Inter-rater concordance of basal cell carcinoma subtypes: influences on reporting format and opportunities for further classification modifications

Diagnosis of basal cell carcinoma (BCC) higher risk subtypes influences management strategies because of their propensity to recur locally. Subtyping is prone to interobserver variability, and subtyping definitions are inconsistently applied. This study sought to compare the interobserver reproducibility of individual BCC subtypes using the 4th edition World Health Organization (WHO) Classification of Skin Tumours (CoST) definitions, with classification into lower and higher risk histological subtype groups. Ninety-one BCC cases were rated by seven pathologists, noting the presence of BCC subtype(s), and providing a higher or lower risk subtype grouping per case. Raters were provided with definitions as per the 4th edition WHO CoST for 10 listed BCC subtypes. Surgical specimen type was noted. Subgroup analysis was performed to exclude cases when the tumour deep front was not well visualised, or there was tangential sectioning (n = 6). Light's kappa was used to assess inter-rater reliability. From the total group (n = 91), five BCC subtypes showed a sufficient number of ratings for computing a K statistic. From these five subtypes, superficial subtype showed substantial inter-rater agreement (K = 0.64), and the other four subtypes showed moderate inter-rater agreement [nodular (K = 0.45), sclerosing/morphoeic (K = 0.45), infiltrating (K = 0.49) and micronodular (K = 0.57)]. Two-tiered rating into either higher or lower risk subtype showed substantial inter-rater agreement (K = 0.72). Our results suggest a need to more precisely define BCC subtypes. We suggest reporting BCC subtype using a twotiered risk grouping, followed by specific subtypes present. Further studies examining the inter-rater reliability of less common BCC subtypes are required.

Automated summary

This study compares the interobserver reproducibility of individual subtypes of basal cell carcinoma (BCC) using the 4th edition World Health Organization (WHO) Classification of Skin Tumours (CoST) definitions. The results show that some BCC subtypes have high interobserver agreement, while others have moderate agreement. The study suggests the need for more precise definitions of BCC subtypes and recommends reporting BCC subtypes using a two-tiered risk grouping method.