There is a known issue in the Dicom Standard. SOP classes related to RT can legitimately be written / transferred using several different Transfer Syntaxes. HOWEVER The length of some of the attributes associated to RT is such that they can not be represented within a 32 bit word. As such it is imposable to represent them using an explicit Transfer Syntax.

Therefore it is often appropriate to limit these SOP classes so that they can only use Implicit VR Little-endian (1.2.840.10008.1.2)

DicomObjects.NET

In the DicomObjects.NET v6 onwards this can be done using the grouping method:

DicomGlobal.ClearOfferedTransferSyntaxes(DicomObjects.Enums.SOPClassGroups.User1);
DicomGlobal.AddToSOPClassGroup(Enums.SOPClassGroups.User1, DicomUIDs.SOPClasses.RT);
DicomGlobal.AddToSOPClassGroup(Enums.SOPClassGroups.User1, DicomUIDs.SOPClasses.RTBeamsTreatmentRecord);
DicomGlobal.AddToSOPClassGroup(Enums.SOPClassGroups.User1, DicomUIDs.SOPClasses.RTBrachyTreatmentRecord);
DicomGlobal.AddToSOPClassGroup(Enums.SOPClassGroups.User1, DicomUIDs.SOPClasses.RTDose);
DicomGlobal.AddToSOPClassGroup(Enums.SOPClassGroups.User1, DicomUIDs.SOPClasses.RTPlan);
DicomGlobal.AddToSOPClassGroup(Enums.SOPClassGroups.User1, DicomUIDs.SOPClasses.RTStructureSet);
DicomGlobal.AddToSOPClassGroup(Enums.SOPClassGroups.User1, DicomUIDs.SOPClasses.RTTreatmentSummaryRecord);
DicomGlobal.AddToAcceptedTransferSyntaxes(DicomObjects.Enums.SOPClassGroups.User1,DicomObjects.DicomUIDs.TransferSyntaxes.ImplicitVRLittleEndian);
DicomGlobal.AddToOfferedTransferSyntaxes(DicomObjects.Enums.SOPClassGroups.User1,DicomObjects.DicomUIDs.TransferSyntaxes.ImplicitVRLittleEndian);