Currently, the description of the Breast Radiology Report reads:

This resource is one of the components of a Breast Radiology Document. Each Breast Radiology Document bundle will contain exactly one Breast Radiology Document instance that is referenced in the document’s ‘Report’ section.

The description doesn't quite fit the definition of the Composition profile. Other parts of the IG describe the "Document" as being a FHIR Bundle containing a Composition as the first resource. The following may be better:

This resource is one of the components of a Breast Radiology Composition. Each Breast Radiology Composition will contain exactly one Breast Radiology Radiology Report instance that is referenced in the composition's ‘Report’ section.