An oncocytic neoplasm is a tumor made up predominantly of oncocytes, cells packed with an unusually large number of mitochondria that give them a distinctive swollen, granular appearance under a microscope. These tumors can arise in the kidney, thyroid, salivary glands, adrenal glands, and other organs. Most are benign or slow-growing, but the term itself does not guarantee a harmless outcome, and telling a benign oncocytic tumor from a malignant one remains one of the trickier problems in surgical pathology.
What Makes a Cell Oncocytic
The word “oncocyte” comes from the Greek onkos, meaning mass or bulk, and it describes an epithelial cell whose cytoplasm is crammed with mitochondria. Under standard staining, that mitochondrial overload makes the cell’s interior appear bright pink and finely granular, a look pathologists call eosinophilic cytoplasm.1PubMed Central. Oncocytic Salivary Gland Tumours: Cytological Features and Diagnostic Pitfalls The cells themselves look bloated compared to their normal counterparts.2PubMed. Oncocytic tumours When a tumor is composed of more than roughly 90% oncocytic cells, it earns the label “oncocytic neoplasm.”3PubMed Central. Fine Needle Aspiration Oncocytic Adrenal Tumors: A Tri-Focal Review with Integrated Cytopathological, Pathological, and Molecular Perspectives
A useful way to think about it: mitochondria are the structures inside cells that generate energy. A normal cell has hundreds of them. An oncocyte has so many that they physically distort the cell, puffing it up and changing the way it looks under a microscope. That visual signature is what first tips off a pathologist, but the real question is always what sits behind it.
Why Mitochondria Pile Up
Researchers have spent decades trying to understand why oncocytes accumulate mitochondria in the first place. The short answer is that the mitochondria inside these cells tend to be defective, and the cell compensates by making more of them. Two overlapping problems drive this.
The first is mutations in mitochondrial DNA. Mitochondria carry their own small genome, separate from the DNA in the cell’s nucleus. In many oncocytic tumors, genes encoding parts of the energy-production machinery, especially complex I of the electron transport chain, are disrupted by frameshift mutations that truncate the resulting proteins and disable them. A study of renal oncocytomas found these mutations in the majority of tumors analyzed, with no detectable normal mitochondrial DNA remaining in the affected tissue.4Clinical Cancer Research. Loss of Complex I due to Mitochondrial DNA Mutations in Renal Oncocytoma Similar mitochondrial DNA deletions have been documented in oncocytic tumors of the adrenal glands and thyroid.3PubMed Central. Fine Needle Aspiration Oncocytic Adrenal Tumors: A Tri-Focal Review with Integrated Cytopathological, Pathological, and Molecular Perspectives
The second problem involves the cell’s cleanup system. Cells normally remove damaged mitochondria through a recycling process called mitophagy. In oncocytic thyroid tumors, researchers found that a protein called MIEAP, which plays a key role in a specialized form of this cleanup, was absent. Without MIEAP, the cell cannot efficiently clear out its broken mitochondria. The broken mitochondria trigger the cell to produce even more, creating a vicious cycle of accumulation.5PubMed Central. Causative role for defective expression of mitochondria-eating protein in accumulation of mitochondria in thyroid oncocytic cell tumors The overall picture across organs is consistent: damaged mitochondria that cannot do their job properly and cannot be removed lead to a compensatory buildup that defines the oncocytic cell.6PubMed. Learning from oncocytic tumors: Why choose inefficient mitochondria?
How Oncocytic Cells Keep Themselves Alive
With so many broken mitochondria, oncocytic cells face an energy crisis. They cannot rely on the normal oxygen-dependent energy pathway the way healthy cells can. Research on oncocytic thyroid cancer cells shows that these cells ramp up glucose uptake and reroute much of that sugar into side pathways, including fat production and the synthesis of the amino acid serine. They also become heavily dependent on glutamine, using it primarily to produce glutathione, an antioxidant that counteracts the oxidative stress caused by all those malfunctioning mitochondria. On top of that, they boost amino acid transport and ramp up autophagy to scavenge building blocks from within.7Scientific Reports. Characterization of metabolic reprogramming by metabolomics in the oncocytic thyroid cancer cell line XTC.UC1
This metabolic rewiring matters for more than academic reasons. It suggests that oncocytic tumors may respond differently to treatments that target energy metabolism compared to conventional cancers. It also helps explain why many oncocytic tumors grow slowly: they are spending a lot of their resources just keeping themselves alive rather than dividing aggressively.
