20 C.F.R. § 30.232
How does a claimant establish that the employee has been diagnosed with a covered illness, or sustained an injury, illness, impairment or disease as a consequence of a covered illness?
(a) To establish that the employee has been diagnosed with a covered illness as required by § 30.230(d), the employee, or his or her survivor(s), must provide the following:
(1) Written medical evidence containing a physician's diagnosis of the employee's covered illness (as that term is defined in § 30.5(s)), and the physician's reasoning for his or her opinion regarding causation; and
(2) Any other evidence OWCP may deem necessary to show that the employee has or had an illness that resulted from an exposure to a toxic substance while working at either a DOE facility or a RECA section 5 facility.
(b) An injury, illness, impairment or disease sustained as a consequence of a covered illness (as defined in § 30.5(s)) must be established with a fully rationalized medical report by a physician that shows the relationship between the injury, illness, impairment or disease and the covered illness. Neither the fact that the injury, illness, impairment or disease manifests itself after a diagnosis of a covered illness, nor the belief of the claimant that the injury, illness, impairment or disease was caused by the covered illness, is sufficient in itself to prove a causal relationship.