Influence of histologic criteria and confounding factors in staging equivocal cases for microscopic perivesical tissue invasion (pT3a): An interobserver study among genitourinary pathologists
Urinary Bladder Neoplasms
Current oncology guidelines and clinical trials consider giving adjuvant chemotherapy to bladder cancer patients with at least microscopic perivesical tissue invasion (MPVTI) (≥pT3a) on cystectomy. The boundary of muscularis propria (MP) and perivesical tissue is commonly ill defined, and hence, when the tumor involves the interface, interpretation of MPVTI is likely to be subjective. In this study, 20 sets of static images that included 1 nontumoral bladder wall for defining MP-perivesical tissue boundary and 19 bladder cancer cases equivocal for MPVTI with confounding factors were sent to 17 expert genitourinary pathologists for review. The confounding factors were "histoanatomic," as defined by the irregular MP-perivesical tissue boundary, and "tumor related," such as fibrosis, dense inflammation, tumor cells at the edge of the outermost MP muscle bundle, and lymphovascular invasion. These equivocal cases were divided into 3 categories according to the following factors: (1) histoanatomic only (7/19), (2) histoanatomic+tumor related (7/19), and (3) tumor related only (5/19). Participating genitourinary pathologists used different criteria to assess MPVTI: (A) drawing a straight horizontal line using the outermost MP muscle bundle edge as the MP-perivesical tissue boundary reference (3/17); (B) drawing multiple straight lines interconnecting the outermost MP muscle bundle edges (9/17); (C) following the curves of every outermost MP muscle bundle edge (4/17). In category 1 cases, most pathologists who used the A criterion called for absence (6/7), whereas those who used the C criterion called for presence (5/7) of MPVTI, which resulted in disparity in 4/7 cases. There was no circumstance in which criteria A and C agreed on the presence or absence of MPVTI but was opposed by the B criterion in category 1 cases. Median pairwise agreement among all pathologists (regardless of criteria) for all cases (regardless of category) was only "fair" (κ=0.281). However, when only the B criterion was assessed for category 1 cases, median agreement was "substantial" (κ=0.696), and pairwise rater comparisons included 6/36 (17%) "near perfect," 13/36 (36%) "substantial," and 11/36 (31%) "moderate" agreements. When all cases with histoanatomic factors (categories 1 and 2) were combined, median pairwise agreements were: (A) κ=0.588, (B) κ=0.423, and (C) κ=0.512, and the B criterion rater comparisons included 0/36 (0%) "near perfect," 6/36 (17%) "substantial," and 16/36 (44%) "moderate" agreements, which showed the confounding effect of tumor-related factors. For category 3 cases, median pairwise agreement for all pathologists was "fair" (κ=0.286), with consensus agreement in only 2/5 of these equivocal cases. Lymphovascular invasion only at the MP-perivesical tissue boundary was not staged as MPVTI by 87.5% of pathologists. In conclusion, this study showed that interpretation of equivocal cases for MPVTI can be made difficult by factors intrinsic to bladder histoanatomy, defined by an irregular MP-perivesical tissue boundary, and factors related to tumor spread. There are at least 3 different approaches to demarcating an irregular outer MP boundary, and agreement is improved on equivocal cases when a common histoanatomic criterion is used. However, inconsistent agreement of anatomic criteria may cause systematic discrepancy in assessing MPVTI. Tumor-related factors such as dense fibrosis or desmoplasia, obscuring inflammation, tumor cells at the edge of the outermost MP muscle bundle, and admixed lymphovascular invasion can also negatively influence the agreement on interpretation of MPVTI. This study highlights the need to adopt common criteria in defining the outer MP boundary. Future studies may identify the most clinically relevant histoanatomic criteria for MPVTI.