¹⁸F-FDG-avid Renal Oncocytoma in a Breast Cancer Patient: A Diagnostic Pitfall on PET/CT

INTRODUCTION

A 66-year-old female is a known case of breast cancer, presented with a huge left breast mass with matted axillary lymph nodes referred for 18F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT) for staging. ¹⁸F-FDG PET/CT shows large hypermetabolic fungating lobulated mass in the outer quadrant of the left breast (primary tumor) and FDG avid metastatic ipsilateral left axillary and internal mammary lymph nodes [Figure 1]. In addition, there is a large FDG-avid ill-defined soft -tissue mass in the right kidney. Finding is suspicious for malignancy, possible renal primary versus metastatic. Further histopathological correlation is recommended. Biopsy showed renal oncocytoma (RO) [Figure 2].

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Figure 1:

(a) 18F-fluorodeoxyglucose positron emission tomography (FDG PET) maximum intensity projection images show large hypermetabolic fungating lobulated mass in the outer quadrant of the left breast (primary tumor) and FDG avid metastatic ipsilateral left axillary and internal mammary lymph nodes (b and c). In addition, transaxial 18F-FDG PET/computed tomography (CT) demonstrates large FDG-avid ill-defined soft -tissue mass (black arrows) in the right kidney. Finding is suspicious for malignancy, renal primary versus metastatic. (d) Contrast CT showed large predominantly solid exophytic renal mass (white arrows) measuring 7.5 cm × 6.9 cm × 6.7 cm with circumscribed margins and in homogeneous enhancement, suggesting renal malignancy

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Figure 2:

Renal core biopsy. (a) H and E staining low power view (10x) shows epithelial neoplasm arranged in nests and sheets. The tumor cells contain abundant eosinophilic granular cytoplasm and small round nuclei showing mild nuclear atypia. (b) Immunohistochemistry shows that tumor cells are positive for PAX8, thus indicating the tumor is of primary renal cell origin. (c) Negative for GATA3, indicating the tumor is not of a primary breast origin. (d) The tumor cells are positive for CD117, supporting the diagnosis of renal oncytoma

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RO is a benign renal neoplasm, with a reported prevalence of 3%–7.3% and most commonly present in the sixth to seventh decades of life.^[1] RO is usually asymptomatic and is observed incidentally during the routine examinations. It usually appears as a solitary tumor measuring between 4 and 8 cm, which may infiltrate peripheral renal tissues and oft en indistinguishable from renal cell carcinomas (RCCs) on imaging, increasing concern for urologists and oncologists.^[2] On CT scan, oncocytoma appears as solid homogeneous lesion with a centrally located scar, and arteriography may reveal a spoke-wheel vascular pattern.

¹⁸F-FDG PET is widely used in cancer for staging, restaging, assessing disease extent, monitoring treatment response, and prognostication.^[3] While highly sensitive for detecting malignancies, its specificity can be affected by benign, inflammatory, and infective conditions.^[4,5] 18F-FDG PET plays a pivotal role in the staging and restaging of various malignancies, including breast cancer. However, its specificity is limited when evaluating incidental renal lesions. ¹⁸F-FDG PET/CT has limited utility for localized renal masses due to variable uptake in primary tumors and high background activity in normal parenchyzma and radiotracer excretion in urine. Most benign tumors typically show low or no ¹⁸F-FDG uptake, ROs represent an important exception.^[6]

Oncocytoma, a benign renal neoplasm composed of cells, can exhibit increased uptake on ¹⁸F-FDG PET mimicking metastatic disease.^[7,8] This uptake is thought to result from the high metabolic activity and mitochondrial content of oncocytic cells.^{[9] 18}F-FDG PET/CT cannot reliably distinguish oncocytomas from RCC due to overlapping metabolic profiles. For example, RCCs also display variable ¹⁸F-FDG uptake, ranging from the background level activity to intense hypermetabolism. Unfortunately, ¹⁸F-FDG uptake patterns cannot reliably distinguish oncocytomas from RCC, and standardized uptake values oft en overlap. In oncology patients, this can lead to diagnostic dilemmas and potentially unnecessary invasive procedures. Thus, additional imaging such as contrast-enhanced CT or magnetic resonance imaging and histopathological confirmation remain essential for the accurate characterization of ¹⁸F-FDG avid renal lesions. Our case delves into the intricacies of false positive ¹⁸F-FDG PET/CT results due to oncocytoma. Careful evaluation of the imaging findings, clinical correlation, and the judicious use of additional imaging modalities for lesion morphology and distribution are helpful for accurate diagnosis and appropriate patient management.