Differential Diagnosis for 69-year-old Male with GPA and Rapidly Expanding Lymphadenopathy

Single Most Likely Diagnosis

• GPA (Granulomatosis with Polyangiitis) with lymph node involvement: The patient's history of GPA, ANCA vasculitis, and necrotizing granulomatosis on biopsy, combined with the rapidly expanding lymphadenopathy, suggests that the lymphadenopathy could be a manifestation of active GPA. GPA can involve various organs, including the lymph nodes, and this presentation is consistent with known disease patterns.

Other Likely Diagnoses

• Lymphoma: Given the rapid expansion of lymphadenopathy, lymphoma should be considered, especially in an elderly patient. The presence of ANCA vasculitis and GPA does not exclude the possibility of a concurrent lymphoma.
• Infection: Infections can cause rapid lymph node enlargement, especially in immunocompromised patients. Given the patient's age and potential immunosuppression due to GPA treatment, an infectious cause should be considered.
• Drug-induced lymphadenopathy: Certain medications, including those used to treat GPA, can cause lymphadenopathy. A review of the patient's medication list is necessary to evaluate this possibility.

Do Not Miss Diagnoses

• Malignancy (other than lymphoma): Other malignancies, such as metastatic cancer, can present with rapidly enlarging lymph nodes. Given the patient's age, a thorough evaluation for other cancers is crucial.
• Tuberculosis or other granulomatous infections: These infections can mimic GPA and cause lymphadenopathy. They are important to rule out due to different treatment approaches.
• Histiocytosis: Although rare, histiocytosis can present with lymphadenopathy and systemic symptoms, making it a "do not miss" diagnosis due to its potential for aggressive disease.

Rare Diagnoses

• Kikuchi-Fujimoto disease: A rare form of lymphadenitis that can present with rapidly enlarging lymph nodes, typically in younger individuals but can occur at any age.
• Kimura disease: A chronic inflammatory disorder that can cause lymphadenopathy and eosinophilia, more common in Asian males.
• Castleman disease: A rare disorder that can cause lymphadenopathy, fever, and systemic symptoms, often associated with HIV infection or other immunodeficiencies.

Each of these diagnoses requires careful consideration of the patient's clinical presentation, laboratory results, and imaging studies to determine the most appropriate diagnostic and therapeutic approach.