Objective: We sought to determine the interobserver reliability of the histopathologic diagnosis of basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) (keratinocyte carcinomas) in the setting of a Department of Veteran Affairs multicenter chemoprevention study. Methods: Interobserver concordance was assessed by blinded review of histopathologic slides by study dermatopathologists. Results: Overall interobserver agreement between the two dermatopathogists was κ = 0.69 (95% confidence interval [CI] 0.67-0.69). The dermatopathologists' interobserver agreement was highest for basal cell carcinoma at κ = 0.88 (95% CI 0.84-0.91) and for a diagnostic category in the SCC-actinic keratosis spectrum at κ = 0.80 (95% CI 0.73-0.86). The largest disagreements between the two reference dermatopathologists were regarding the categories of invasive SCC at κ = 0.62 (95% CI 0.52-0.72), SCC in situ at κ = 0.42 (95% CI 0.29-0.56), and actinic keratosis at κ = 0.51 (95% CI 0.40-0.62). Agreement between the local pathologists and central reference dermatopathologists were similar to the agreement between the central dermatopathologists. The morphea subtype of basal cell carcinoma was the only reliably diagnosed subtype (κ = 0.79, 95% CI 0.51-1.00), and tumor depth was reliably measured. Limitations: A limitation of this study was the use of only two reference dermatopathologists. Conclusion: Because of the impact on physician decision making and patient care, researchers and clinicians need to be aware of reliability of histopathology results, particularly pertaining to the SCC and actinic keratosis spectrum.
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