The Definition Source Sequence (0008,1156) references SOP Instances of First or Second Generation Radiotherapy IODs as the source of the information which has been transcoded to the current SOP Instance up to the capability of the current SOP Class. The Definition Source Sequence shall not be used when the current SOP Instance represents a derivation or successor of the source Instance. The source Instance shall not contain a reference to the current Instance.

Typical use cases are: A device (e.g. a treatment planning system or treatment delivery system) is creating Second Generation SOP Instances and additionally encoding them in First Generation SOP Instances for other receivers supporting First Generation RT IODs only. Another use case is that an application receives Second Generation SOP Instances and transcodes them to First Generation SOP Instances to make the content available to receivers supporting First Generation IODs only.

The same applies for the reverse use cases when the source Instance is a First Generation SOP Instance and the current SOP Instance is a transcoded Second Generation SOP Instance.