A number assigned to patients in a cancer registry in the order they are entered is called a(n):

An accession number is a unique identifier assigned to each patient or case when they are entered into a cancer registry. This number is crucial for the systematic tracking and management of cancer cases, allowing healthcare professionals to maintain accurate and organized records.

The accession number is typically assigned sequentially based on the order of entry, ensuring that each patient has a distinct identifier that can be used for referencing their data throughout the registry's processes. This helps in maintaining the integrity of the registry and facilitates efficient data retrieval for analysis, reporting, and follow-up of cancer cases.

The other terms listed, such as follow-up number, reference number, and tracking number, are generally not specific to the initial recording of patients in a registry. They may refer to other aspects of patient management or case tracking but do not convey the same meaning or purpose as an accession number in the context of a cancer registry.