Imaging Differential of Adrenal Mass Malignancy

Anatomic Imaging

Adrenal mass malignancy imaging differential refers to the radiologic assessment of whether an adrenal lesion is more consistent with adrenocortical carcinoma (ACC) or with another benign or malignant process. Within ACC care, this topic sits at the interface of incidentaloma evaluation, endocrine testing, operative planning, and oncologic staging under Anatomic Imaging. The central diagnostic problem is not simply benign versus malignant, but distinction among ACC, adenoma, pheochromocytoma, metastasis, lymphoma, myelolipoma, collision tumors, and occasional nonadrenal masses that project into the adrenal bed.¹²³

Cross-sectional imaging provides the main anatomic framework for this differential, but imaging alone usually cannot establish ACC with histologic certainty. Features that raise concern for malignancy include large size, irregular or lobulated contour, heterogeneity, necrosis, hemorrhage, calcification, and evidence of local invasion; however, these findings overlap substantially with other adrenal malignancies and with some benign lesions.⁴³ Standard CT tools such as unenhanced attenuation and contrast washout remain useful for recognizing many adenomas, yet they are less reliable in indeterminate masses and do not by themselves exclude ACC.¹³

The evidence base is constrained by the rarity of ACC. Much of the literature consists of retrospective surgical series, selected oncology cohorts, and case reports describing unusual mimics or delayed diagnoses, so reported imaging patterns are often suggestive rather than validated diagnostic rules.⁵⁶⁷ In clinical practice, the value of imaging is therefore greatest when interpreted alongside hormonal status, prior malignancy history, interval growth, and the consequences of competing management strategies such as surveillance, biopsy in selected non-ACC scenarios, or adrenalectomy.⁸⁹

Diagnostic context

ACC usually enters the imaging differential when an adrenal mass appears large, heterogeneous, high in attenuation, necrotic, calcified, hemorrhagic, or invasive. These features reliably signal that a lesion is not a straightforward benign adenoma, but they do not reliably identify cortical carcinoma specifically because similar appearances occur in metastases, pheochromocytoma, lymphoma, and mixed benign lesions.⁴³

Initial evaluation commonly begins with unenhanced CT attenuation and, when appropriate, washout assessment. These measures are well established for identifying many lipid-rich adenomas, but their performance declines in lipid-poor or otherwise indeterminate lesions. In particular, attenuation above 20 HU supports further workup as an indeterminate or potentially malignant lesion, yet it should be treated as a nonspecific risk marker rather than proof of ACC.¹³

Because many adrenal masses remain unresolved after baseline characterization, the differential typically shifts from algorithmic adenoma exclusion to a more contextual comparison of major mimics.

Major imaging mimics of ACC

Adenoma, lipid-poor adenoma, and fat-containing lesions

Adenoma is the most common competing diagnosis in adrenal mass imaging. Lipid-rich adenomas are often diagnosed confidently, but lipid-poor adenomas may overlap with ACC and other malignancies on attenuation and enhancement measurements. Techniques intended to improve adenoma detection, such as CT histogram analysis, are not fully specific because negative-pixel findings may also be seen in ACC, pheochromocytoma, and metastases.¹

Macroscopic fat also does not fully resolve the differential. Although it often suggests myelolipoma, fat-containing adrenal lesions may be large, heterogeneous, hemorrhagic, or calcified and can therefore mimic features traditionally considered suspicious for ACC.⁵⁴ The reliable point is that clearly benign fat-rich morphology is helpful, but partial fat content or mixed morphology is not sufficient to exclude malignancy; clinically, atypical fat-containing lesions still require full adrenal and oncologic assessment.

Metastasis and collision tumors

Metastatic disease is a major alternative diagnosis, especially in patients with a known extra-adrenal malignancy. In that setting, pretest probability from the cancer history may be as important as morphology, because heterogeneity, mixed attenuation, or variable washout may represent metastasis, adenoma, or a collision lesion containing both benign and malignant elements.⁸¹⁰

Disease-specific studies further show that apparent benignity on one imaging parameter may be misleading. In hepatocellular carcinoma and clear cell renal cell carcinoma, adrenal metastases may overlap with adenoma on washout or chemical-shift techniques, while heterogeneity, T2 signal characteristics, or multiphase enhancement patterns may offer additional clues only within those specific populations.¹¹¹²¹³ What is reliable is the broader principle that metastases can mimic adenoma on standard adrenal protocols; what is not reliable is extrapolation of narrow disease-specific thresholds to the general ACC differential. The practical implication is that indeterminate adrenal lesions in oncology patients often need integration with systemic staging and, in selected cases, tissue confirmation or surgery.¹⁴¹⁰

Pheochromocytoma, lymphoma, and other malignant adrenal lesions

Pheochromocytoma is an especially important mimic because it may resemble ACC in size, heterogeneity, and enhancement behavior. Reported dynamic enhancement and washout patterns are variable, limiting the reliability of enhancement kinetics as a discriminator among pheochromocytoma, adenoma, and ACC.¹⁵ The clinically dependable implication is not radiologic but biochemical: pheochromocytoma should be excluded when imaging is suspicious but nonspecific.

