A 62-year-old woman with no smoking history presents with progressive dyspnoea and chest pain. Chest X-ray shows a 4 cm well-circumscribed mass in the right lower lobe periphery with a halo sign on CT. PET-CT demonstrates moderate FDG uptake (SUVmax 4.2) in the mass and no distant metastases. Bronchoscopy shows no endobronchial lesion. What is the most likely diagnosis?

## Peripheral Lung Mass in Non-Smoker: Adenocarcinoma **Key Point:** The combination of peripheral location, well-defined margins, halo sign on CT, moderate FDG uptake, and non-smoking status strongly suggests **adenocarcinoma**, particularly the lepidic (formerly bronchioloalveolar) variant. ### Imaging Features Distinguishing Adenocarcinoma from Other Peripheral Masses | Feature | Adenocarcinoma | Squamous Cell | Hamartoma | Aspergilloma | |---------|----------------|---------------|-----------|-------------| | **Location** | Peripheral (distal lung) | Central | Variable | Cavitary, upper lobe | | **Margins** | Well-defined, sometimes spiculated | Spiculated, central | Sharply demarcated | Cavitary with fungal ball | | **Halo sign** | Present (early stage, lepidic) | Absent | Absent | Absent | | **FDG uptake** | Moderate to high | High | Minimal/none | Minimal | | **Smoking association** | Weak (non-smokers at risk) | Strong | None | Immunocompromised | | **Bronchoscopy** | Negative (peripheral) | Positive (endobronchial) | Negative | Negative | **High-Yield:** **Halo sign** = ground-glass opacity surrounding a nodule, seen in early-stage adenocarcinoma (especially lepidic pattern) due to alveolar spread and surrounding inflammation. It is a sign of peripheral, slow-growing adenocarcinoma. **Clinical Pearl:** Adenocarcinoma is the most common lung cancer in non-smokers and has a strong predilection for the **peripheral lung**, particularly the lower lobes. The lepidic variant grows along alveolar walls without invasion, producing the characteristic well-defined, halo-positive appearance. ### Why Adenocarcinoma Fits This Case 1. **Peripheral location** in right lower lobe — adenocarcinoma's classic site 2. **Non-smoking history** — adenocarcinoma is the histotype in non-smokers 3. **Halo sign** — pathognomonic for lepidic adenocarcinoma (ground-glass halo from alveolar spread) 4. **Moderate FDG uptake** — consistent with slower-growing adenocarcinoma (SUVmax 4.2 is not hypermetabolic) 5. **Negative bronchoscopy** — confirms peripheral location, rules out endobronchial squamous cell 6. **Well-circumscribed margins** — lepidic pattern grows along alveolar walls with minimal invasion