In examining an order from the hearing examiner, we must determine if the order is supported by substantial evidence on the record as a whole. Santos v. District of Columbia Dep't of Employment Servs., 536 A.2d 1085, 1088 (D.C. 1988). Substantial evidence is "such relevant evidence as a reasonable mind might accept as adequate to support a conclusion." Ferreira v. District of Columbia Dep't of Employment Servs., 667 A.2d 310, 312 (D.C. 1995) (citations and internal quotations omitted). Under our authorities, the testimony of a treating physician is ordinarily preferred over that of a physician retained solely for litigation purposes. See Stewart v. District of Columbia Dep't of Employment Servs., 606 A.2d 1350, 1353 (D.C. 1992). The hearing examiner, however, acts as the judge of credibility, and may reject the testimony of a treating physician due to conflicting evidence. Canlas v. District of Columbia Dep't of Employment Servs., 723 A.2d 1210, 1212 (D.C. 1999) (examiner must give reasons for rejecting a treating physician's testimony). See McKinley v. District of Columbia Dep't of Employment Servs., 696 A.2d 1377, 1386 (D.C. 1997) (citing Robinson, supra, 683 A.2d at 488). In McKinley, the court noted the equivocal nature of the treating physician's testimony was a sufficient basis for the hearing examiner to reject that evidence and conclude that the claimant's emotional injury was not work-related. 696 A.2d at 1384-85.