Giant Adrenal Oncocytoma: A Case Report and Literature Review

ABSTRACT

Introduction

Adrenal oncocytomas are rare, nonfunctioning tumors composed of eosinophilic oncocytic cells whose nonspecific imaging often impedes preoperative diagnosis. We report a case of a large nonfunctioning adrenal oncocytoma with borderline malignant potential, incidentally discovered during cardiac symptom evaluation.

Case Presentation

A 77‐year‐old man with an incidentally discovered retroperitoneal tumor was referred to Urology for further evaluation. Contrast‐enhanced CT revealed a 93‐mm lobulated mass with heterogeneous enhancement abutting the left kidney's upper pole. Given the tumor's size, open left nephrectomy and adrenalectomy were performed. Gross and histopathology revealed a yellowish, solid adrenal tumor composed predominantly of oncocytic cells with no invasion of the kidney. Immunohistochemistry revealed a low Ki‐67 index (approximately 2%). The tumor was classified as having borderline malignant potential (Lin–Weiss–Bisceglia criteria).

Conclusion

We present a rare case of nonfunctioning adrenal oncocytoma with borderline malignant potential. Long‐term follow‐up is recommended due to the uncertain malignant potential.

Keynote Message

Adrenal oncocytoma is extremely rare and often radiologically indistinguishable from adrenocortical carcinoma (ACC). We present a typical case with borderline malignant potential per the Lin–Weiss–Bisceglia (LWB) criteria. Histopathological assessment using the LWB system and Ki‐67 index is essential for prognostic evaluation and postoperative surveillance planning.

Abbreviations

ACC

adrenocortical carcinoma

CECT

contrast‐enhanced computed tomography

CT

computed tomography

HPF

high power field

LWB

Lin–Weiss–Bisceglia

MRI

magnetic resonance imaging

1. Introduction

Oncocytomas are composed of large polygonal cells with abundant eosinophilic, mitochondria‐rich cytoplasm. Although most common in the kidneys, they also occur in the thyroid, salivary glands, pituitary glands, lungs, liver, gastrointestinal tract, and skin [1, 2]. Adrenal oncocytomas are extremely rare, first classified as a distinct entity in 1986; approximately 200 cases have been reported worldwide as of 2024. Most are hormonally non‐functional and incidentally discovered on imaging. In rare instances, hormonal activity resembling Cushing's syndrome or pheochromocytoma is present. Tumors are typically large (5–15 cm) yet frequently asymptomatic. Radiological differentiation from cortical adenomas, ACCs, or pheochromocytomas is challenging, rendering histopathological evaluation essential [3, 4]. Malignant potential is classified by the LWB criteria as benign, borderline, or malignant [5]. Given poorly defined clinical guidelines, we report an incidentally detected adrenal oncocytoma with a literature‐based overview of its features.

2. Case

A 77‐year‐old man with thyroid and left submandibular gland tumors and hip osteoarthritis presented with dyspnea and nausea. Transthoracic echocardiography identified severe aortic stenosis. Non‐contrast chest and abdominal CT revealed a mass suspicious for a left renal or adrenal tumor with suspected hepatic and mediastinal metastases, prompting referral.

Adrenal hormone evaluation showed: serum cortisol 21.8 μg/dL, plasma renin activity 0.6 ng/mL/h, aldosterone 42.5 pg/mL, adrenaline 0.04 ng/mL, noradrenaline 0.41 ng/mL, dopamine 0.05 ng/mL, urinary metanephrine 0.13 mg/L, and normetanephrine 0.32 mg/L, with no evidence of hormonal hypersecretion.

Contrast‐enhanced abdominal CT revealed a 93‐mm lobulated mass with heterogeneous enhancement and well‐defined margins at the upper pole of the left kidney (Figure 1A). Early‐phase dynamic CT showed enhancement less intense than the normal renal cortex with delayed‐phase washout, raising suspicion for a renal primary; renal cell carcinoma could not be excluded (Figure 1B). The hepatic lesion was considered a hemangioma and the mediastinal lesion a thymic cyst or thymoma. No distant metastasis was identified. Open left nephrectomy and adrenalectomy were performed.

FIGURE 1.

(A) Sagittal section image obtained during the arterial phase of contrast‐enhanced CT. The arrow indicates the adrenal tumor. (B) Dynamic contrast‐enhanced CT in axial section.

Operative time was 2 h 43 min with 520 mL blood loss. The tumor and kidney were resected en bloc; the kidney measured 120 × 50 × 45 mm and the yellowish solid tumor 110 × 60 × 50 mm (Figure 2A). No continuity between tumor and renal parenchyma was identified; adrenal tissue adhered to the dorsal surface (Figure 2B). Histologically, the tumor consisted predominantly (> 90%) of eosinophilic oncocytic cells with irregular nuclei and prominent nucleoli in trabecular, solid, nested, and glandular patterns. Mitoses were infrequent (< 1/50 HPF) (Figure 2B,C). No capsular, vascular, periadrenal fat, or renal parenchymal invasion was found. Ki‐67 index was approximately 2% (Figure 2D). Meeting only one LWB minor criterion (size > 10 cm), the tumor was classified as borderline malignant potential. The patient was recurrence‐free at 3 years postoperatively.

FIGURE 2.

Pathological findings of the adrenal oncocytoma. (A) Gross photograph of the left kidney and the left adrenal tumor. (B) H&E staining showing the boundary between the oncocytoma and the compressed adrenal gland. Scale bar: 200 μm. (C) H&E staining of the oncocytoma. Scale bar: 50 μm. (D) Immunohistochemical staining for Ki‐67. Scale bar: 50 μm.

