What is the best imaging modality to monitor bilateral axillary (armpit) adenopathy in an immunocompromised patient with a history of severe bacterial and fungal infection in the lungs, who has undergone computed tomography (CT) scan and positron emission tomography (PET) scan?

Monitoring Bilateral Axillary Adenopathy After Treated Pulmonary Infection

For monitoring bilateral axillary adenopathy in this clinical context, axillary ultrasound is the optimal imaging modality, as it provides excellent characterization of lymph node morphology, is non-invasive, avoids radiation exposure, and can be repeated serially to assess response. 1, 2

Clinical Context and Interpretation

The minimal FDG uptake on PET scan combined with the discovery and treatment of severe bacterial-fungal lung infection strongly suggests the axillary adenopathy is reactive rather than malignant. 2 Benign reactive lymphadenopathy from infectious and inflammatory processes is a common cause of FDG-avid axillary nodes, with infectious etiologies including granulomas and various infections. 2 The bilateral presentation further supports a systemic process such as infection, inflammatory conditions, or reactive changes rather than malignancy. 1, 3

Recommended Monitoring Strategy

Primary Imaging Modality

• Axillary ultrasound should be used for serial monitoring because it is the primary modality of choice for evaluating lymph node morphology and determining whether masses are solid or cystic. 1, 2, 3
• Ultrasound allows assessment of specific morphologic features that distinguish benign from malignant nodes without radiation exposure. 1
• Serial ultrasound examinations can track changes in node size, cortical thickness, and preservation of fatty hilum over time. 1

Timing of Follow-up Imaging

• Do not repeat imaging less than 7 days after completion of antimicrobial therapy, as lymph nodes may initially increase in size despite effective treatment, similar to the pattern seen with pulmonary fungal infections. 4
• Consider initial follow-up ultrasound at 4-6 weeks after completion of treatment to allow adequate time for resolution of reactive changes. 4

When to Consider Alternative Imaging

Chest CT without IV contrast should be obtained if:

• The patient develops new pulmonary symptoms during follow-up, as CT is superior for detecting recurrent or persistent lung infiltrates. 4
• Lymph nodes demonstrate suspicious morphologic features on ultrasound (loss of fatty hilum, cortical thickening >3mm, rounded rather than oval shape). 1
• There is clinical concern for recurrent infection, as high-resolution CT is preferred for primary diagnosis in high-risk immunocompromised patients. 4

PET/CT has limited utility for routine monitoring because FDG uptake is nonspecific and cannot distinguish between infection, inflammation, and malignancy. 2, 5 While FDG-PET/CT may be helpful in selected patients with unexplained fever to rule out undetected infection, it should not be used for routine surveillance of known reactive adenopathy. 4

Critical Decision Points

Indications for Tissue Diagnosis

Proceed to ultrasound-guided biopsy if: 1, 3

• Lymph nodes persist or enlarge beyond 8-12 weeks after completion of antimicrobial therapy
• Nodes develop suspicious morphologic features on serial ultrasound
• New systemic symptoms develop suggesting lymphoma or other malignancy
• Clinical context changes (new fever, night sweats, weight loss)

The specificity of ultrasound-guided core needle biopsy or fine-needle aspiration is 98-100%, providing definitive diagnosis when needed. 3

Common Pitfalls to Avoid

• Never rely on physical examination alone, as both sensitivity and specificity are limited for assessing lymph node characteristics. 1, 3
• Do not assume benign etiology without tissue diagnosis if nodes persist beyond 3 months, as bilateral presentation can represent lymphoma despite initial reactive appearance. 1, 3
• Do not interpret persistent or enlarging nodes in the first week of treatment as treatment failure, as this paradoxical enlargement is well-described with effective antimicrobial therapy. 4
• Avoid repeat PET/CT for routine monitoring, as the nonspecificity of FDG uptake provides limited additional information in this clinical scenario. 2, 5

Expected Clinical Course

In the setting of treated severe pulmonary infection, reactive axillary adenopathy should gradually decrease in size over 6-12 weeks. 4 Serial ultrasound examinations at 4-6 week intervals allow objective documentation of resolution without unnecessary radiation exposure or cost. 1 If nodes have not substantially decreased by 12 weeks post-treatment, tissue diagnosis should be strongly considered to exclude alternative diagnoses including lymphoma. 3