Right, it would make more sense to use HL7 FHIR (possibly along with CQL) as a starting point instead of reinventing the wheel. Talk to the CodeX accelerator about writing an Implementation Guide in this area. The PlanDefinition resource type should be a good fit for modeling cancer guidelines.
https://codex.hl7.org/
https://www.hl7.org/fhir/plandefinition.html