# FDG PET/CT in Lung Cancer

## Indications

### Non-Small Cell Lung Cancer (NSCLC)

FDG PET/CT plays a central role in the management of non-small cell lung cancer across multiple clinical scenarios. Initial staging of known or suspected NSCLC is the most common indication. PET/CT is essential for mediastinal lymph node evaluation when surgical planning is being considered, for detection of distant metastases in the adrenals, bone, liver, contralateral lung, and brain, and for restaging after neoadjuvant therapy. It is also used for treatment response assessment, surveillance for recurrence, and radiation therapy planning through target volume delineation.

### Small Cell Lung Cancer (SCLC)

In small cell lung cancer, FDG PET/CT is used for staging, particularly to distinguish limited from extensive disease. PET/CT upstages 10 to 20% of patients initially thought to have limited disease based on conventional imaging. Although less well studied than in NSCLC, its use in SCLC is increasing.

## Staging with FDG PET/CT

### Primary Tumor (T Stage)

FDG PET characterizes the metabolic activity of the primary tumor, and the SUVmax correlates with tumor aggressiveness and prognosis. PET may identify additional pulmonary nodules not seen on CT or help characterize indeterminate nodules detected on the CT component. However, PET cannot differentiate T1 through T4 categories based on size and invasion as accurately as CT, so the anatomic CT component provides the detail necessary for T staging. Ground-glass nodules, which are often lepidic-predominant adenocarcinomas, may have low FDG avidity and carry a risk of false-negative results.

### Mediastinal Lymph Nodes (N Stage)

Mediastinal lymph node evaluation is the most important role of FDG PET/CT in lung cancer staging. CT criteria for positive nodes, defined as a short axis exceeding 1 cm, have limited sensitivity (approximately 60%) and specificity (approximately 75%). FDG PET substantially improves accuracy, achieving a sensitivity of 80 to 90%, specificity of 85 to 95%, and a negative predictive value of approximately 95% for mediastinal nodes. The clinical significance of accurate nodal staging is substantial: N2 disease (ipsilateral mediastinal nodes) or N3 disease (contralateral or supraclavicular nodes) generally precludes primary surgical resection. False-positive results occur with granulomatous disease (sarcoidosis, histoplasmosis, tuberculosis), reactive lymphadenopathy, silicosis, and anthracosis. False-negative results can occur with microscopic metastases, low-grade tumors, and mucinous adenocarcinoma. Current guidelines recommend that PET-positive mediastinal nodes should be confirmed by tissue sampling through EBUS-TBNA or mediastinoscopy before denying a patient surgery.

### Distant Metastases (M Stage)

FDG PET/CT detects unsuspected distant metastases in 10 to 20% of patients initially thought to have localized disease. Common metastatic sites include the adrenal glands, where FDG PET differentiates benign adenomas (low FDG uptake) from metastases (high FDG uptake), with an adrenal lesion SUV exceeding liver uptake suggesting metastasis. Bone metastases are detected with higher sensitivity than bone scintigraphy, particularly for lytic and mixed lesions, though purely sclerotic lesions may be less FDG-avid. Hepatic metastases missed on CT can be identified, and synchronous tumors or metastatic deposits in the contralateral lung are detectable. A critical limitation is that FDG PET has poor sensitivity for brain metastases because high normal cortical FDG uptake masks lesions. Brain MRI, not PET, is required for brain staging.

| Staging Role | PET/CT Performance | Key Limitations |
|---|---|---|
| Mediastinal nodes (N stage) | Sensitivity 80–90%, Specificity 85–95%, NPV ~95% | False positives: granulomatous disease, reactive nodes |
| Distant metastases (M stage) | Detects unsuspected disease in 10–20% | Poor for brain metastases (use MRI) |
| Adrenal lesions | SUV > liver suggests metastasis | Benign adenomas may show mild uptake |
| Bone metastases | Superior to bone scan for lytic lesions | Purely sclerotic lesions may be less FDG-avid |
| Solitary pulmonary nodule | Sensitivity ~95%, Specificity ~80% | Not reliable for nodules <8 mm or ground-glass |

### Impact on Management

FDG PET/CT changes management in 25 to 40% of NSCLC patients, most commonly by detecting unsuspected metastases that upstage the disease. This prevents futile thoracotomy in patients with unresectable disease and is cost-effective when accounting for the avoided unnecessary surgeries.

## Solitary Pulmonary Nodule (SPN) Evaluation

### Role of FDG PET

FDG PET helps distinguish malignant from benign indeterminate pulmonary nodules in the 8 to 30 mm size range. An SUVmax above 2.5 suggests malignancy with a sensitivity of approximately 95%, while an SUVmax below 2.5 suggests a benign etiology, though this is not definitive. The overall diagnostic profile reflects high sensitivity (approximately 95%) but limited specificity (approximately 80%). False-positive results arise from granulomas, active infections, and inflammatory conditions. False-negative results can occur with carcinoid tumors, lepidic-predominant adenocarcinoma (formerly bronchioloalveolar carcinoma pattern), and small nodules below 8 mm where the partial volume effect causes underestimation of SUV.

### Limitations

FDG PET is not recommended for nodules smaller than 8 mm, which fall below PET spatial resolution and are subject to substantial partial volume effects. It is also not recommended for pure ground-glass nodules due to their low metabolic activity. In regions endemic for granulomatous infections such as histoplasmosis (Ohio and Mississippi River valleys) and coccidioidomycosis, the false-positive rate is high. Importantly, PET does not replace biopsy when clinical suspicion for malignancy is high.

