Purpose: The American Society of Clinical Oncology (ASCO)/ American Board of Internal Medicine (ABIM) Choosing Wisely

(CW) measures aim to reduce the use of interventions that lack evidence of benefit in cancer care. The study presented here characterized adherence to the 2012 ASCO/ABIM CW recom-mendations by linking health plan claims data with a regional cancer registry and sought to identify areas for research inter-ventions to improve adherence. Methods: SEER records for patients diagnosed with cancer in Western Washington State between 2007 and 2014 were linked with enrollment and claims from a large regional commercial insurance plan. Using claims and SEER records, algorithms were developed to characterize adherence to each CW measure. In addition, we calculated differences in total reimbursements and procedure-specific reimburse-ments for patients receiving adherent and nonadherent care. Results: A total of 22,359 unique individuals with cancer were linked with insurance enrollment records and met basic eligibility criteria. Overall adherence varied from 53 % (breast surveillance) to 78 % (breast staging). Within each measure, adherence varied sub-stantially by stage at diagnosis and by cancer site in situations in which the CW measure affected multiple types of cancer. The dif-ference in reimbursements between adherent and nonadherent populations across all fivemeasureswas approximately $29million. Conclusion: Adherence to the ASCO/ABIM CW measures varies widely, as does the cost implication of nonadherence. A structured approach to evaluating adherence and cost impact is needed before developing programs aimed at improving adher-ence to the ASCO/ABIM CW measures