Diagnostic Value of Fluorine-18-fluorodeoxyglucose Positron Emission Tomography/Computed Tomography in Staging Histological Subtypes of Invasive Bladder Cancer.

BACKGROUND AND

OBJECTIVE

Up to 25% of bladder cancer cases harbour histological subtypes (HSs) distinct from conventional urothelial carcinoma (UC), often with more aggressive behaviour and variable treatment response. Accurate staging is essential, but the optimal approach for HSs remains unclear. Fluorine-18-fluorodeoxyglucose positron emission tomography/computed tomography ([18F]FDG-PET/CT) may improve staging in invasive bladder cancer (IBC), though its value in HSs is unestablished. This study evaluates the diagnostic performance and utility of [18F]FDG-PET/CT in staging HSs.

METHODS

We included 112 patients with histologically confirmed IBC containing ≥75% HS content, treated at a national comprehensive cancer centre (2012-2024). An expert pathological review was performed. All patients underwent standard staging with contrast-enhanced CT (CECT) and [18F]FDG-PET/CT. The primary endpoint was visualisation of the primary tumour. The secondary endpoints included incremental diagnostic value (change in nodal or metastatic stage compared with CECT), exploratory comparison with a matched UC cohort, and pathological correlation assessment.

KEY FINDINGS AND LIMITATIONS: The predominant HSs were neuroendocrine carcinoma (40%), squamous cell carcinoma (34%), adenocarcinoma (11%), and sarcomatoid carcinoma (11%). The primary tumour was FDG-avid in 87%, and FDG-negative in 5%. In 20%, [18F]FDG-PET/CT modified staging, mainly through upstaging. Differences in FDG avidity and staging impact across HSs and between pure (≥95%) and mixed (75-94%) content were small. A matched UC cohort showed similar rates of [18F]FDG-PET/CT-induced stage modification. Pathological confirmation was limited to a small selected subgroup, restricting the assessment of diagnostic accuracy.

CONCLUSIONS AND CLINICAL IMPLICATIONS: The use of [18F]FDG-PET/CT demonstrated high visualisation rates for primary tumour detection and altered staging in one of five patients, supporting its consideration as a complementary modality in HS staging.