Most of the Course, Phase and Plan level profiles include a description for .basedOn that looks something like "Should Reference a Radiotherapy Plan Prescription and the Radiotherapy Planned Phase to which this Treatment Plan belongs." 

But few actually flag .basedOn as Must Support (for example, Radiotherapy Treated Plan does mark it as MS but not other profiles). As well, while each calls out specific profiles in the description, none seem to constrain the References to those profiles (rather, they are all still generic CarePlan or ServiceRequest resources). It seems important to maintain linkages between the different parts of the treatment course. Should all .basedOn elements be MS and reference specific profiles?