Beam Area Limit Sequence Attribute
| Tag | (300A,0689) |
|---|---|
| Type | Conditionally Required (1C) |
| Keyword | BeamAreaLimitSequence |
| Value Multiplicity | 1 |
| Value Representation | Sequence (SQ) |
Area within which the treatment beam must be contained, for example when using MLC tracking for a moving target.
Only a single Item shall be included in this Sequence.
Required if beam shall be limited.
See Section C.36.2.2.5.1.
Section C.36.2.2.5.1
C.36.2.2.5.1 RT Control Point Attribute Concept
The treatment-modality Modules use a common formalism to represent parameters that define the behavior of a delivery device during delivery of radiation. These parameters are communicated as a sequence of values, organized as Control Points (see Section C.36.1.1.1) and represented as RT Control Points. The resolution of RT Control Points depends on the level of detail required to define the behavior of the delivery device.
A Control Point is a point on a time line of a delivery process. RT Control Points are sequenced using an index number starting with 1, e.g., 1, 2, 3, 4. The RT Control Point parameters reflect the state of the delivery device at that point in time. The Control Point Cumulative Meterset reflects the dose that has been delivered from the beginning of the delivery process up to that point in time.
For all beam deliveries there are at least two RT Control Points, corresponding to the start and end of delivery. E.g., for a simple Static Beam delivery with a constant field aperture, only two RT Control Points are needed to define the start and end, as there are no changes in-between. For a dynamic delivery, in which the MLC leaves are changing while radiation is delivered, the number of Control Points will be higher to provide enough detail to define the leaf movement with sufficient resolution to achieve the radiation fluence distribution expected for the prescribed dose.
DICOM does not specify the behavior of the machine parameters between Control Points. The planning system needs to know the hardware-specific characteristics of the delivery system for which the plan is being created.
C.36.2.2.5.1.1 Requirements for Changing Values within RT Control Point Sequence Attributes
This Section specifies when individual Attributes shall be present in a Sequence.
The RT Control Point Sequence specifies a certain order of execution.
At each RT Control Point the value of various Attributes may be specified as an explicit value (which in the case of a type 2C Attribute may be a null value) and if absent remain at the same value as specified previously. There are physical and mechanical implications of specifying a new value as opposed to staying at the same value, for example gear lash, floating point jitter, etc.
At the first Sequence Item in RT Control Point Sequences (i.e., with an RT Control Point Index (300A,0600) equal to 1) all Attributes affected by this Section shall be present if applicable conditions are met.
For Sequence Items other than the first Sequence Item, Attributes shall be present if applicable conditions are met and the value is different from the previously populated value for the same Attribute (in the case of a type 2C Attribute, a null value is considered as a value). The previously populated value is the value from the Item where the Attribute was present with the greatest value of RT Control Point Index (300A,0600) less than the value of the RT Control Point Index (300A,0600) in the current Item.
This means that for an Item in which an Attribute is absent, the application stays at the value of the previously populated Item.
For Sequences inside a RT Control Point Sequence Item, the Sequence shall be present if any of the nested Attributes affected by this Section differ from the corresponding previously populated Item.
For multi-valued Attributes, such as Parallel RT Beam Delimiter Positions (300A,064A), all values shall be present if any value changes.
C.36.2.2.5.1.2 Control Point Attribute Example
The following examples illustrate RT Control Points:
-
Static Beam delivery:
RT Control Point Index (300A,0600)
Cumulative Meterset (300A,063C)
All other parameters
1
0
<defined>
2
76
<not present>
At completion this beam delivers 76 Monitor Units using a fixed static set of treatment parameters defined in RT Control Point 1.
-
Arc delivery:
RT Control Point Index (300A,0600)
Cumulative Meterset (300A,063C)
Source Roll Angle (300A,067A)
All other parameters
1
0
<initial angle>
<defined>
2
56
<final angle>
<not present>
At completion this delivers 56 Monitor Units while rotating the gantry from initial angle to final angle.
-
Dynamic delivery of three segments, including a Source Roll Angle change during the second segment:
RT Control Point Index (300A,0600)
Cumulative Meterset (300A,063C)
Source Roll Angle (300A,067A)
Number of RT Beam Limiting Device Openings (300A,0657)
Parallel RT Beam Delimiter Positions (300A,064A)
X
Referenced Device Index 1
Parallel RT Beam Delimiter Positions(300A,064A)
Y
Referenced Device Index 2
RT Beam Limiting Device Angle (300A,0679)
All other parameters
1
0
0
2
2\2
2\2
30
<defined>
2
40
<not present>
1
<not present>
4\4
<not present>
<not present>
3
45
7
0
<not present>
<not present>
<not present>
<not present>
4
80
<not present>
1
4\4
<not present>
<not present>
<not present>
At completion this delivers 80 Monitor Units while first increasing the Y opening and then increasing the X opening, while the beam limiting device angle stays fixed. For the RT Beam Limiting Device Opening Sequence (300A,0656) this results in having three Items for the first Control Point and only one for Control Points 2 (Referenced Device Index 2 only) and 3 (Referenced Device Index 1 only). Control Point 3 demonstrates the case where no changes occur in the RT Beam Limiting Device Openings Sequence, requiring the Number of RT Beam Limiting Device Openings to be present with a 0 value. See also Figure C.36.2.2.5.1-1.
Figure C.36.2.2.5.1-1. Control Points Sub-Sequence Attribute Presence
-
Dynamic Delivery of two unequally weighted segments with a step change of 5 degrees in the positive direction of the Patient Support Angle:
Note
Patient Support Angle is represented by the Image to Equipment Mapping Matrix (0028,9520). The table contains the effective angle and not the complete matrix.
RT Control Point Index (300A,0600)
Cumulative Meterset (300A,063C)
Image to Equipment Mapping Matrix (0028,9520)
Source Roll Angle (300A,067A)
All other parameters
1
0
0
-90
<defined>
2
30
<not present>
<not present>
<not present>
3
<not present>
5
0
<not present>
4
90
<not present>
<not present>
<not present>
At completion this delivers 90 Monitor Units. Between RT Control Point 2 and 3 the Patient Support Angle and Source Roll Continuous Angle are changed and no radiation is delivered.
C.36.2.2.5.1.3 Cumulative Meterset
The Meterset at a given Control Point is specified by Cumulative Meterset (300A,063C). This value is specified in units defined by Radiation Dosimeter Unit Sequence (300A,0658) in the Section C.36.12 RT Delivery Device Common Module. The Meterset values are intended to correspond to the values produced by the primary or the single Meterset-measuring device of a RT Radiation Delivery Device.