DICOM PS3.3 2021d - Information Object Definitions

C.36.2.2.8 RT Beam Limiting Devices Definition Macro (Current)
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##### C.36.2.2.8 RT Beam Limiting Devices Definition Macro

This Macro describes the configuration of Beam Limiting Devices which cannot vary during delivery.

Table C.36.2.2.8-1. RT Beam Limiting Devices Definition Macro Attributes

Attribute Name | Tag | Type | Attribute Description
--- | --- | --- | ---
Number of RT Beam Limiting Devices | (300A,0641) | 1C | Number of RT Beam Limiting Devices in the RT Beam Limiting Device Definition Sequence (300A,064D). Required if RT Radiation Physical and Geometric Content Detail Flag (300A,0638) equals FULL. May be present otherwise.
RT Beam Limiting Device Definition Sequence | (300A,064D) | 1C | Beam limiting device (collimator), such as jaw or leaf (element) sets. The number of Items included in this Sequence shall equal the value of Number of RT Beam Limiting Devices (300A,0641). Required if Number of RT Beam Limiting Devices (300A,0641) is present and has a non-zero value.
>Device Index | (3010,0039) | 1 | Index of the Device in this Sequence. The value shall start at 1 and increase monotonically by 1.
>Referenced Defined Device Index | (300A,0602) | 1C | Device Index value that links the device defined by this Sequence Item to the corresponding device in an RT Radiation Instance. The device identification of the two devices may or may not be the same. The value is the index of a device in the RT Beam Limiting Device Definition Sequence (300A,064D) within the single SOP Instance referenced by Referenced RT Instance Sequence (300A,0631). Required if the Instance referenced in Referenced RT Instance Sequence (300A,0631) contains the device that corresponds to the device defined by this Sequence Item. See Section C.36.2.2.8.1.5.
>Include Table C.36.2.2.3-1 “RT Accessory Device Identification Macro Attributes”. | CID is specified at invocation.
>Beam Modifier Orientation Angle | (300A,0645) | 1 | Angle in degrees of the Beam Modifier Coordinate System with respect to the Base Beam Modifier Coordinate System. The angle is a Continuous Rotation Angle, see Section C.36.1.1.5. If Device Type Code Sequence (3010,002E) contains either (130331, DCM, "Leaf Pairs"), or (130333, DCM, "Single Leaves") the motion of the RT Beam Delimiters is along the x-axis of the Beam Modifier Definition Plane. See Section C.36.1.1.9.
>RT Beam Limiting Device Proximal Distance | (300A,0642) | 2 | Distance in mm from the reference location as specified by RT Device Distance Reference Location Code Sequence (300A,0659) to the proximal end of beam limiting device (collimator) along the beam axis. See Section C.36.2.2.8.1.4.
>RT Beam Limiting Device Distal Distance | (300A,0643) | 2 | Distance in mm from the reference location as specified by RT Device Distance Reference Location Code Sequence (300A,0659) to the distal end of beam limiting device (collimator) along the beam axis. See Section C.36.2.2.8.1.4.
>Parallel RT Beam Delimiter Device Sequence | (300A,0647) | 1C | Device that uses parallel beam delimiters to limit the beam. Required if Device Type Code Sequence (3010,002E) contains either (130331, DCM, "Leaf Pairs") or (130333, DCM, "Single Leaves"). Only a single Item shall be present in the Sequence.
>>Number of Parallel RT Beam Delimiters | (300A,0648) | 1 | Number of beam delimiters parallel to the axis of motion. E.g., a beam limiting device jaw pair is represented as 1 parallel delimiter, an MLC with 100 leaf pairs or with 100 single leaves is represented as 100 parallel delimiters. See Section C.36.2.2.8.1.3.
>>Parallel RT Beam Delimiter Device Orientation Label Code Sequence | (300A,0644) | 1 | A code used to identify the orientation of the beam limiting device. Only a single Item shall be present in the Sequence.
>>>Include Table 8.8-1 “Code Sequence Macro Attributes”. | DCID 9547 “RT Beam Limiting Device Orientation Labels”. See Section C.36.2.2.8.1.1.
>>Parallel RT Beam Delimiter Opening Mode | (300A,064E) | 1 | The operation mode of Parallel RT Beam Delimiters used to define a treatment aperture. Enumerated Values: BINARY leaf positions constrained to two states: open and closed VARIABLE any leaf position may be specified
>>Parallel RT Beam Delimiter Boundaries | (300A,0649) | 1 | Boundaries in mm of parallel beam delimiters. These are defined along the axis perpendicular to the motion of the delimiters of the RT Beam Limiting Device Type (300A,00B8) with respect to the Beam Modifier Coordinate System. The order of values shall increase monotonically. See Section C.36.2.2.8.1.2. N+1 values shall be provided, where N is the Number of Parallel RT Beam Delimiters (300A,0648).
>>Parallel RT Beam Delimiter Leaf Mounting Side | (300A,064F) | 1C | Specifies the mounting side identified by the direction from the tip to the tail of the delimiter parallel to the axis specified by Device Type Code Sequence (3010,002E). Enumerated Values: P Positive mounting side. The axis intercept of the leaf tip is less than the axis intercept of the leaf tail. N Negative mounting side. The axis intercept of the leaf tip is greater than the axis intercept of the leaf tail. M values shall be provided, where M is the Number of Parallel RT Beam Delimiters (300A,0648), in the order of the Parallel RT Beam Delimiter Boundaries (300A,0649). Required if Device Type Code Sequence (3010,002E) contains (130333, DCM, "Single Leaves"). See Section C.36.2.2.8.1.3.
>Fixed RT Beam Delimiter Device Sequence | (300A,0646) | 1C | Device that uses a fixed aperture to limit the beam. Required if Device Type Code Sequence (3010,002E) is part of CID 9545 Fixed Beam Limiting Device Types. Only a single Item shall be included in this Sequence.
>>Include Table 10.38-1 “Outline Definition Macro Attributes”. | The Outline is defined on the Beam Modifier Definition Plane.

