microUSG for CaP: The New Kid on the Block

Microultrasound (microUSG), most widely implemented via the 29 MHz ExactVu™ system, delivers ~70 µm axial resolution—comparable to prostatic duct size—allowing detailed visualization of disrupted glandular architecture. This contrasts with the ~300 µm resolution of multiparametric MRI (mpMRI). A side-fire transducer enables a standard transrectal approach with minimal retraining and ergonomic handling. While its field of view is slightly narrower than conventional low-frequency TRUS, microUSG provides superior detail of the near and mid-zones. However, penetration is limited in larger prostates, particularly anteriorly, sometimes requiring probe tilting or a transperineal approach.

The PRI-MUS (Prostate Risk Identification using MicroUltrasound) scoring system (Score 1–2 benign, 3 equivocal, 4–5 suspicious) standardizes lesion characterization, enabling real-time targeting without MRI-fusion. With its higher resolution, microUSG demonstrates high accuracy (82%), excellent sensitivity (91%), and strong negative predictive value (97%), but low specificity (49). This makes it highly effective for ruling out prostate cancer when PRI-MUS ≤ 2, though scores of 3–5 require caution due to a tendency to overcall benign lesions.

Advantages

MicroUSG offers multiple advantages in prostate cancer diagnostics:

• Real-time, high-resolution targeting for a single-sitting “see-and-sample” workflow.

• Reduced delays and patient anxiety by minimizing the imaging-to-biopsy gap.

• Optimized resource use by reserving MRI for staging and complex workups.

• Enhanced accessibility by bypassing MRI bottlenecks, contrast requirements, and fusion logistics.

• Operator familiarity with ultrasound, intuitive interface, and short learning curve.

• High overlap with MRI findings, with most MRI-visible targets also detectable on microUSG, allowing real-time targeting without cognitive or software fusion.

Limitations

Challenges include operator-dependent interpretation, evolving inter-reader agreement, and reduced penetration due to high frequency, which limits anterior and apical lesion assessment in large prostates. Calcification-related shadowing can also hinder evaluation. Consequently, microUSG is best viewed as a complementary tool, with MRI remaining superior for comprehensive local staging, particularly in assessing extracapsular extension and neurovascular bundle involvement.

Current Evidence

The multicenter, international OPTIMUM trial demonstrated microUSG to be non-inferior to MRI/fusion for clinically significant prostate cancer detection (non-inferiority P<0.001). Additional meta-analyses and prospective trials further confirmed its comparable accuracy, with each modality identifying lesions occasionally missed by the other.

Parameter	microUSG	mpMRI
Resolution	70 µm	≈300 µm
Approach	Transrectal, side-fire probeTransperineal possible	Imaging followed by fusion-guided biopsy
Workflow	See-and-sample (In-office, real-time targeting)	Multi-step (MRI → reporting → fusion biopsy)
Time to Biopsy	Immediate (same session)	Delayed (Days-weeks)
Cost and Accessibilty	Lower, Portable	Higher, need MRI suite
Contrast Agents	No	Yes
Accuracy	82%	Comparable (80-85%)
Sensitivity	91%	Comparable (89-93%)
Specificity	49%	Higher (65-75%)
Negative Predictive Value	97%	Comparable (90-95%)

Conclusion

For urologists, microUSG represents a practical, fast, and accurate advancement in prostate cancer diagnostics. It offers a frontline, urologist-led pathway, particularly in settings where MRI access is limited or timely diagnosis is critical.

Dr. Harshdeep Singh, VMMC & SJH, New Delhi