Where Oncocytic Neoplasms Appear
Oncocytic tumors can show up in several organs, and each site brings its own diagnostic wrinkles.
In the kidney, the classic oncocytic tumor is the renal oncocytoma, a benign mass that accounts for a meaningful fraction of kidney tumors found incidentally on imaging. The diagnostic headache is that it looks a lot like chromophobe renal cell carcinoma, a low-grade malignancy, on both scans and biopsy slides. Pathologists distinguish the two partly by nuclear features: oncocytomas tend to have round, uniform nuclei, while chromophobe carcinomas show wrinkled, raisin-like nuclei and more frequent binucleation.8American Journal of Clinical Pathology. Discriminant Nuclear Features of Renal Oncocytoma and Chromophobe Renal Cell Carcinoma A newer category called “low-grade oncocytic tumor” has been introduced in recent kidney cancer classifications for lesions that do not fit neatly into either box. Genomic work has linked these tumors to mutations in the mTOR signaling pathway, and they consistently lack the large-scale chromosomal losses seen in chromophobe carcinoma.9Modern Pathology. Germline and sporadic mTOR pathway mutations in low-grade oncocytic tumor of the kidney
In the thyroid, oncocytic cells have historically been called Hürthle cells, and the tumors they form were until recently lumped under the umbrella of follicular thyroid neoplasms. The WHO now recognizes them as a distinct category. Oncocytic thyroid carcinomas tend to be less responsive to radioiodine therapy than conventional thyroid cancers, which is clinically important because radioiodine is a mainstay of treatment for most thyroid malignancies. A study of Hürthle cell carcinoma found that about half of cases were multifocal at diagnosis, though lymph node involvement was uncommon.10PubMed Central. Epidemiology and prognostic factors of Hürthle-oncocytic cell carcinoma of the thyroid
In the salivary glands, the most common oncocytic tumor is Warthin tumor, a benign growth of the parotid gland. Oncocytomas also occur in the salivary glands but are far rarer, and they can be confused with Warthin tumors, pleomorphic adenomas, and other salivary neoplasms because of overlapping appearances.11PubMed Central. Submandibular Gland Oncocytoma: A Rare Tumor Case Series-Its Diagnosis and Treatment Based on Clinicopathology A spectrum of oncocytic salivary lesions exists, ranging from focal oncocytic change (essentially benign age-related accumulation of oncocytes) through hyperplasia to true benign and malignant neoplasms.12PubMed. Non-Warthin’s tumor oncocytic lesions
In the adrenal glands, oncocytic tumors are uncommon, and assessing whether they are benign or malignant requires a different scoring system than the one used for ordinary adrenal cortical tumors. The standard Weiss criteria, which pathologists normally use to diagnose adrenal cortical carcinoma, are not reliable for oncocytic variants. Instead, a separate set of major and minor criteria has been proposed, where the presence of even a single major criterion points toward malignancy and the absence of all criteria suggests a benign tumor.13PubMed. Adrenocortical oncocytic tumors: report of 10 cases and review of the literature
Why Diagnosis Is Difficult
The central challenge across all organs is that oncocytic cells look similar whether they belong to a benign or a malignant tumor. On fine-needle aspiration, the samples tend to be highly cellular, with large uniform cells full of granular pink cytoplasm, scattered bare nuclei, and cytoplasmic debris in the background.14Modern Pathology. Selected Problems in Fine Needle Aspiration of Head and Neck Masses – Section: Large-Cell Epithelial Neoplasms of Low Nuclear Grade A needle biopsy of a salivary oncocytoma can look almost identical to a Warthin tumor, and one large series found that fine-needle aspiration correctly identified Warthin tumors only about three-quarters of the time.15PubMed. Diagnostic accuracy and pitfalls in fine-needle aspiration interpretation of Warthin tumor