###### C.36.2.2.8.1 RT Beam Limiting Device Definition Macro Attribute Description

###### C.36.2.2.8.1.1 Parallel RT Beam Delimiter Device Orientation Label Code

The value of Parallel RT Beam Delimiter Device Orientation Label Code Sequence (300A,0644) shall be chosen as follows:

When the value of Beam Modifier Orientation Angle (300A,0645) equals zero the code shall be (130334, DCM, "X Orientation").

When the value of Beam Modifier Orientation Angle (300A,0645) equals 90 the code shall be (130335, DCM, "Y Orientation").

When the value of Beam Modifier Orientation Angle (300A,0645) is not zero or 90, the label should be chosen to best reflect the user perception or another code may be used.

###### C.36.2.2.8.1.2 Parallel RT Beam Delimiter Boundaries

The Parallel RT Beam Delimiter Boundaries (300A,0649) shall be the positions of the mechanical boundaries (projected on the Beam Modifier Definition Plane defined by the RT Beam Modifier Definition Distance (300A,0688)) between beam delimiter elements. These are fixed for a given beam limiting device. Parallel RT Beam Delimiter Positions (300A,064A) are values specific to a given Control Point, specifying the beam limiting device element openings.

###### C.36.2.2.8.1.3 Number of Parallel RT Beam Delimiters

Figure C.36.2.2.8.1-1. Number of Parallel RT Beam Delimiters for X Leaf Pairs

Figure C.36.2.2.8.1-2. Number of Parallel RT Beam Delimiters for X Single Leaves

In example in Figure C.36.2.2.8.1-2 the delimiters labeled 1, 3 and 5 have a Parallel RT Beam Delimiter Leaf Mounting Side (300A,064F) value of N (negative direction) and the delimiters labeled 2 and 4 have a Parallel RT Beam Delimiter Leaf Mounting Side value of P (positive direction).

###### C.36.2.2.8.1.4 RT Beam Limiting Device Proximal Distance and RT Beam Limiting Device Distal Distance

Figure C.36.2.2.8.1-3 shows the RT Beam Limiting Device Proximal Distance (300A,0642) and RT Beam Limiting Device Distal Distance (300A,0643).

In this example the reference location specified by the RT Device Distance Reference Location Code Sequence (300A,0659) has the value (130358, DCM, "Nominal Radiation Source Location").

Figure C.36.2.2.8.1-3. RT Beam Limiting Device Proximal and Distal Distance

###### C.36.2.2.8.1.5 Referenced Defined Device Index

The Referenced Defined Device Index (300A,0602) provides the facility to relate devices from one Instance to the other. Device Macros such as the RT Beam Limiting Device Definition Macro are used in Sequences which list the devices used in the context of a SOP Instance. Each device is identified by the Device Index (3010,0039). These devices may be described in other related SOP Instances. The values of Device Index (3010,0039) are not required to be the same in different SOP Instances.

For example, an RT Radiation Instance may contain the RT Beam Limiting Device Definition Sequence (300A,064D), listing the Beam Limiting Devices to be used for treatment. A related RT Radiation Record Instance for the same type of delivery device will contain the same Sequences. However, the collection of devices used may not be the same:

E.g., the RT Radiation Instance may describe a treatment that contains three Items with the following indices:

- Device Index = 1: Type (130331, DCM, "Leaf Pairs") with one leaf pair and the orientation (130334, DCM, "X Orientation"), representing X-Jaws

- Device Index = 2: Type (130331, DCM, "Leaf Pairs") with one leaf pair and the orientation (130335, DCM, "Y Orientation"), representing Y-Jaws

- Device Index = 3: Type (130331, DCM, "Leaf Pairs") with 80 leaf pairs and the orientation (130334, DCM, "X Orientation"), representing an X-MLC

However, the treatment may have been executed on a different machine which has the X-Jaws and the X-MLC, but a fixed collimator in the Y direction instead of the Y-Jaws. Therefore, the Y-Jaws will not be recorded as they have not been used. Also, the fixed collimator in Y direction is not recorded, as this is not a device which is part of the RT Beam Limiting Device Definition Sequence (300A,064D). The Referenced Defined Device Index (300A,0602) will then point to the indices in the referenced RT Radiation Instance to annotate which device in the RT Radiation Record corresponds to the device in the RT Radiation. In this case, the RT Radiation Record would contain the devices as listed in the following:

- Device Index = 1: Type (130331, DCM, "Leaf Pairs") with one leaf pair and the orientation (130334, DCM, "X Orientation"), representing X-Jaws-> Referenced Defined Device Index = 1

- Device Index = 2: Type (130331, DCM, "Leaf Pairs") with 80 leaf pairs and the orientation (130334, DCM, "X Orientation"), representing an X-MLC-> Referenced Defined Device Index = 3

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DICOM PS3.3 2021d - Information Object Definitions