Are hypermetabolic nodules on PET (Positron Emission Tomography) scans typically malignant?

From the Guidelines

Hypermetabolic nodules on PET scans are often suspicious for malignancy, but they are not always malignant, and further investigation is usually required to determine the nature of the nodule. The likelihood of malignancy depends on several factors, including:

• SUV (Standardized Uptake Value): Higher SUVs are more concerning for malignancy, as noted in the study by 1
• Size of the nodule: Larger nodules are more likely to be malignant, as suggested by the guidelines in 1 and 1
• Location: Some areas are more prone to malignancies than others
• Patient history: Risk factors like smoking or previous cancer increase suspicion, as mentioned in 1

To determine if a hypermetabolic nodule is malignant, further investigation is usually required, which may include:

1. Correlation with other imaging modalities (CT, MRI)
2. Biopsy of the nodule
3. Follow-up imaging to assess for changes over time

It's essential to note that PET scans can have false positives due to inflammation, infection, or other benign processes, as discussed in 1 and 1. Conversely, some slow-growing cancers may not appear hypermetabolic on PET. In practice, hypermetabolic nodules should always be taken seriously and warrant further investigation, but they should not be automatically assumed to be malignant without additional evidence, as recommended by the guidelines in 1 and 1.

The most recent and highest quality study, 1, suggests that surgical biopsy is recommended in individuals with a solid, indeterminate nodule that measures > 8 mm in diameter, and the clinical probability of malignancy is high (> 60%), or the nodule is intensely hypermetabolic as assessed by PET. Therefore, in cases where the clinical probability of malignancy is high, or the nodule is intensely hypermetabolic, surgical biopsy is recommended, as stated in 1.

From the Research

Hypermetabolic Nodules on PET Scans

• Hypermetabolic nodules on PET scans can be either malignant or benign, and their diagnosis requires careful evaluation and follow-up 2, 3, 4, 5, 6
• A study published in 2004 found that 71% of unexpected foci of hypermetabolism on whole-body FDG PET scans were either malignant or premalignant tumors 2
• Another study published in 2016 found that both malignant and benign fat-containing masses and masslike lesions can show hypermetabolic activity on PET/CT, highlighting the need for careful interpretation of imaging findings 3

Malignancy Risk

• A 2022 study found that more than half of focal hypermetabolic thyroid incidentalomas on F-18 FDG PET-CT were revealed to be malignant lesions, and SUVmax was the best parameter for discriminating between malignant and benign disease 4
• A 2017 study found that the lgSUVmax of malignant thyroid nodules was significantly higher than that of benign nodules, and that (18)F-FDG PET-CT imaging had a certain clinical value for the identification of benign and malignant nodules 5
• However, a 2016 study found that SUVmax values did not add relevant information to cytological categories, and that the prevalence of malignancy in thyroid nodules with a SUVmax <2.5 was similar to the general population 6

Diagnostic Evaluation

• The diagnosis of hypermetabolic nodules on PET scans requires a combination of imaging, clinical evaluation, and histopathological confirmation 2, 3, 4, 5, 6
• Ultrasonography and fine-needle aspiration cytology are recommended for thyroid nodules with high FDG uptake (hypermetabolism) that are 1 cm or greater in size 4
• The use of SUVmax and other PET parameters can aid in the diagnosis and discrimination between malignant and benign lesions, but should be interpreted in the context of clinical and cytological findings 4, 5, 6