Primary adrenal lymphoma is another comparator, particularly when masses are bilateral or associated with nodal disease. Comparative data suggest that bilateral involvement, relative homogeneity, lack of calcification, and malignant regional lymphadenopathy may favor lymphoma over ACC, but overlap remains substantial and imaging alone is not definitive.³ Thus, these features may guide prioritization of the differential, but they do not replace tissue diagnosis or broader hematologic evaluation when lymphoma is suspected.

Nonadrenal and anatomic mimics

Some lesions mimic adrenal malignancy mainly because of location rather than adrenal tumor biology. Reported examples include adrenohepatic fusion causing a benign adrenal lesion to appear contiguous with the liver and upper abdominal cystic lesions that cannot be cleanly separated from the adrenal region on initial imaging.¹⁶² These observations are based largely on case reports, so they are best understood as reminders to confirm organ of origin with multiplanar review rather than as common pathways in adrenal oncology.

Imaging modalities and discriminative features

CT remains the main anatomic imaging modality because it supports lesion detection, attenuation measurement, assessment of local invasion, and operative planning. Its strengths are availability and integration into standard adrenal workflows; its main limitation is that many suspicious CT features are shared by several benign and malignant entities.¹³

MRI may provide additional tissue characterization when CT findings are indeterminate, particularly through chemical-shift and signal-pattern assessment. However, MRI is also vulnerable to overlap, as some metastases may demonstrate signal loss usually associated with adenoma, and pheochromocytomas may show variable enhancement behavior.¹⁵¹³ Specialized approaches such as proton MR spectroscopy have shown potential to separate ACC from adenoma, pheochromocytoma, and metastasis in selected studies, but the evidence remains limited and does not establish a routine replacement for standard CT-based assessment.⁶ Overall, the most reliable interpretation is multimodal and contextual rather than based on any single imaging metric.

Limitations and common pitfalls

Across the literature, ACC may be both overcalled and underrecognized. It may be overcalled because benign lesions can reproduce alarming findings such as hemorrhage, calcification, vascular abutment, heterogeneity, or fat-containing mixed architecture.⁵⁴ It may be underrecognized because some ACCs initially present as only indeterminate nodules without dramatic early growth.⁹⁷

A recurring pitfall is overreliance on short-term stability. Limited evidence from case reports and review material suggests that some indeterminate adrenal lesions may remain stable for prolonged periods before later accelerated enlargement, so stability over one to two years does not completely exclude malignancy in a persistently suspicious lesion.⁹⁷ This is not a validated universal surveillance rule, but it has practical importance when imaging features remain atypical or when management would differ substantially if ACC were missed.

Role in management and research

The main role of imaging differential diagnosis is to guide management rather than to supply definitive histology. Compared with immediate surgery, careful imaging interpretation may reduce unnecessary adrenalectomy for benign mimics; compared with surveillance alone, suspicious but nonspecific findings may justify endocrine evaluation, multidisciplinary review, or broader oncologic staging.¹⁶³

For suspected ACC, imaging is most useful for estimating malignant potential, identifying plausible alternative diagnoses, and defining local extent and resectability. The current literature supports cautious, context-dependent interpretation: imaging may substantially narrow the differential, but pathology, biochemical testing, and clinical course remain central when decisions diverge among ACC, metastasis, pheochromocytoma, lymphoma, and benign adrenal lesions.⁸⁶⁷

Included Articles

• PMID 7059929: This report highlights adrenal myelolipoma as an imaging differential for suspected ACC, noting that nonfunctional, low-density or radiolucent adrenal masses with little or no angiographic vascularity may still be adrenocortical carcinoma. The authors state there were no satisfactory specific radiographic criteria to reliably distinguish myelolipoma from ACC.⁵

• PMID 15914530: In adrenal masses arising in patients with prior extraadrenal cancer, collision/composite tumors can mimic primary adrenocortical lesions on scintigraphy and hormone assays. The report highlights CT heterogeneity with distinct low- and high-attenuation components, and differing washout behavior, as a useful clue to coexisting benign adenoma and metastatic tissue.⁸

• PMID 16794176: This CT study found that histogram analysis based on negative-pixel counts is not specific for adrenal adenoma because negative pixels also occurred in metastases, pheochromocytomas, and adrenocortical carcinomas. For ACC specifically, both unenhanced and enhanced lesions could contain negative pixels, limiting the method’s discriminatory value.¹

