If your TUE application is denied, it means that your application doesn’t currently meet the criteria outlined in the ISTUE, not necessarily that your treatment isn’t considered valid.
The decision letter provided by the CCES will explain why it was denied and which criteria were not met. The letter will also list which elements were missing and could be submitted in a new application to be reconsidered by the TUE Committee (TUEC). Under the rules of the CADP, you may appeal a denial decision.