MARK SIENKO, M.D. appears in the CMS NPPES registry as a Medical Oncology Physician provider holding M.D. credentials at 844 N 5TH AVE, SEQUIM, WA, 98382, with a listed phone of (360) 683-9895. NPI 1932146552 was issued on 06/01/2006. Because NPPES data is self-reported by the provider and refreshed monthly, the address, phone, and specialty reflect what SIENKO most recently submitted to CMS rather than a vetted directory listing, which is why patients should always confirm the practice is still at this location before scheduling.

Medicare Part D records for calendar year 2023 show 1,166 prescription claims written by this provider, covering 188 Medicare beneficiaries and totaling roughly $1.7 million in drug spend, with an opioid prescribing rate of 8.8%. These figures capture only Medicare Part D — commercial insurance, Medicaid, and cash-pay prescriptions are not included, and any drug-beneficiary cell under 11 is suppressed by CMS for patient privacy. On the CMS Merit-based Incentive Payment System, this provider earned a Final Score of 80.9739/100 for the 2023 performance year (Quality 89.6387, Cost 64.4409), compared with the national average of 83.1.

Medical Oncology Physician is a narrow specialty nationwide, with 3,792 enrolled providers across 53 states and an average of 689 Part D claims per prescriber, so the context for interpreting these metrics differs meaningfully from a primary-care baseline. Important: PlainDoctor is a directory of publicly available government data, not medical advice, a malpractice database, or a quality rating. Inclusion here does not imply a recommendation, and absence of complaints or sanctions on this page does not mean none exist — always verify credentials through the NPPES NPI Registry, your state medical board, and the ABMS or AOA before making healthcare decisions.