• PMID 17917001: This case series highlights that adrenal myelolipoma occurring within adrenocortical adenoma can produce large heterogeneous masses with hemorrhage, calcification, enhancement, and apparent proximity to major vessels, thereby mimicking radiologic features commonly considered suspicious for adrenocortical carcinoma.⁴

• PMID 18024453: A prospective study of 60 adrenal masses found that in vivo proton MR spectroscopy could help distinguish adrenocortical carcinoma from adenoma, pheochromocytoma, and metastasis using metabolite ratios, with combined choline-creatine and 4.0-4.3 ppm/creatine measures providing the best lesion separation.⁶

• PMID 19447543: This case report highlights an imaging pitfall in which adrenohepatic fusion made a lipid-rich adrenal cortical adenoma mimic a malignant hepatic tumor or ACC. On retrospective CT review, unenhanced attenuation of 6 HU and continuity with the right adrenal gland on multiplanar reformations supported benign adrenal origin and could have avoided unnecessary surgery.¹⁶

• PMID 37908218: This case shows that a heterogeneous adrenal mass with macroscopic fat and only mild FDG avidity can still harbor metastatic malignancy within a cortisol-producing adrenal adenoma. In patients with a known extra-adrenal cancer, benign-appearing imaging features should not exclude metastatic disease or adrenal collision tumors.¹⁰

• PMID 39021177: A case report describes a non-secreting adrenal incidentaloma with indeterminate baseline CT features, including 20 HU density, that remained radiologically stable for two years before enlarging dramatically to a 14.5 cm lesion later confirmed as adrenocortical carcinoma. It highlights that indeterminate adrenal lesions may warrant ongoing imaging follow-up despite apparent early stability.⁹

• PMID 41113707: In a differential-diagnosis study of indeterminate adrenal masses, primary adrenal lymphoma more often showed bilateral involvement, homogeneous appearance, absent calcifications, and more frequent malignant regional lymph nodes than ACC and other comparators. The article reinforces that CT density above 20 HU is nonspecific and can overlap across malignant adrenal lesions including ACC.³

• PMID 41603379: This review and case series describes rare adrenal lesions with initially benign or indeterminate imaging features, including long-term size stability, that later entered an accelerated growth phase and were diagnosed as ACC. It emphasizes that prolonged dimensional stability alone is not sufficiently reassuring for radiologically indeterminate adrenal nodules.⁷

• PMID 16277009: A case report describes an adult gastric duplication cyst that was considered preoperatively in the differential of a presumed adrenal-region mass on CT because it could not be fully separated from the gastric fundus. For ACC imaging differential, it adds a rare example of a nonadrenal upper abdominal cystic lesion that can mimic adrenal pathology by location rather than by classic malignant imaging features.²

• PMID 38456897: A 2024 retrospective study in patients with hepatocellular carcinoma found that lipid-poor adrenal adenomas and adrenal metastases remain difficult to separate with standard features alone; in this setting, multiphase hepatic CT parameters outperformed chemical-shift MRI, with the best performance from combining relative enhancement ratio and washout measures.¹²

• PMID 11933633: A 2002 case series of three patients with operable non-small cell lung cancer and synchronous solitary adrenal masses found two adrenal metastases and one benign cortical adenoma after laparoscopic adrenalectomy, underscoring how indeterminate adrenal lesions can alter staging and operative planning. Its relevance to ACC is indirect and mainly reinforces the need for cautious differential diagnosis when an extra-adrenal primary malignancy is present.¹⁴

• PMID 16373781: A 2006 Radiology letter argued that pheochromocytomas may show highly variable enhancement and washout behavior on dynamic MR imaging, including frequent rapid washout that can overlap with adenoma-like patterns. The exchange highlights modality-specific and methodological limits but supports describing pheochromocytoma as an imaging chameleon in the ACC differential.¹⁵

• PMID 23478006: In a retrospective HCC cohort, unenhanced CT attenuation differentiated many adrenal adenomas from HCC metastases better than lesion size, with no metastasis reported below 17 HU. The study adds HCC-specific nuance but remains limited by follow-up-based reference standards and indirect relevance to ACC.¹¹

• PMID 27007886: A retrospective canine adrenal tumor study found that triple-phase CT morphology and washout-related measures differed among cortical adenoma, cortical adenocarcinoma, and pheochromocytoma, supporting the general concept that dynamic CT may aid preoperative differentiation. Its relevance to ACC is indirect because the data are veterinary and not directly applicable to human diagnostic thresholds.¹⁷

