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Study Type - Prognosis (inception cohort) Level of Evidence 2a What's known on the subject? and What does the study add? Tumour stage is a powerful predictor of clinical outcomes and the most important factor driving clinical decision-making after radical nephroureterectomy (RNU) in upper tract urothelial carcinoma (UTUC). It has been suggested that renal pelvic pT3 subclassification into microscopic infiltration of the renal parenchyma (pT3a) versus macroscopic infiltration or invasion of peripelvic adipose tissue (pT3b) has strong prognostic value. This is an external validation study of the prognostic value of pT3 subclassification of renal pelvic UTUC in a large international cohort of patients treated with RNU. pT3b UTUC is associated with features of aggressive tumour biology, disease recurrence and cancer-specific mortality. However, pT3 subclassification is not an independent predictor of clinical outcomes. OBJECTIVE To externally validate the prognostic value of subclassification of pT3 renal pelvic upper tract urothelial carcinoma (UTUC) in a large international cohort of patients treated with radical nephroureterectomy (RNU)., PATIENTS AND METHODS The RNU specimens with pT3 UTUC of the renal pelvis from 284 patients at 11 centres located in Asia, North America and Europe were retrospectively evaluated. All specimens were reviewed by genitourinary pathologists at each institution. Tumours were categorized as pT3a (microscopic infiltration of the renal parenchyma) or pT3b (macroscopic infiltration of the renal parenchyma and/or infiltration of peripelvic adipose tissue)., RESULTS Overall, 148 (52%) tumours were classified as pT3a and 136 (48%) as pT3b. Patients with pT3b disease were more likely to have high-grade tumours and sessile tumour architecture (all P≤ 0.02). Patients with pT3b tumours were at increased risk of disease recurrence (5-year estimates: 55% versus 42%, P= 0.012) and cancer-specific mortality (CSM) (5-year estimates: 48% versus 40%, P= 0.04). Lymph node status, tumour architecture and tumour grade were independently associated with disease recurrence, whereas lymph node status, tumour architecture and lymphovascular invasion were independently associated with CSM. Subclassification of pT3 tumours was not associated with recurrence or CSM in multivariable analyses., CONCLUSION Patients with pT3b UTUC were more likely to have tumours with aggressive pathological features and were at higher risk of disease recurrence and CSM after RNU compared with patients with pT3a disease. However, the pT3 subclassification did not remain an independent predictor of disease recurrence or CSM after controlling for tumour grade, lymph node status, tumour architecture and lymphovascular invasion. [ABSTRACT FROM AUTHOR]