Can you use an ICD-10-CM code for the PrimaryCancerCondition code and an ICD-O-3 code for the HistologyMorphology extension?
There are no constraints in the spec stopping you from doing that but it really doesn’t make sense to mix and match having just ICD-10-CM and an ICD-O-3 morphology code without a topology code.
That is why we placed the table in the Representing Cancer Diagnosis section of the homepage. The intent is that if a source application was going to send a cancer patient record to a registry (e.g., SEER) then ICD-O-3 is sent and not ICD-10-CM. And since the FHIR condition resource requires a Condition.code, we need to have some placeholder for its representation.