3. Discussion

Adrenal oncocytoma is extremely rare; approximately 200 cases have been reported worldwide [1]. The mean age at diagnosis is 44 years, with female (female‐to‐male ≈2.5:1) and left‐sided (left‐to‐right ≈3.5:1) predominance [1, 2]. Most cases are nonfunctioning and incidentally detected.

Radiological distinction from ACC is difficult [3]. Imaging criteria for malignancy—size ≥ 5 cm, indistinct margins, irregular shape, and heterogeneous enhancement—overlap with ACC, limiting preoperative differentiation [1]. MRI shows T1 isointensity and mild T2 hyperintensity, though nonspecific [1]. Approximately 20% of oncocytic tumors are hormonally functional, yet no imaging feature predicts endocrine activity; biochemical evaluation is therefore essential [4]. Histopathological examination remains indispensable for definitive diagnosis.

The Weiss system overestimates malignancy in oncocytic variants; the LWB criteria are recommended [5]. In a series of 10 oncocytic adrenocortical tumors, 4 were borderline, 4 carcinoma, and 2 benign [5]. Major criteria—high mitotic rate (> 5/50 HPF), atypical mitoses, venous invasion—indicate malignancy; minor criteria—necrosis, sinusoidal/capsular invasion, and size > 10 cm/> 200 g—indicate borderline malignancy. No association between hormonal activity and malignant potential has been established; prognosis relies primarily on LWB classification.

The Ki‐67 index is an important prognostic biomarker [6]. The 2025 WHO Classification stratifies risk as < 10% (low), 10%–20% (intermediate), and > 20% (high). Here, Ki‐67 (≈2%) indicated low risk; tumor size was the sole LWB minor criterion. Recurrence or metastasis occurs in 0%, 8%, and 75% of benign, borderline, and malignant tumors, with mortality of 0%, 3%, and 40% [7]. Five‐year survival is 100%, 88%, and 47% [7]. Our patient was recurrence‐free at 3 years. No standardized surveillance protocol exists; long‐term imaging follow‐up is appropriate, with endocrine evaluation for hormonally active cases and interval guided by LWB classification.

A review of nine Japanese cases confirmed female and left‐sided predominance (Table 1). Some showed elevated DHEA‐S (Cases 1, 2) [1, 8]. In Case 1, ACC was the leading differential diagnosis based on CT/MRI and there was no obvious renal invasion. Open nephroadrenalectomy was chosen for the tumor size of over 10 cm. Japanese guidelines do not specify clear criteria regarding tumor size for determining whether to perform open adrenalectomy or laparoscopic adrenalectomy for benign adrenal tumors, but open adrenalectomy is recommended when ACC is suspected. Case 3 showed atypical low T2 signal [8]. Case 4 involved primary aldosteronism with an adjacent oncocytic adenoma [9]. Cases 5–6 were IL‐6–producing tumors resolving after resection [10, 11]. Cases 7–8 showed FDG‐PET uptake attributed to mitochondrial abundance [12, 13]. None showed recurrence where follow‐up was reported; the present case remains recurrence‐free at 3 years. Nephroadrenalectomy was favored when malignancy or renal invasion was suspected; in our case, early‐phase enhancement with delayed‐phase washout on CT precluded exclusion of a renal primary.

TABLE 1.

Summary of the literature review.

Case	Age/Sex	Chief complaint	Side	Size (cm)	Surgery	Functionality	Ki‐67	LWB classification	Follow up period	Outcome
1	20s/M	None	Left	13 × 11 × 8	Open	DHEA‐S elevation	0.4%	Benign	2 years and 9 months	No recurrence
2	45/F	None	Left	7.5 × 6.5 × 6	No data	DHEA‐S elevation	1%	Borderline	2 years	No recurrence
3	40s/F	None	Left	6 × 5 × 5	Laparoscopic	Nonfunctioning	0.4%	Benign	2 years and 5 months	No recurrence
4	48/F	Hypertension	Left	5.4 × 3.7	No data	PAC 20.8 ng/dL, PRA 0.8 ng/mL/h	No data	Benign	No data	No data
5	38/F	Fever	Left	4.5 × 3.5 × 2.5	Laparoscopic	Nonfunctioning (IL‐6 elevation)	No data	Borderline	8 years and 9 months	No recurrence
6	6/F	Fever, Weight loss	Left	4.5 × 4.5 × 2.5	No data	Nonfunctioning (IL‐6 elevation)	No data	Benign	7 years	No recurrence
7	36/F	Back pain	Left	6 × 3.8	Open	Nonfunctioning	No data	Benign	2 years	No recurrence
8	37/F	None	Left	3.7 × 3 × 3.8	Laparoscopic	PAC 12.7 ng/dL, PRA 2.7 ng/mL/h	< 1.5%	Benign	No data	No data
9	45/M	Abdominal pain	Left	7.5 × 6 × 4.5	No data	Nonfunctioning	No data	Malignancy	No data	No data
Current case	77/M	Dyspnea, Nausea	Left	11 × 6 × 5	Open	Nonfunctioning	2%	Borderline	3 years	No recurrence

4. Conclusion

We report a rare large nonfunctioning adrenal oncocytoma with borderline malignant potential, incidentally discovered and resected without complications. Concomitant nephrectomy was performed as imaging could not exclude renal cell carcinoma. Histopathological findings confirmed the diagnosis; the borderline classification necessitates close surveillance. This case highlights the importance of including adrenal oncocytoma in the differential diagnosis of adrenal or upper renal pole masses.

Funding

The authors have nothing to report.

Consent

Written informed consent for publication was obtained from the patient.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.