When to perform a CT (Computed Tomography) scan in a patient with known cancer presenting with sudden dyspnea and suspected pulmonary embolism?

CT Scan for Suspected Pulmonary Embolism in Cancer Patients

Patients with known cancer who present with sudden dyspnea and suspected pulmonary embolism should undergo immediate CT pulmonary angiography (CTPA) if they have high pretest probability of PE, without waiting for D-dimer testing. 1

Risk Assessment Algorithm

1. Assess pretest probability using validated clinical prediction tools:

   • Wells score or Geneva score
   • Cancer patients have inherently higher risk of PE

2. Decision pathway based on pretest probability:

   • High pretest probability:

     • Proceed directly to CTPA without D-dimer testing 1, 2
     • Do not delay imaging as this could increase mortality risk

   • Low to intermediate pretest probability:

     • Obtain D-dimer test
     • If D-dimer negative: PE can be safely excluded
     • If D-dimer positive: Proceed to CTPA
     • For patients >50 years: Use age-adjusted D-dimer threshold (age × 10 ng/mL) 1, 2

Special Considerations in Cancer Patients

• Cancer patients have higher risk of PE than general population
• Incidental PE in cancer patients is not necessarily less severe than symptomatic PE 3
• Cancer patients with PE have higher risk of sudden death (26% within 30 days in one study) 4
• PE may present atypically in cancer patients, mimicking other conditions
• Consider pulmonary tumor embolism as differential diagnosis, which can present similarly to thromboembolism but requires different management 5, 6

When to Consider Alternative Imaging

• For patients with contraindication to CTPA (severe renal impairment, contrast allergy):

  • Ventilation-perfusion (V/Q) scan can be used 1
  • Note: V/Q scanning may not be useful in patients with COPD, pneumonia, or pulmonary edema 1

• For patients with history of multiple CTs for PE:

  • Consider lower-extremity venous ultrasonography, especially with lower extremity symptoms 1, 2
  • Positive proximal DVT can establish need for anticoagulation without additional CT 1

Pitfalls to Avoid

1. Overuse of CTPA without proper risk stratification:

   • Following validated decision protocols like Wells Criteria can significantly decrease unnecessary CTPA use 7
   • One study found only 5.7% of cancer patients who underwent CTPA in the ED actually had PE 7

2. Ignoring radiation exposure risks:

   • Research shows 5% of patients evaluated for PE had 5 or more CTs within 5 years 1
   • Educate patients about radiation risks from multiple CTs

3. Missing alternative diagnoses:

   • CTPA can identify other causes of dyspnea when PE is not present
   • In cancer patients, consider progression of malignancy, treatment-related toxicity, infection

4. Delaying diagnosis in high-risk patients:

   • Immediate CTPA is warranted in high-risk scenarios without waiting for D-dimer results
   • Mortality increases with delayed diagnosis and treatment

By following this evidence-based approach, clinicians can appropriately utilize CT scanning for suspected PE in cancer patients, balancing the need for prompt diagnosis with concerns about radiation exposure and resource utilization.