title: Multicenter trials using ¹⁸F-fluorodeoxyglucose (FDG) PET to predict chemotherapy response: effects of differential measurement error and bias on power calculations for unselected and enrichment designs.
abstract: BACKGROUND
  Clinical validation of a predictive biomarker is especially difficult when the biomarker cannot be assessed retrospectively. A cost-effective, prospective multicenter replication study with rapid accrual is warranted prior to further validation studies such as a marker-based strategy for treatment selection. However, it is often unknown how measurement error and bias in a multicenter trial will differ from that in single-institution studies.
  
  
  OBJECTIVE
  Power calculations using simulated data may inform the efficient design of a multicenter study to replicate single-institution findings. This case study used serial standardized uptake value (SUV) measures from (18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) to predict early response to breast cancer neoadjuvant chemotherapy. We examined the impact of accelerating accrual through increased inclusion of secondary sites with greater levels of measurement error and bias. We also examined whether enrichment designs based on breast cancer initial uptake could increase the study power for a fixed budget (200 total scans).
  
  
  METHODS
  Reference FDG PET SUV data were selected with replacement from a single-institution trial; pathologic complete response (pCR) data were simulated using a logistic regression model predicting response by mid-therapy percent change in SUV. The impact of increased error for SUV measurements in multicenter trials was simulated by sampling from error and bias distributions: 20%-40% measurement error, 0%-40% bias, and fixed error/bias values. The proportion of patients recruited from secondary sites (with higher additional error/bias compared to primary sites) varied from 25% to 75%.
  
  
  RESULTS
  Reference power (from source data with no added error) was 0.92 for N = 100 to detect an association between percentage change in SUV and response. With moderate (20%) simulated measurement error for 3/4, 1/2, and 1/4 of measurements and 40% for the remainder, power was 0.70, 0.61, and 0.53, respectively. Reduction of study power was similar for other manifestations of measurement error (bias as a percentage of true value, absolute error, and absolute bias). Enrichment designs, which recruit additional patients by not conducting a second scan in patients with unsuitable pre-therapy uptake (low baseline SUV), did not lead to greater power for studies constrained to the same total cost.
  
  
  CONCLUSIONS
  Simulation parameters could be incorrect, or not generalizable. Under a different logistic regression model relating mid-therapy percent change in SUV to pCR (with no relationship for patients with low baseline SUV, rather than the modest point estimate from reference data), the enrichment design did have somewhat greater power than the unselected design.
  
  
  CONCLUSIONS
  Even moderate additional measurement error substantially reduced study power under both unselected and enrichment designs.
authors.affiliation: aFred Hutchinson Cancer Research Center, Seattle, WA, USA.
authors.forename: Brenda F
authors.initials: BF
authors.lastname: Kurland
authors.forename: Robert K
authors.initials: RK
authors.lastname: Doot
authors.forename: Hannah M
authors.initials: HM
authors.lastname: Linden
authors.forename: David A
authors.initials: DA
authors.lastname: Mankoff
authors.forename: Paul E
authors.initials: PE
authors.lastname: Kinahan
mesh_terms: D000970:Antineoplastic Agents / Q000627:therapeutic use; D015982:Bias; D001943:Breast Neoplasms / Q000000981:diagnostic imaging / Q000188:drug therapy; D005260:Female; D019788:Fluorodeoxyglucose F18; D006801:Humans; D016015:Logistic Models; D015337:Multicenter Studies as Topic / Q000379:methods*; D020360:Neoadjuvant Therapy; D049268:Positron-Emission Tomography; D011446:Prospective Studies; D019275:Radiopharmaceuticals; D012107:Research Design; D018401:Sample Size; D016896:Treatment Outcome
chemical_list: D000970:Antineoplastic Agents; D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18