle (300A,0122) in the previous examples.
Continuous arc delivery could include a change of gantry angle direction in which case each control point shall define the Gantry Rotation Direction (300A,011F).
Continuous arc delivery could include some segments with the gantry remaining stationary in which case each control point shall define the Gantry Rotation Direction (300A,011F) with the Value NONE for the first control point of a stationary segment.
For continuous arc delivery a Value of NONE for the Gantry Rotation Direction (300A,011F) does not explicitly require that the gantry has to physically stop moving, but it is necessary to avoid requiring that the gantry performs a full clockwise rotation (360 degrees) as expected from Section C.8.8.14.8. DICOM does not specify the behavior of the treatment machine within a segment or between segments.
| DICOM PS3.3 2025b - Information Object Definitions |
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