Atypical Findings for a Typical Diagnosis: A Spectrum of Imaging Patterns for Oncocytoma

Objectives: Oncocytomas are benign renal neoplasms that are difficult to differentiate from renal cell carcinomas (RCC) on imaging. Biopsy is not always reliable, as oncocytic elements may be seen in RCC variants. Without a reliable less invasive option, patients with biopsy-proven oncocytomas typically undergo partial or complete nephrectomy. Due to stark differences in management and prognosis, there has been a robust effort to develop accurate preoperative imaging techniques to differentiate oncocytomas from malignant neoplasms. While MRI can assess a lesion’s histological components, it cannot characterize cellular makeup, rendering oncocytoma and RCC virtually indistinguishable on MRI. The advent of SPECT/CT has allowed for differentiation on a molecular level. We present a spectrum of SPECT/CT and MRI imaging findings of four patients at our institution with biopsy-proven oncocytomas in an effort to draw awareness to vast differences in imaging patterns to aid in diagnosis.

Methods: Four patients with biopsy-proven oncocytomas were selected to undergo [^99mTc]-sestamibi SPECT/CT to confirm their biopsy results. Per protocol, a bolus of intravenous 22-26 mCi [^99mTc]-sestamibi was injected. Dynamic flow and delayed sequential static images were obtained one hour after injection. SPECT/CT images were additionally acquired for accurate lesion localization on a 4-slice hybrid SPECT/CT system. Qualitative analysis was performed by a dual board-certified nuclear radiologist who categorized tumors as compatible with oncocytoma (increased uptake) or not compatible with oncocytoma (no uptake). All planar and SPECT/CT images were viewed on vendor provided software. Renal protocol MRIs were performed for further tumor characterization. MRI was performed using a 1.5 or 3.0-Tesla MRI scanner via the following protocol: 5-7 mm slice thickness, field-of-view of 38-44 cm. For contrast enhancement sequences, 0.1 mmol/kg of a gadolinium chelate was intravenously injected. Results: SPECT/CT incorrectly identified 2/4 biopsy proven renal oncocytomas. On one case, the oncocytoma on MRI had no enhancement, likely due to necrosis, a highly atypical finding for oncocytoma (figure 1). The remaining two biopsy proven oncocytomas did have significant [^99mTc]-sestamibi uptake on SPECT/CT, characteristic for oncocytoma. One of these had a central stellate scar on post-contrast MRI, characteristic of oncocytoma. However, a prior MRI of the same case was interpreted as favoring clear cell renal carcinoma rather than oncocytoma. The second SPECT/CT positive case was interpreted on MRI as suggestive of RCC. Both SPECT/CT positive cases for oncocytoma had MRI findings more characteristic of RCC. The two other biopsy proven oncocytoma cases had findings strongly suggestive of RCC on MRI and did not have uptake on SPECT/CT. Conclusion: We observed that characterization of a lesion as a benign oncocytoma on SPECT/CT is not as straightforward as currently believed. Our experience further supported the consensus that MRI is not a reliable modality to confirm oncocytoma. A limitation of our study, as well as others on this topic, is a low sample size. Future multi-institutional studies would be beneficial in more accurately identifying the subtle imaging characteristics of benign oncocytomas.

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