3:54 PM Abstract No. 187 Renal cell carcinoma local recurrence vs. non-malignant imaging mimics post-radiofrequency ablation: incidence, imaging features, and natural history

Abstract

To compare the frequency, imaging characteristics and natural history of non-malignant imaging mimics of tumor recurrence following successful radiofrequency ablation (RFA) of RCC with confirmed local recurrence. A single-center, retrospective case-control study evaluated all patients with successful RFA treatment of biopsy-confirmed RCC or suspicious oncocytic neoplasm between April 2004 and June 2016. Inclusion criteria included minimum post-RFA follow-up of 10 months and at least 2 contrast-enhanced studies post-RFA. 256 patients and 268 tumors were included (mean follow-up 4.2 ± 2.6 years). All RFA procedures were performed under conscious sedation using a multi-tined RF electrode. Post-RFA imaging follow-up was with either multiphase CT, MRI or contrast-enhanced US. All patients flagged by the radiologist as suspicious for local recurrence were analyzed and stratified into 1) Local recurrence (LR) or 2) Non-malignant changes (NMC). Patients were categorized as NMC only if a biopsy confirmed no viable tumor or if the abnormal features resolved on follow-up imaging. Local recurrence and non-malignant changes occurred in 3.4% (9/268) and 2.6% (7/268) of all treated tumors. Mean age at time of RFA (LR 64.7 ± 14.1, NMC 69.9 ± 8.5 years), tumor size (LR 2.9 ± 0.8, NMC 2.4 ± 0.4 cm), and time post-RFA to imaging abnormality detection (LR 29.0 ± 19.5, NMC 40.3 ± 17.7 months) were not significantly different between the groups. Respective tumor histologies were LR: clear cell 8 (89%); papillary 1 (11%) and NMC: clear cell 4 (57%); papillary, chromophobe & oncocytic neoplasm 1 each (14%). Imaging characteristics differed between the two groups as the amount of CT enhancement above the precontrast baseline was significantly higher (p<0.001) for the LR group (155 ± 38 HU) than the NMC group (28 ± 8 HU). Post-ablation nodular enhancement either centrally within the region of coagulation or along the extra-renal ablation margin was more commonly associated with NMC (4/5 tumors), while nodular enhancement along the renal parenchyma margin was always LR (8/8 tumors). Non-malignant changes post-RFA can mimic RCC recurrence on imaging and should be recognized to minimize unnecessary treatment.