What imaging studies are recommended for diagnosing penile lymphedema?

Imaging Modalities for Diagnosing Penile Lymphedema

Magnetic Resonance Imaging (MRI) is the preferred imaging modality for diagnosing penile lymphedema, as it provides detailed anatomical visualization of lymphatic channels, soft tissue changes, and can detect lymphatic malformations without requiring contrast agents. 1, 2

Primary Imaging Recommendations

• MRI (non-contrast): Should be the first-line imaging modality for penile lymphedema, using heavily T2-weighted sequences that highlight lymphatic vessels while suppressing background tissue signal 3
• Ultrasound: Can be used as an alternative when MRI is unavailable, particularly useful for initial assessment of soft tissue changes and to guide any potential fine-needle aspiration cytology (FNAC) 4

MRI Protocol for Penile Lymphedema

• Technique: Use heavily T2-weighted fast spin echo sequences that provide nearly complete signal loss in tissue background while highlighting lymphatic vessels 3
• Processing: Raw data should be processed with maximum intensity projection algorithm to obtain anatomic representation of lymphatic structures 3
• Findings: MRI can identify characteristic features including:
  • Diffuse dermal and subcutaneous edema
  • Serpiginous "channels" or "lakes" consistent with dermal collateral lymphangiectasis
  • Sequestered lymph and increased subcutaneous fat 1

Classification Based on MRI Findings

MRI allows classification of lymphedema into three categories:

• Aplasia: No collecting vessels demonstrated
• Hypoplasia: Small number of lymphatic vessels present
• Hyperplasia: Increased number of lymphatic vessels with greater and abnormal diameter 3

Additional Imaging Modalities

• Lymphoscintigraphy: Useful for functional assessment of lymphatic drainage but less detailed anatomically than MRI; shows dermal diffusion ("backflow"), cross-over with retrograde tracer backflow, and delayed tracer transport 1
• High-resolution ultrasound: Can identify low-density spaces in subcutaneous tissue corresponding to dilated lymphatic spaces 5
• 18FDG-PET/CT: Reserved for cases where malignancy is suspected as a cause of lymphedema or to exclude distant metastases in penile cancer patients 4

Diagnostic Algorithm

1. Initial assessment: Physical examination of the penis, documenting morphology and extent of swelling 4
2. First-line imaging: Non-contrast MRI of the penis using T2-weighted sequences 3
3. If MRI unavailable: High-resolution ultrasound to assess soft tissue changes 5
4. For suspected infection/inflammation: Consider ultrasound-guided FNAC of any associated enlarged inguinal lymph nodes 6
5. For suspected malignancy: Consider 18FDG-PET/CT to evaluate for metastatic disease 4

Clinical Pearls and Pitfalls

• Differential diagnosis: Important to distinguish between primary lymphedema (congenital lymphatic dysplasia) and secondary causes (post-infectious, post-surgical, malignancy) 7
• Common pitfall: Up to 50% of palpable inguinal nodes may be due to inflammatory changes rather than metastatic disease, requiring careful correlation of imaging findings with clinical history 6
• Follow-up imaging: Non-contrast MRI is safe for repeated evaluations to assess treatment response 3
• Surgical planning: MRI can help determine the extent of lymphedematous tissue requiring excision in cases needing surgical intervention 2