Carcinoma in situ of the bladder

 Flat lesion composed of cells in mid to upper epithelium with high cytologic grade
 By definition, no invasion into lamina propria

Note: "high grade non-invasive papillary lesions" are NOT designated "carcinoma in situ" to avoid confusion

Microscopy

 Flat lesion composed of cells with large, irregular, hyperchromatic nuclei, prominent nuclear pleomorphism, high N/C ratio, mitotic figures in mid to upper epithelium
 Atypia may not be full thickness
 Epithelium is often denuded
 Nuclear size is 5x that of lymphocytes vs 2x lymphocytes for normal urothelium (Hum Pathol 2001;32:997)
 Also (but less important) loss of polarity, nuclear crowding, irregular thickness of urothelium
 Cells are not cohesive, leading to shedding into urine
 Occasionally present in prostatic ducts, spreads by intramucosal extension

Variants

 Large cells with pleomorphism,
 large cells without pleomorphism,
 small cell,
 clinging (single layer of atypical cells on denuded urothelium),
 cancerization of urothelium (pagetoid - Hum Pathol 1993;24:1199, undermining or overriding)
 Microinvasion (2 mm or less) demonstrates invasive cells with retraction artifact mimicking vascular invasion (77% of cases of microinvasion)
 Also nests or irregular cords, rarely invades as isolated single cells with or without desmoplasia (Am J Surg Pathol 2001;25:356)

Note: "high grade non-invasive papillary lesions" are NOT designated as "carcinoma in situ" to avoid confusion