# The Incidental Finding: Adrenal, Renal, Liver, and Thyroid

## Introduction

Incidental findings are abnormalities discovered on imaging studies performed for **unrelated clinical indications**. With the increasing use of cross-sectional imaging, incidental findings are encountered daily in radiology practice. The ACR Incidental Findings Committee has published evidence-based white papers to guide management and reduce unnecessary follow-up.

## General Principles

Incidental findings are found on up to **40%** of abdominal CT studies. Management must balance the risk of missing a significant lesion against the harms of **unnecessary workup** (anxiety, cost, radiation, procedural risk). The appropriate ACR white paper recommendations should be applied based on lesion characteristics. Patient factors (age, comorbidities, life expectancy) should influence management decisions. The finding and recommendation must be documented clearly in the radiology report.

## Adrenal Incidentalomas

### Prevalence and Differential

Adrenal incidentalomas are found in approximately **4-5%** of abdominal CT studies. The most common diagnosis is **adrenal adenoma** (70-80% of incidental adrenal lesions). Other considerations include myelolipoma, cyst, pheochromocytoma, adrenal metastasis, and adrenocortical carcinoma.

### Characterization by Imaging

| Criterion | Value | Interpretation |
|-----------|-------|---------------|
| Unenhanced CT attenuation | <=10 HU | Diagnostic for lipid-rich adenoma |
| Absolute washout | >60% | Adenoma |
| Relative washout | >40% | Adenoma |
| Chemical shift MRI | Signal dropout on opposed-phase | Intracellular lipid (adenoma) |
| Macroscopic fat | Negative HU on CT | Myelolipoma (pathognomonic) |
| Size >4 cm | Any attenuation | Surgical referral recommended |

**Unenhanced CT attenuation** is the single most useful measurement: adenomas are lipid-rich, and a threshold of **10 HU or less** is diagnostic (sensitivity approximately 71%, specificity approximately 98%). **Contrast washout** with absolute washout greater than 60% and relative washout greater than 40% on 15-minute delayed images characterizes adenomas. **Chemical shift MRI** shows signal dropout on out-of-phase compared to in-phase images, indicating intracellular lipid (adenoma). **Myelolipoma** contains macroscopic fat (negative HU on CT) and is pathognomonic. Lesions **greater than 4 cm** have higher risk for adrenocortical carcinoma, and surgical referral is recommended regardless of imaging characteristics.

### Management Algorithm

A homogeneous lesion measuring 10 HU or less and less than 4 cm is a benign adenoma requiring no follow-up. An indeterminate lesion (10-30 HU) less than 4 cm should undergo contrast washout CT or chemical shift MRI. Lesions greater than 4 cm or showing growth warrant surgical referral. In patients with a known cancer history, full characterization is needed; PET/CT may be useful for distinguishing adenoma from metastasis.

![Axial non-contrast CT showing a left adrenal nodule measuring 5 HU consistent with a lipid-rich adenoma](images/adrenal-adenoma-ct.jpg)

## Renal Incidentalomas

### Bosniak Classification (Updated 2019)

**Bosniak I** is a simple cyst with a thin wall, homogeneous water attenuation, and no enhancement; no follow-up is needed. **Bosniak II** is a minimally complex cyst with few thin septa, fine calcification, or a homogeneous high-attenuation lesion 3 cm or smaller; no follow-up is needed. **Bosniak IIF** is minimally complex and requires follow-up; it may have multiple thin septa, minimal smooth thickening, or be a high-attenuation lesion greater than 3 cm. **Bosniak III** is indeterminate with thickened irregular septa or wall with measurable enhancement; management includes surgical or active surveillance. **Bosniak IV** is a clearly malignant cystic mass with an enhancing soft tissue component requiring surgical management.

### Solid Renal Masses

Incidental solid enhancing renal masses are malignant in approximately **80-85%** of cases. Small renal masses (less than 4 cm) are increasingly managed with **active surveillance** in elderly or comorbid patients. A **homogeneous, well-defined, avidly enhancing** small mass may represent oncocytoma or angiomyolipoma (AML). The presence of **macroscopic fat** (negative HU) is essentially diagnostic of AML. MRI with subtraction imaging is the problem-solving tool for indeterminate renal lesions.

