Leslie W. Nelson

Retroperitoneal diseases encompass a broad and heterogeneous spectrum, ranging from benign conditions to aggressive malignancies. Clinical presentation is often nonspecific, and physical examination is limited by the deep location of the retroperitoneum. Consequently, cross-sectional imaging—primarily computed tomography (CT) and magnetic resonance imaging (MRI)—is central to detection, characterization, and staging, playing a key role in guiding management and surgical planning. Imaging evaluation is challenging due to overlapping appearances, difficulty determining the site of origin and extent of disease in large or infiltrative processes, and the complex, incompletely understood retroperitoneal anatomy. A structured, anatomy-based interpretive framework fosters improved diagnostic accuracy. Initial assessment focuses on localization and lesion composition. Distinguishing between pathology arising from retroperitoneal organs and primary retroperitoneal processes, combined with characterization of composition (e.g., solid, cystic/fluid, fat, calcification, gas, or mixed), allows generation of a focused differential diagnosis. Integration of key imaging and clinical features then refines and prioritizes diagnostic considerations. Common pitfalls in interpretation are related to technique, mischaracterization of lesion location or composition, and underestimation of disease extent. Recognition of these potential sources of error, coupled with a systematic approach to image interpretation, can improve diagnostic accuracy and better inform clinical decision-making in retroperitoneal pathology.

Background/Objectives: Percutaneous microwave (MW) ablation is a nephron sparing treatment for localized renal cell carcinoma (RCC). We compared perioperative, renal functional, and oncologic outcomes for clinical stage 1 RCC treated with MW ablation, PN, or RN. Methods: Adults with clinical T1 kidney masses treated with MW ablation, PN, or RN from 2001–2025 were identified. Outcomes included: 90-day overall and major complication rate, 30-day readmission rate, length of hospital stay (LOS), change in renal function, local recurrence-free survival (LRFS), metastasis-free survival (MFS), and cancer-specific survival (CSS). Univariable and multivariable analyses evaluated outcomes adjusted for confounders. Results: A total of 2201 patients with renal masses ≤ 7 cm and no evidence of locally advanced or metastatic disease were treated with MW ablation (708), PN (729), or RN (764). MW ablation patients were older and more comorbid compared to both PN/RN, whereas RN patients had larger, higher-grade tumors. Ninety-day overall complications were lowest after MW ablation (8.9% vs. 20.3% PN, p < 0.001 and 8.9% vs. 19.9% RN, p < 0.001). LOS was shortest after MW ablation (median 1 day vs. 3 days PN/RN, p < 0.001 for each). Six-month eGFR decline was similar after MW ablation and PN (−5.2% and −4.7%, p = 0.84) but greater after RN (−32.9%, p < 0.001). Local recurrences were more common with MW ablation, with five-year LRFS 96.4% versus 99.7% for PN (p < 0.001). Five-year MFS (99.5% vs. 99.7%, p = 0.24) and CSS (99.3% vs. 99.7%, p = 0.71) did not differ between MW ablation and PN. Conclusions: Percutaneous MW ablation has comparable metastasis free and cancer specific survival with lower perioperative morbidity and comparable renal preservation to PN, despite worse baseline comorbidity and renal function. These findings support MW ablation as an effective nephron-sparing option for appropriately selected patients with clinical T1 RCC when performed at an experienced center.

RSNA Case Collection is a collection of peer-reviewed radiology cases.