(a) Any employee of the agency who has not yet entered retirement status on the basis of age and service and who becomes partially disabled by injury, illness or disease resulting from any occupational risk or hazard inherent in or peculiar to the services required of employees of the agency or incurred pursuant to or while the employee was engaged in the performance of his or her duties as an employee of the agency shall, if, in the opinion of the board, he or she is, by reason of that cause, unable to perform adequately the duties required of him or her as an employee of the agency, but is able to engage in other gainful employment in a field other than law enforcement, be retired from active service by the board. The retirant thereafter is entitled to receive annually from the fund in equal monthly installments during his or her lifetime, or until the retirant attains the age of fifty-five or until the disability eligibility sooner terminates, one or the other of two amounts, whichever is greater:
(1) An amount equal to six tenths of the base salary received in the preceding twelve-month employment period: Provided, That if the member had not been employed with the agency for twelve months prior to the disability, the amount of monthly salary shall be annualized for the purpose of determining the benefit; or