## Hepatic Incidentalomas

### In Patients Without Known Malignancy or Chronic Liver Disease

A **simple cyst** (homogeneous, water attenuation, no enhancement) requires no follow-up at any size. A **hemangioma** with peripheral nodular enhancement and centripetal fill-in requires no follow-up if classic. **Focal nodular hyperplasia (FNH)** shows homogeneous arterial enhancement with a central scar and typically requires no follow-up. A **hepatic adenoma** is relevant in young women on oral contraceptives and requires follow-up and potential resection if greater than 5 cm. A lesion **less than 1 cm** in a patient without risk factors is generally too small to characterize, and no follow-up is recommended.

### In Patients With Known Malignancy

Any new hepatic lesion in an oncology patient should be evaluated for **metastatic disease**. Lesions too small to characterize (less than 1 cm) should be noted and followed on the next staging study. MRI with hepatobiliary contrast (gadoxetate) is the most sensitive tool for liver metastasis detection.

### In Patients With Chronic Liver Disease or Cirrhosis

The **LI-RADS** classification system should be applied for all observations. Screening with ultrasound every 6 months is standard, with CT/MRI for characterization of detected lesions. An arterially enhancing lesion with washout and capsule is classified as LI-RADS 5 (definite HCC).

![Contrast-enhanced CT showing a small hypodense hepatic lesion in a patient without liver disease, consistent with a simple cyst](images/liver-incidentaloma-ct.jpg)

## Thyroid Incidentalomas

### Prevalence and Context

Incidental thyroid nodules are found on approximately **16-18%** of CT and **1-2%** of PET/CT studies. The vast majority are **benign**, with a malignancy risk of approximately 5-10% overall. FDG-avid thyroid incidentalomas on PET carry a higher malignancy risk of approximately **30-35%**.

### ACR White Paper Recommendations

| Scenario | Size Threshold | Recommendation |
|----------|---------------|----------------|
| CT/MRI, patient <35 years | >1.5 cm | Thyroid ultrasound |
| CT/MRI, patient >=35 years | >2.5 cm | Thyroid ultrasound |
| Focal FDG-avid on PET | Any size | Thyroid ultrasound |
| Diffuse FDG uptake on PET | N/A | No further imaging (likely thyroiditis) |

On CT/MRI, a nodule **greater than 1.5 cm** in patients under 35 or **greater than 2.5 cm** in patients 35 and older warrants thyroid ultrasound. Focal FDG-avid thyroid uptake on PET warrants thyroid ultrasound regardless of size. Diffuse FDG thyroid uptake on PET is typically thyroiditis and does not require further imaging workup. **ACR TI-RADS** on ultrasound is used for risk stratification and biopsy decision.

![Incidental FDG-avid thyroid nodule on PET/CT with corresponding ultrasound showing a hypoechoic solid nodule](images/thyroid-incidentaloma-pet.jpg)

## Key Clinical Pearls

Apply **ACR Incidental Findings Committee** white papers systematically; they reduce unnecessary follow-up by up to 50%. For adrenal nodules, unenhanced CT attenuation of 10 HU or less is the single most useful measurement; it confirms a benign adenoma and eliminates the need for follow-up. In patients without cancer or cirrhosis, hepatic lesions less than 1 cm that do not meet criteria for a simple cyst or hemangioma generally do not require follow-up. Always include a specific, actionable recommendation in the report for every incidental finding; vague language like "clinical correlation recommended" is insufficient.

## References

1. Mayo-Smith WW, et al. Management of incidental adrenal masses: a white paper of the ACR Incidental Findings Committee. *J Am Coll Radiol*. 2017;14(8):1038-1044.
2. Silverman SG, et al. Bosniak classification of cystic renal masses, version 2019. *Radiology*. 2019;292(2):475-488.
3. Gore RM, et al. Incidental hepatic lesions: management recommendations. *J Am Coll Radiol*. 2017;14(11S):S429-S435.
4. Hoang JK, et al. Managing incidental thyroid nodules detected on imaging: white paper of the ACR Incidental Thyroid Findings Committee. *J Am Coll Radiol*. 2015;12(2):143-150.