## Treatment Response Assessment

### During/After Neoadjuvant Therapy

FDG PET/CT assesses response to neoadjuvant chemotherapy or chemoradiation before surgery. A metabolic response, reflected by a decrease in SUV, predicts both pathologic response and survival. Imaging is typically performed 4 to 6 weeks after completing neoadjuvant therapy. A key challenge is that post-radiation inflammation can cause false-positive FDG uptake.

### RECIST vs. PERCIST

RECIST 1.1 provides anatomic response criteria based on tumor size measurement, while PERCIST provides metabolic response criteria based on SULpeak measurement. PERCIST classifies responses as complete metabolic response (CMR, resolution of all FDG-avid lesions), partial metabolic response (PMR, 30% or greater decrease in SULpeak), stable metabolic disease (SMD, no significant change), or progressive metabolic disease (PMD, 30% or greater increase in SULpeak or new FDG-avid lesions). Metabolic response often precedes anatomic response, enabling earlier assessment of treatment efficacy.

### Immunotherapy Response Assessment

Immune checkpoint inhibitors such as pembrolizumab, nivolumab, and atezolizumab can cause pseudoprogression, in which an initial increase in tumor size or FDG uptake occurs due to immune cell infiltration, followed by subsequent response if treatment is effective. Modified response criteria (iRECIST and imPERCIST) require confirmation of apparent progression on follow-up imaging before concluding that treatment has failed. The optimal timing of post-immunotherapy PET remains controversial, but imaging is typically performed 8 to 12 weeks after initiation.

## Radiation Therapy Planning

### Target Volume Delineation

FDG PET/CT is used for radiation treatment planning to define the gross tumor volume (GTV) and identify involved lymph node stations. SUV threshold or gradient-based methods are applied for tumor boundary delineation. Incorporating PET data reduces interobserver variability in target contouring and allows dose escalation to metabolically active tumor while sparing uninvolved tissue. FDG PET/CT changes radiation treatment volumes in 30 to 60% of cases.

<image>A staging FDG PET/CT in a patient with NSCLC showing the primary right upper lobe mass with high FDG avidity, ipsilateral mediastinal lymph node uptake (station 4R and 7, indicating N2 disease), and an unsuspected FDG-avid left adrenal metastasis (M1b disease). Include the CT, PET, and fused images for each finding. Show the maximum intensity projection (MIP) image with arrows pointing to all abnormal foci. Annotate with the clinical significance: upstaged from potentially resectable to stage IVA.</image>

<image>A comparison panel showing FDG PET/CT findings in three scenarios of mediastinal lymph node evaluation. Panel A: true-positive mediastinal node (confirmed N2 disease on EBUS biopsy) with high FDG uptake. Panel B: false-positive mediastinal node (granulomatous disease/sarcoidosis) with moderate FDG uptake that proved benign on biopsy. Panel C: false-negative mediastinal node (micrometastatic disease in a normal-sized, non-FDG-avid node confirmed at mediastinoscopy). Emphasize the need for tissue confirmation of PET-positive mediastinal nodes.</image>

<image>A series of FDG PET/CT maximum intensity projection images showing treatment response assessment in NSCLC. Baseline scan with extensive primary tumor and mediastinal disease, interim scan after 2 cycles of chemotherapy showing partial metabolic response (decreased SUV), and end-of-treatment scan showing complete metabolic response. Include SUV values at each time point and label with PERCIST response categories.</image>

## Clinical Pearls

FDG PET/CT changes management in 25 to 40% of NSCLC patients, primarily by detecting unsuspected distant metastases. It should be performed in all patients being considered for curative-intent therapy.

PET-positive mediastinal lymph nodes must be confirmed by tissue sampling (EBUS or mediastinoscopy) before denying surgery. False-positive rates of 15 to 20% from granulomatous disease and reactive nodes are too high to rely on PET alone.

Brain MRI, not FDG PET, is required for brain metastasis evaluation. High normal cortical FDG uptake masks brain metastases and makes PET unreliable for this purpose.

Ground-glass predominant nodules (lepidic-pattern adenocarcinoma) and carcinoid tumors are known causes of false-negative FDG PET. Low metabolic activity does not exclude malignancy in these histologies.

An SUVmax above 2.5 in a solitary pulmonary nodule suggests malignancy, but in regions endemic for granulomatous infections (Ohio and Mississippi River valleys for histoplasmosis), the false-positive rate is high.

Pseudoprogression after immunotherapy can cause false-positive FDG PET. Use iRECIST criteria and confirm with follow-up imaging before changing treatment.

FDG PET/CT should be performed at least 4 to 6 weeks after completion of chemotherapy and 8 to 12 weeks after radiation therapy to minimize false-positive inflammatory uptake.

## References

- Silvestri GA, et al. Methods for staging non-small cell lung cancer: ACCP evidence-based clinical practice guidelines. *Chest*. 2013;143(5_Suppl):e211S-e250S.
- De Wever W, et al. Role of integrated FDG-PET/CT in the staging of non-small cell lung cancer. *J Thorac Oncol*. 2010;5(9):S448.
- Wahl RL, et al. From RECIST to PERCIST: evolving considerations for PET response criteria in solid tumors. *J Nucl Med*. 2009;50(Suppl 1):122S-150S.
- Postmus PE, et al. Early and locally advanced non-small-cell lung cancer: ESMO clinical practice guidelines. *Ann Oncol*. 2017;28(suppl_4):iv1-iv21.