What does a rounded densely calcified structure on the left aspect of the urinary bladder represent?

Rounded Densely Calcified Structure at Left Bladder: Differential Diagnosis

A rounded densely calcified structure on the left aspect of the urinary bladder most likely represents either a bladder calculus (stone) or a phlebolith, though other important differential diagnoses include calcified bladder tumor, calcified urachal remnant, or incrusted cystitis.

Primary Differential Considerations

Bladder Calculus (Most Common)

• Bladder stones typically contain calcium and appear as uniformly radiopaque structures that may be laminated, faceted, spiculated, or seedlike in appearance 1
• Location within the bladder lumen (rather than the bladder wall) favors a stone
• Clinical correlation with urinary symptoms, history of bladder outlet obstruction, or recurrent UTIs supports this diagnosis 1

Phlebolith (Common Mimic)

• Phleboliths are benign calcified thrombi in pelvic vasculature that occur at frequencies equal to or greater than ureteral stones on CT imaging 2
• The close proximity of pelvic vessels to the bladder can make differentiation challenging 2
• The "comet tail sign" (irregular tapering soft tissue mass) suggests a phlebolith rather than a stone, though this sign has high specificity but low sensitivity 2
• Phleboliths are often less opaque centrally compared to the uniform radiopacity of true calculi 1

Critical Diagnostic Approach

Imaging Strategy

• CT scan without contrast is the gold standard for evaluating calcifications in this region, with 97% sensitivity for urolithiasis 3
• Multi-planar reformatted (MPR) images and maximum intensity projection (MIP) images are essential to determine the exact anatomic location of the calcification 4
• Review of prior imaging is crucial to assess for interval change 4

Key Distinguishing Features to Assess

Location determination:

• Is the calcification within the bladder lumen (stone) or within the bladder wall (tumor, incrusted cystitis)?
• Is it actually outside the bladder in adjacent vasculature (phlebolith)?
• Is it at the bladder dome (consider urachal pathology)? 1

Morphology assessment:

• Uniform density throughout suggests stone 1
• Central lucency suggests phlebolith 1
• Nodular or plaque-like surface calcification suggests transitional cell carcinoma 5
• Multiple fine punctate calcifications within a mass suggest mucinous adenocarcinoma 5

Less Common But Important Differential Diagnoses

Calcified Bladder Tumor

• Calcifications occur in approximately 8% of bladder tumors 5
• Transitional cell carcinoma shows nodular or plaque-like calcifications on the tumor surface 5
• Mucinous adenocarcinoma demonstrates multiple fine punctate calcifications scattered within the mass 5
• Urachal carcinoma typically occurs at the bladder dome with associated tumor calcification 1

Calcified Urachal Remnant

• Can mimic a bladder calculus, particularly in the dependent portion of the bladder on supine imaging 4
• Location at the anterior bladder dome/wall is characteristic 4
• MPR images are essential to distinguish this from a true bladder stone 4

Incrusted Cystitis

• Presents with calcified plaques in the bladder mucosa that can mimic a tumor 6
• Associated with highly alkaline urine and ureolytic bacteria 6
• Clinical presentation includes severe urinary symptoms 6
• Biopsy shows calcified necrotic mucosa without malignancy 6

Clinical Pitfalls to Avoid

• Do not rely on KUB alone for diagnosis—it has poor sensitivity (53-62%) and specificity (67-69%) for detecting calculi and cannot determine the cause of calcification 7
• Always review MPR and MIP images to confirm anatomic location before assuming a calcification represents a bladder stone 4
• Do not mistake calcified bladder tumors for benign stones—biopsy may be necessary if imaging features are atypical 5, 6
• Consider phleboliths in the differential, especially if the "rim sign" (circumferential ureteral soft tissue edema) is absent 2

Recommended Diagnostic Algorithm

1. Obtain non-contrast CT with MPR and MIP reconstructions to definitively localize the calcification 7, 4
2. Assess for secondary signs:
   • Hydronephrosis or hydroureter (suggests obstructing stone)
   • Bladder wall mass (suggests tumor)
   • Comet tail sign (suggests phlebolith) 2
3. If imaging is equivocal, consider cystoscopy with biopsy to exclude malignancy and incrusted cystitis 5, 6
4. Correlate with clinical history: urinary symptoms, alkaline urine, recurrent UTIs, hematuria 6