In Hebden v. Workmen's Compensation Appeal Board (Bethenergy Mines, Inc.), 534 Pa. 327, 330, 632 A.2d 1302, 1304 (1993), our Supreme Court explained the operation of the res judicata doctrine in the context of a proceeding to terminate partial disability benefits after it had been determined in a prior proceeding that Hebden suffered from occupationally acquired pneumoconiosis. The Court reiterated the long-standing principle that an employer may not relitigate, by way of a termination or modification petition, the medical diagnosis that formed the basis of a claimant's original compensation award. Id. The Court further explained: We do not lose sight of the fact that the Workers' Compensation Act Act at Section 413 (77 P.S. 772) expressly provides that an award may be terminated based upon changes in the employee's disability. But that raises the logical question of whether an employee's disability is changeable in a given case. If it is, an employee's condition may be re-examined at a later time to see if he is still disabled or not. If it is not, an attempt to re-examine the employee's condition is merely a disguised attempt to relitigate what has already been settled. We think that the latter is what occurred here. Id. at 331, 632 A.2d at 1304.