Background <p>Primary adrenal lymphoma (PAL) is a rare and aggressive extranodal lymphoma that may radiologically mimic primary adrenal carcinoma, often leading to diagnostic uncertainty and potentially unnecessary surgical intervention. Early tissue diagnosis is essential for accurate management.</p> Case Presentation <p>A 71-year-old man was evaluated for persistent left upper quadrant abdominal pain and progressive fatigue. Cross-sectional imaging revealed a large, heterogeneous mass originating from the left adrenal gland with radiologic features suggestive of local invasion. Based on the initial radiologic impression, primary adrenal carcinoma was suspected, and surgical intervention was planned accordingly. However, further review by an experienced radiologist raised suspicion of a lymphoproliferative disorder. A computed tomography-guided core needle biopsy was subsequently performed, and histopathological analysis confirmed the diagnosis of high-grade B-cell lymphoma with a Ki-67 proliferation index of approximately 80%. The patient received six cycles of well-tolerated R-CHOP chemotherapy (rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone). Post-treatment imaging revealed complete resolution of the adrenal mass. At the 24-month follow-up, the patient remained in complete remission, with no evidence of disease recurrence.</p> Conclusions <p>This case underscores the importance of multidisciplinary assessment and image-guided biopsy in the evaluation of adrenal masses. Avoiding unnecessary surgery through early tissue diagnosis enabled effective systemic treatment. In selected patients, R-CHOP chemotherapy may result in long-term disease control, even in elderly individuals with large unilateral adrenal tumors.</p>