## Imaging Pattern Recognition in Lung Cancer **Key Point:** The imaging findings—**peripheral location**, **spiculated margins**, and **pleural indentation**—are classic hallmarks of **adenocarcinoma**, the most common lung cancer in smokers and non-smokers alike. ### Characteristic Features of Major Lung Cancer Subtypes | Feature | Adenocarcinoma | Squamous Cell | Small Cell | Large Cell | |---------|----------------|---------------|------------|------------| | **Location** | Peripheral | Central (hilum/proximal bronchus) | Central/perihilar | Peripheral | | **Margins** | Spiculated, irregular | Irregular, may cavitate | Ill-defined, rapid growth | Ill-defined | | **Cavitation** | Rare | Common (20–40%) | Rare | Rare | | **Pleural involvement** | Pleural indentation/tag — hallmark | Less common | Less common | Variable | | **Hilar LN** | Variable | Prominent, early | Prominent, bulky | Variable | | **Growth rate** | Slower | Intermediate | Rapid | Rapid | | **Smoking association** | Yes (but also non-smokers) | Strongly associated | Strongly associated | Associated | **Clinical Pearl:** Adenocarcinoma is the **most common primary lung malignancy** overall and the most common in smokers presenting with a **peripheral spiculated mass with pleural indentation**. The spiculated margins reflect desmoplastic stromal reaction and lepidic spread, while pleural indentation (pleural tag) is a CT hallmark caused by fibrous strands pulling the visceral pleura inward — a feature most strongly associated with adenocarcinoma (Harrison's Principles of Internal Medicine, 21st ed.). **High-Yield:** **Peripheral location + spiculated margins + pleural indentation = Adenocarcinoma** until proven otherwise on imaging. ### Why Adenocarcinoma Fits This Presentation 1. **Peripheral location** in the right upper lobe — adenocarcinoma arises from peripheral bronchioles/alveolar epithelium, not central airways 2. **Spiculated margins** — reflects desmoplastic fibrosis, irregular infiltration, and lepidic spread along alveolar walls 3. **Pleural indentation (pleural tag)** — fibrous strands tethering the visceral pleura; this is the **most specific CT sign** for adenocarcinoma 4. **Haemoptysis** — can occur in adenocarcinoma due to tumour vascularity, though more classically associated with squamous cell 5. **Hilar lymphadenopathy** — occurs in adenocarcinoma; 1.5 cm ipsilateral hilar node is consistent with N1 disease ### Why Squamous Cell Carcinoma Is Less Likely - Squamous cell carcinoma classically arises from **central airways** (segmental/lobar bronchi), presenting as a **hilar or perihilar mass**, often with post-obstructive atelectasis or pneumonia - It is strongly associated with cavitation (20–40%), which is absent here - A **peripheral** mass with pleural indentation is atypical for squamous cell carcinoma - While haemoptysis is a classic feature of squamous cell (endobronchial ulceration), it also occurs in adenocarcinoma ### Why Other Options Are Incorrect - **Small cell lung cancer (C):** Presents as a **central/perihilar mass** with bulky mediastinal/hilar lymphadenopathy and rapid growth; rarely presents as a solitary peripheral nodule with spiculated margins - **Large cell carcinoma (A):** Typically a large peripheral mass with ill-defined margins; lacks the specific pleural indentation sign; diagnosis of exclusion after ruling out other subtypes **Reference:** Harrison's Principles of Internal Medicine, 21st ed.; Felson's Principles of Chest Roentgenology, 5th ed. — peripheral spiculated mass with pleural tag is the imaging archetype of pulmonary adenocarcinoma. 
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