A 62-year-old man with advanced lung cancer and recent chemotherapy presents with acute dyspnea. CTPA is performed and shows multiple emboli. Which is the most common location of thrombus within the pulmonary arterial tree on CTPA?

Explanation

## Most Common Location of PE Within the Pulmonary Arterial Tree **Key Point:** Segmental and subsegmental pulmonary arteries are the most common sites of thrombus lodgement on CTPA, accounting for the majority of clinically detected pulmonary emboli. ### Distribution of Thrombus Within Pulmonary Circulation | Location | Frequency | CTPA Visibility | Clinical Significance | |----------|-----------|-----------------|----------------------| | Segmental & subsegmental pulmonary arteries | ~65–75% | Clearly visible (segmental); visible but small (subsegmental) | Most common; moderate emboli; often multiple | | Lobar pulmonary arteries | 20–30% | Easily visible | Moderate to large emboli; hemodynamically significant | | Main pulmonary artery | 5–10% | Easily visible | Massive PE; hemodynamic instability | | Peripheral/distal arteries | <5% | Difficult to visualize | Rare; very small emboli | ### Anatomical Basis for Segmental/Subsegmental Predominance 1. **Thrombus caliber matching** — emboli originating from lower extremity DVT fragment as they travel through the pulmonary circulation; by the time they reach the segmental and subsegmental branches, vessel diameter matches thrombus size, causing lodgement. 2. **Branching geometry** — the progressive narrowing of the pulmonary arterial tree means the majority of emboli are trapped at the segmental level, with smaller fragments continuing to subsegmental vessels. 3. **CTPA sensitivity** — modern multidetector CTPA has high sensitivity for segmental and even subsegmental emboli, making these the most frequently reported sites in large imaging series (Remy-Jardin et al., Radiology; PIOPED II data). **Clinical Pearl:** Massive PE (main PA involvement) is less common (~5–10%) but carries the highest mortality. The vast majority of clinically significant emboli lodge at the segmental level and are readily detected on CTPA. Subsegmental PE detection has debated clinical significance and may not require anticoagulation in low-risk patients without proximal DVT. **High-Yield (per PIOPED II and standard radiology references):** Segmental pulmonary arteries are the single most common site of PE on CTPA. Option C (lobar only) is incorrect because lobar arteries, while commonly involved, are not the *most* common site — segmental and subsegmental arteries collectively account for the majority of emboli detected. **Mnemonic: SLMP** — **S**egmental (most common), **L**obar (second), **M**ain PA (massive, rare), **P**eripheral (very rare).