Frequency of Low Grade T1 Bladder Cancer Has Decreased But Continues to Vary by Institution
Low grade but invasive urothelial cancers are rare and in 2004, the World Health Organization switched from a three-tier to the current two-tier grading system: high-grade (HG) and low-grade (LG) to reduce tumor heterogeneity. We sought to evaluate the incidence and factors associated with the diagnosis of LG T1 bladder cancer since the adoption of the 2004 grading system.
The Surveillance, Epidemiology, and End Reports (SEER: 2004-2014) and the National Cancer Database (NCDB: 2004-2015) were queried for all patients diagnosed with T1 bladder cancer. The proportion of T1 patients with LG disease was trended over time. Logistic regression was used to determine factors associated with a diagnosis of LG (compared to HG), including year of diagnosis. Overall survival (OS) and cancer-specific (CSS) were compared between the groups.
Within SEER, 34,735 patients were diagnosed with T1 bladder cancer. The percentage of LG cancers decreased from 33.4% in 2004 to 15.6% in 2014 (chi-squared p < 0.001). Patients with HG T1 had worse 10-year CSS and OS, 70.9% and 42.6%, compared to LG, 83.1% and 51.5%, respectively (both log-rank p < 0.001). The NCDB identified 58,092 patients with T1, with 26.5% LG. The percentage of LG decreased over time from 38.9% to 18.5% (OR 0.38 [95% CI, 0.35-0.41, p < 0.001] for 2013 vs 2004). On logistic regression, each successive year, the type of institution (academic or comprehensive compared to community) and distance from treatment facility greater than 60 miles were associated with lower rates of LG. Within NCDB institutions, the proportion of LG T1 ranged from 0% to 92% (Figure 1). Among LG T1 patients, there was no difference in survival when stratified by year of diagnosis.
Since the adoption of the current bladder cancer grading system, the percentage of T1 diagnoses documented as LG has declined substantially over time but continues to vary widely by institution. This variability raises concerns for grading misclassification within the current system. Given the worse OS and CSS for HG T1, the misclassification of T1 disease as LG could have a negative impact on survival.