Immunohistochemistry, in which pathologists stain tissue for specific proteins, helps in some cases but is not a silver bullet. A study evaluating immunohistochemical markers for oncocytic renal tumors found substantial overlap between oncocytoma and chromophobe carcinoma staining patterns, concluding that the final diagnosis still had to rely heavily on traditional microscopic features and, where available, molecular testing.16PubMed. Immunohistochemistry for the differential diagnosis of renal tumors with oncocytic features Newer markers are emerging, though. In salivary gland tumors, a protein called BSND has been validated as a way to reliably separate benign from malignant oncocytic lesions, with all cases in a recent series being correctly reclassified using this marker.17PubMed. BSND: An emerging immunohistochemical marker that reliably distinguishes benign from malignant oncocytic salivary gland tumors
Molecular testing adds another layer. In the thyroid, genomic analysis of fine-needle aspirates classified as oncocytic (Hürthle cell) can identify cases that turn out to be non-neoplastic conditions like autonomously functioning nodules or oncocytic metaplasia rather than true tumors, potentially sparing patients from unnecessary surgery. Among confirmed oncocytic thyroid neoplasms, about half show genome-wide chromosomal copy number alterations, a pattern that can help predict which tumors are more likely to behave aggressively.18PubMed. Impact of molecular testing on detecting mimics of oncocytic neoplasms in thyroid fine-needle aspirates diagnosed as follicular neoplasm of Hürthle cell (oncocytic) type
The Role of Imaging
Standard imaging with CT or MRI can detect an oncocytic tumor but usually cannot tell you whether it is benign. Renal oncocytomas, for example, show avid arterial enhancement on CT and sometimes have a central scar, which sounds like it should be a giveaway.19PubMed Central. Imaging spectrum of renal oncocytomas: a pictorial review with pathologic correlation The trouble is that chromophobe renal cell carcinoma can have the same features. An MRI study found no significant difference between oncocytoma and chromophobe carcinoma for findings like central scar, hemorrhage, cysts, or segmental enhancement inversion.20PubMed. MRI features of renal oncocytoma and chromophobe renal cell carcinoma Enhancement patterns, heterogeneity, and calcification can help stratify risk, but no single imaging sign is definitive.21PubMed. Renal Oncocytic Neoplasms: Review of Classification Updates, Imaging, and Management
One specialized scan has changed the game for kidney oncocytic tumors. A nuclear medicine test using the radiotracer Tc-99m sestamibi, originally developed for heart imaging, takes advantage of the fact that oncocytomas are loaded with mitochondria and tend to retain the tracer more than most malignant tumors.22PubMed Central. 99mTc-sestamibi SPECT/CT for the characterization of renal masses: a pictorial guide A systematic review and meta-analysis found that this scan had a pooled sensitivity of about 92% and specificity of about 88% for distinguishing renal oncocytoma from other kidney tumors. When the question was narrowed to separating the entire benign oncocytic spectrum from malignant lesions, specificity reached roughly 95%.23PubMed. Diagnostic accuracy of 99mTc-sestamibi SPECT/CT for detecting renal oncocytomas and other benign renal lesions: a systematic review and meta-analysis Those are strong enough numbers that some centers now use a positive sestamibi scan as a reason to offer active surveillance rather than surgery.
