A 58-year-old smoker has a 3 cm peripheral lung mass with a CT density of +10 HU at baseline and +35 HU post-contrast. On PET-CT, the SUVmax is 2.2. Which statement BEST characterizes this lesion?

• A Enhancement <15 HU excludes malignancy regardless of PET findings
• B Enhancement >15 HU excludes benign causes; PET SUVmax >2.5 is required for malignancy
• C Enhancement >15 HU strongly suggests malignancy; PET SUVmax <2.5 is borderline indeterminate ✓
• D Enhancement <15 HU and SUVmax <2.5 together confirm benign nodule

Explanation

On contrast-enhanced CT, enhancement >15 HU in a solitary pulmonary nodule strongly suggests malignancy or active inflammation (sensitivity ~98% for malignancy). This lesion enhanced by 25 HU (35-10), meeting the malignant threshold. PET-CT SUVmax ≥2.5 is the traditional threshold for malignancy; SUVmax 2.2 is borderline indeterminate and does not exclude malignancy in an at-risk smoker. Enhancement <15 HU has high NPV but does not absolutely exclude malignancy, particularly in carcinoid or early adenocarcinoma.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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