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Recently, \u201chybrid\u201d methods have been developed through which measured confounding is first attenuated via propensity scores and unmeasured confounding is addressed through (Bayesian) dynamic borrowing. Most efforts to date have focused on augmenting control arms with historical controls. Here we consider augmenting treatment arms through \u201cexpanded access\u201d, which is a pathway of nontrial access to investigational medicine for patients with seriously debilitating or life\u2010threatening illnesses. Motivated by a case study on expanded access, we developed a novel method (the ProPP) that provides a conceptually simple and easy\u2010to\u2010use combination of propensity score weighting and the modified power prior. Our weighting scheme is based on the estimation of the average treatment effect of the patients in the trial, with the constraint that external patients cannot receive higher weights than trial patients. The causal implications of the weighting scheme and propensity\u2010score integrated approaches in general are discussed. In a simulation study, our method compares favorably with existing (hybrid) borrowing methods in terms of precision and type I error rate. We illustrate our method by jointly analyzing individual patient data from the trial and expanded access program for vemurafenib to treat metastatic melanoma. Our method provides a double safeguard against prior\u2010data conflict and forms a straightforward addition to evidence synthesis methods of trial and real\u2010world (expanded access) data.<\/jats:p>","DOI":"10.1002\/sim.70168","type":"journal-article","created":{"date-parts":[[2025,8,26]],"date-time":"2025-08-26T14:39:16Z","timestamp":1756219156000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":2,"title":["Augmenting Treatment Arms With External Data Through Propensity\u2010Score Weighted Power Priors: An Application in Expanded Access"],"prefix":"10.1002","volume":"44","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-2720-3479","authenticated-orcid":false,"given":"Tobias B.","family":"Polak","sequence":"first","affiliation":[{"name":"Department of Biostatistics Erasmus MC  Rotterdam the Netherlands"},{"name":"Department of Epidemiology Erasmus MC  Rotterdam the Netherlands"},{"name":"RWD Department myTomorrows  Amsterdam the Netherlands"},{"name":"School of Health Policy &amp; Management Erasmus University  Rotterdam the Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jeremy A.","family":"Labrecque","sequence":"additional","affiliation":[{"name":"Department of Epidemiology Erasmus MC  Rotterdam the Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Carin A.","family":"Uyl\u2010de Groot","sequence":"additional","affiliation":[{"name":"School of Health Policy &amp; Management Erasmus University  Rotterdam the Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9187-244X","authenticated-orcid":false,"given":"Joost","family":"van Rosmalen","sequence":"additional","affiliation":[{"name":"Department of Biostatistics Erasmus MC  Rotterdam the Netherlands"},{"name":"Department of Epidemiology Erasmus MC  Rotterdam the Netherlands"},{"name":"Julius Center for Health Sciences and Primary Care University Medical Center Utrecht, Utrecht University  Utrecht the Netherlands"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2025,8,26]]},"reference":[{"key":"e_1_2_12_2_1","volume-title":"Leveraging Real\u2010World Treatment Experience From Expanded Access Protocols","author":"Reagan\u2010Udall Foundation","year":"2018"},{"key":"e_1_2_12_3_1","doi-asserted-by":"publisher","DOI":"10.1002\/cpt.2479"},{"key":"e_1_2_12_4_1","doi-asserted-by":"publisher","DOI":"10.1177\/0962280217694506"},{"key":"e_1_2_12_5_1","doi-asserted-by":"publisher","DOI":"10.1002\/pst.1589"},{"key":"e_1_2_12_6_1","unstructured":"FDA \u201cRare Diseases: Natural History Studies for Drug Development\u2014FDA \u201d2019Technical Report Food and Drug Administration (FDA)."},{"key":"e_1_2_12_7_1","doi-asserted-by":"publisher","DOI":"10.1002\/sim.8134"},{"key":"e_1_2_12_8_1","doi-asserted-by":"publisher","DOI":"10.1080\/15265161.2020.1820113"},{"key":"e_1_2_12_9_1","doi-asserted-by":"publisher","DOI":"10.1002\/jrsm.1122"},{"key":"e_1_2_12_10_1","doi-asserted-by":"publisher","DOI":"10.1080\/19466315.2018.1497533"},{"key":"e_1_2_12_11_1","doi-asserted-by":"publisher","DOI":"10.1093\/BIOMET\/70.1.41"},{"key":"e_1_2_12_12_1","doi-asserted-by":"publisher","DOI":"10.1080\/19345747.2014.911396"},{"key":"e_1_2_12_13_1","doi-asserted-by":"publisher","DOI":"10.1002\/pst.1807"},{"key":"e_1_2_12_14_1","doi-asserted-by":"publisher","DOI":"10.1002\/sim.3722"},{"key":"e_1_2_12_15_1","doi-asserted-by":"publisher","DOI":"10.1177\/1740774509356002"},{"key":"e_1_2_12_16_1","doi-asserted-by":"publisher","DOI":"10.1111\/biom.12242"},{"key":"e_1_2_12_17_1","doi-asserted-by":"publisher","DOI":"10.1016\/S0378\u20103758(99)00140\u20108"},{"key":"e_1_2_12_18_1","unstructured":"Y.Duan \u201cA Modified Bayesian Power Prior Approach With Applications in Water Quality Evaluation \u201d2005. 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