• PMID 28673446: In a biopsy-referenced CT study of 55 small cirrhotic liver nodules assessed with LI-RADS, one hepatic metastasis from ACC was falsely classified as LI-RADS 5. This indirectly extends the note’s HCC-specific pitfalls by showing that metastatic ACC can rarely mimic imaging criteria used for confident HCC diagnosis.¹⁸

• PMID 28742373: A retrospective MRI study of adrenal lesions in patients with clear cell renal cell carcinoma found that some adrenal metastases can show chemical-shift signal loss overlapping with adenoma, while greater T2 hyperintensity and heterogeneity were more helpful distinguishing features. This extends the note’s discussion of metastasis-specific imaging pitfalls, particularly in patients with a known extra-adrenal primary.¹³

References

Footnotes

1. CT histogram analysis in pathologically proven adrenal masses.. AJR Am J Roentgenol. 2006. PMID: 16794176. Local full text: 16794176.md ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

2. Gastric duplication cyst in an adult: case report.. In Vivo. 2005. PMID: 16277009. Local full text: 16277009.md ↩ ↩² ↩³

3. Primary adrenal lymphoma and its mimics: clinico-radiological differential diagnosis.. Front Endocrinol (Lausanne). 2025. PMID: 41113707. Local full text: 41113707.md ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹

4. Myelolipoma with adrenocortical adenoma: an unusual combination that can resemble carcinoma.. Endocr Pathol. 2007. PMID: 17917001. Local full text: 17917001.md ↩ ↩² ↩³ ↩⁴ ↩⁵

5. Myelolipoma: an unusual surgical lesion of the adrenal gland.. Cancer. 1982. PMID: 7059929. Local full text: 7059929.md ↩ ↩² ↩³ ↩⁴

6. Adrenal masses: characterization with in vivo proton MR spectroscopy—initial experience.. Radiology. 2007. PMID: 18024453. Local full text: 18024453.md ↩ ↩² ↩³ ↩⁴

7. Is adrenal adenoma to carcinoma transformation possible? - Illustrative cases and literature review.. J Clin Endocrinol Metab. 2026. PMID: 41603379. Local full text: 41603379.md ↩ ↩² ↩³ ↩⁴ ↩⁵

8. Collision/composite tumors of the adrenal gland: a pitfall of scintigraphy imaging and hormone assays in the detection of adrenal metastasis.. J Clin Endocrinol Metab. 2005. PMID: 15914530. Local full text: 15914530.md ↩ ↩² ↩³ ↩⁴

9. From Adenoma to Carcinoma: the Unexpected Evolution of an Apparently Stable Adrenal Lesion.. Endocr Metab Immune Disord Drug Targets. 2024. PMID: 39021177. Local full text: 39021177.md ↩ ↩² ↩³ ↩⁴

10. Metastatic Merkel Cell Carcinoma Within a Cortisol-producing Adrenal Adenoma.. JCEM Case Rep. 2023. PMID: 37908218. Local full text: 37908218.md ↩ ↩² ↩³

11. Differentiation of adrenal tumors in patients with hepatocellular carcinoma: adrenal adenoma versus metastasis.. Eur J Radiol. 2013. PMID: 23478006. Local full text: 23478006.md ↩ ↩²

12. Diagnostic performance of hepatic CT and chemical-shift MRI to discriminate lipid-poor adrenal adenomas from hepatocellular carcinoma metastases.. Abdom Radiol (NY). 2024. PMID: 38456897. Local full text: 38456897.md ↩ ↩²

13. Utility of MRI to Differentiate Clear Cell Renal Cell Carcinoma Adrenal Metastases From Adrenal Adenomas.. AJR Am J Roentgenol. 2017. PMID: 28742373. Local full text: 28742373.md ↩ ↩² ↩³

14. [Operable lung cancer and synchronous adrenal masses: role of laparoscopic adrenalectomy combined with pulmonary resection].. Ann Chir. 2002. PMID: 11933633. Local full text: 11933633.md ↩ ↩²

15. Enhancement characteristics of pheochromocytomas.. Radiology. 2006. PMID: 16373781. Local full text: 16373781.md ↩ ↩² ↩³

16. Cortical adenoma in adrenohepatic fusion tissue: clue to making a correct diagnosis at preoperative computed tomography examination.. Eur Urol. 2009. PMID: 19447543. Local full text: 19447543.md ↩ ↩² ↩³

17. Preoperative Differential Diagnosis of Canine Adrenal Tumors Using Triple-Phase Helical Computed Tomography.. Vet Surg. 2016. PMID: 27007886. Local full text: 27007886.md ↩

18. Diagnostic efficacy of the Liver Imaging-Reporting and Data System (LI-RADS) with CT imaging in categorising small nodules (10-20 mm) detected in the cirrhotic liver at screening ultrasound.. Clin Radiol. 2017. PMID: 28673446. Local full text: 28673446.md ↩