Muscularis propria invasion in the urinary bladder

The presence or absence of muscularis propria invasion in the urinary bladder is a very important determination, as the clinical management changes between T1 and T2:

• T1: usually conservative treatment (local excision).
• T2: radical treatment (cystectomy or cystoprostatectomy).

A thin layer of discontinous muscularis mucuosae (MM) is present and, especially if hypertrophic, may be confused with muscuaris propria (MP).

General

Comparing muscularis propria and muscularis mucosae

A comparison between muscularis propria and muscularis mucosae - adapted from Paner et al.:

Feature	Muscularis mucosae	Muscularis propria
Outline/border	typically irregular (frayed edges)	usually regular (circumscribed)
Size of bundles ‡	classically "small", often "large" (hypertrophic)	usually "large"
Isolated fibres	yes	no
Location in bladder	less common in trigone, dome very common	everywhere
Depth †	superficial, occ. deep	deep

Notes:

• † The lamina propria thickness varies with location. It is thinnest in the trigone (0.5-1.6 mm) and thickest in the dome (1.0-3.1 mm).
• ‡ Small is defined as <4 muscle fibres; large >= 4 muscle fibres.
  • The presence of hyperplastic bundles ranges from ~20% in the trigone to ~70% in the dome.

Images

• 

  MM - intermed. mag. (wc)

• 

  MM - high mag. (WC)

• 

  MM - very high mag. (WC)

Microscopic

Rational assessment of muscularis propria invasion

To call muscularis propria invasion:

1. Definite tumour must be between muscle.
2. Muscle bundles must be thick.
3. Multiple bundles must be adjacent to one another.
4. Should not be superficial - surface epithelium if present should be distant.

DDx:

• Invasion of hypertrophic muscularis mucosae.

Images

• 

  MIUC - very low mag.

• 

  MIUC - low mag.

• 

  MIUC - intermed. mag.

• 

  MIUC - intermed. mag.

• 

  MIUC - high mag.

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Cannot exclude MP invasion

Urinary Bladder Tumour, Transurethral Resection:
- INVASIVE HIGH-GRADE UROTHELIAL CARCINOMA.
-- Extensive invasion into at least the lamina propria.
- Negative for definite muscularis propria, see comment.

Comment:
The tumour is seen around bundles of smooth muscle that are favoured 
to represent muscularis mucosa; however, obliterated muscularis 
propria cannot be excluded. 

Correlation with the clinical findings is required. Additional transurethral
sampling and/or imaging to assess the muscularis propria is suggested.

See also

• Urothelial carcinoma.
• Cancer staging systems.