Management of Renal Oncocytic Tumors
Because renal oncocytomas are benign and the newer low-grade oncocytic tumors behave indolently, there has been a growing push to avoid surgery when the diagnosis is reasonably secure. A comparative study of patients managed with surgery, thermal ablation, or active surveillance found that among those on surveillance, the median tumor growth was just over a millimeter per year. No patient in the surveillance group developed metastatic disease. For tumors four centimeters or smaller, the five-year cancer-specific survival was 100% regardless of management approach.24PubMed. Comparative Analysis of Surgery, Thermal Ablation, and Active Surveillance for Renal Oncocytic Neoplasms
A separate study of biopsy-confirmed renal oncocytomas under surveillance reported that about a quarter of patients eventually crossed over to active treatment, most commonly because of tumor growth, with a median time to crossover of nearly four years. No patient in that series had metastatic progression or died of the disease.25PubMed. Active Surveillance for Biopsy Proven Renal Oncocytomas: Outcomes and Feasibility When compared with chromophobe carcinoma patients on surveillance, oncocytoma patients were far less likely to need intervention, at about 13% versus 35%.26PubMed. Active Surveillance of Biopsy-Confirmed Oncocytic Renal Tumors: Growth Dynamics and Impact on Renal Function
The practical takeaway is that if you have a small kidney mass and a biopsy or sestamibi scan points strongly to an oncocytoma, your urologist may recommend periodic imaging instead of immediate surgery. Removing part or all of a kidney is not a trivial procedure, and sparing someone that recovery and the permanent loss of kidney tissue is a real benefit when the tumor is almost certainly benign.
When Oncocytic Means Malignant
Not all oncocytic neoplasms are harmless. Oncocytic carcinomas exist in the thyroid, salivary glands, adrenal cortex, and kidney, and distinguishing them from their benign counterparts is the whole reason so much diagnostic effort goes into these tumors. In the salivary glands, the spectrum runs from focal oncocytic change, which is not a tumor at all, through benign oncocytomas to frank oncocytic carcinomas.12PubMed. Non-Warthin’s tumor oncocytic lesions The Milan reporting system for salivary gland cytology now includes a specific category for “oncocytic salivary gland neoplasm of uncertain malignant potential,” acknowledging that a needle biopsy alone often cannot resolve the question.27PubMed Central. Low-grade oncocytoid neoplasm in the Milan system: Tips for the diagnosis of Warthin tumor and other differential diagnostic considerations
For adrenal oncocytic tumors, the three-tier system described earlier (benign, borderline, malignant) based on major and minor criteria reflects the same uncertainty.13PubMed. Adrenocortical oncocytic tumors: report of 10 cases and review of the literature In the thyroid, papillary oncocytic carcinoma can be particularly deceptive on fine-needle aspiration because the cells may lack the classic nuclear features, like grooves and pseudoinclusions, that normally signal papillary carcinoma. In one case series, none of the tumor cells showed nuclear grooves or intranuclear cytoplasmic inclusions, making the cytologic diagnosis especially challenging.28Diagnostic Cytopathology. Cytopathology of papillary oncocytic carcinoma of the thyroid in fine-needle aspiration biopsy
Oncocytic Change Without a Tumor
It is worth knowing that oncocytic transformation does not always mean cancer, or even a tumor. Scattered oncocytes can appear in aging salivary glands, thyroid tissue, and other organs as a benign, age-related phenomenon. In the thyroid, nodules flagged as oncocytic on biopsy sometimes turn out to be autonomously functioning thyroid nodules or areas of oncocytic metaplasia rather than neoplasms.18PubMed. Impact of molecular testing on detecting mimics of oncocytic neoplasms in thyroid fine-needle aspirates diagnosed as follicular neoplasm of Hürthle cell (oncocytic) type The presence of oncocytic cells is a description of what the cells look like, not a diagnosis of what they are doing. Whether those cells form a benign nodule, a borderline mass, or a cancer depends on additional pathologic and sometimes molecular evaluation.
If you have been told that a biopsy shows oncocytic features, the single most important next step is understanding whether the pathologist is describing a true neoplasm, and if so, whether additional testing can clarify its behavior. The name sounds alarming, but in many cases the prognosis is excellent, and the field’s diagnostic tools are getting sharper with each passing year.