DICOM PS3.3 2023a - Information Object Definitions
C.8.8.14.18 Presence of Items within Sequences in the Control Point Sequence

Items within Sequences in the Control Point Sequence shall be present in the first Control Point or if the value of any Attribute in an Item changes during the Beam.

If an Item is present, all Attributes of that Item shall be present if the Attribute requirements apply, even if the value of the Attribute does not change during the Beam.

Example:

A beam may be delivered with two MLCs "A" and "B", where the values of MLC positions for MLC "A" do not change during the Beam, while the values for MLC "B" are changing. Each MLC has a constant value for its RT Beam Limiting Device Offset (300A,064B).

The Item describing MLC "A" is present in the first Control Point only.

The Items describing MLC "B" will be present in all Control Points containing the values of Parallel RT Beam Delimiter Positions. The values of RT Beam Limiting Device Offset (300A,064B) for that MLC will be also present in all Items, even if these values remain constant during the Beam.

DICOM PS3.3 2023a - Information Object Definitions