How do the clinical presentations of a COPD exacerbation versus acute radiation‑induced lung injury differ in an adult smoker with chronic obstructive pulmonary disease who has received thoracic radiotherapy?

Distinguishing COPD Exacerbation from Acute Radiation-Induced Lung Injury

COPD exacerbation presents with increased dyspnea, sputum volume and purulence, and cough in a patient with known COPD history, while acute radiation pneumonitis occurs within 6 months of thoracic radiotherapy and presents with dry cough, dyspnea, and fever without purulent sputum production. 1, 2, 3

Temporal Relationship to Radiation Therapy

The most critical distinguishing feature is timing:

• Radiation pneumonitis occurs in the early phase (<6 months post-radiotherapy), typically 1-3 months after completing thoracic radiation 3, 4
• COPD exacerbation has no temporal relationship to radiation therapy and is triggered by infections (viral or bacterial), cardiovascular comorbidities, or environmental exposures 1, 5

Clinical Presentation Differences

COPD Exacerbation Characteristics:

• Increased sputum production with change in volume and purulence (often yellow or green) 1, 5
• Productive cough with potentially blood-streaked sputum 5
• Wheeze and increased cough frequency 1
• Symptoms build over days, often preceded by upper respiratory infection 1
• History of similar prior episodes in the patient's COPD course 6

Acute Radiation Pneumonitis Characteristics:

• Dry, non-productive cough as the predominant symptom 2, 7
• Absence of purulent sputum production 2, 4
• Fever is common (often low-grade) 7, 4
• Chest pain may be present 7
• Progressive dyspnea that develops insidiously over weeks 4
• Symptoms confined to or worse in the radiation field 4

Diagnostic Approach Algorithm

Step 1: Establish Radiation History

• Confirm whether patient received thoracic radiotherapy within the past 6 months 3, 4
• If no radiation history exists, radiation pneumonitis is excluded 4

Step 2: Assess Sputum Characteristics

• Purulent sputum with increased volume strongly suggests COPD exacerbation rather than radiation injury 1, 5
• Dry cough without sputum production points toward radiation pneumonitis 2, 7

Step 3: Chest Radiography (Mandatory)

• COPD exacerbation: May show hyperinflation, hyperlucent areas, or be normal; must exclude pneumonia, pneumothorax, pulmonary edema, and heart failure 6, 5
• Radiation pneumonitis: Shows ground-glass opacities or consolidation confined to the radiation field with sharp demarcation at field borders 4
• The geographic distribution matching the radiation portal is pathognomonic for radiation injury 4

Step 4: Exclude Critical Mimics

Both conditions require exclusion of:

• Pneumonia (fever, focal consolidation, leukocytosis) 1, 5
• Pulmonary embolism (sudden onset, pleuritic pain, risk factors) 1, 5
• Heart failure (peripheral edema, elevated BNP/NT-proBNP, pulmonary congestion) 5
• Lung cancer progression (especially relevant in radiation pneumonitis patients) 5, 4

Key Distinguishing Features Summary

Feature	COPD Exacerbation	Radiation Pneumonitis
Timing	Any time, often infection-triggered	<6 months post-RT [3]
Cough	Productive, increased frequency [1]	Dry, non-productive [2,7]
Sputum	Increased volume, purulent [1,5]	Absent or minimal [2]
Fever	Variable, suggests infection [1]	Common, low-grade [7]
Chest X-ray	Hyperinflation or normal [6]	Ground-glass in radiation field [4]
Wheeze	Prominent [1]	Typically absent [4]

Critical Pitfalls to Avoid

• Do not assume all dyspnea in a COPD patient post-radiation is exacerbation—radiation pneumonitis is a diagnosis of exclusion requiring systematic evaluation 5, 4
• The presence of underlying COPD does not prevent radiation pneumonitis—patients can develop both conditions, and radiation injury may be superimposed on chronic lung disease 4
• Viral infections can trigger COPD exacerbations and predispose to bacterial superinfection, so absence of purulent sputum early does not completely exclude exacerbation, but development of purulent sputum strongly favors COPD exacerbation over radiation injury 1
• Cardiovascular comorbidities (heart failure, pulmonary embolism, atrial fibrillation) can precipitate acute respiratory decompensation mimicking either condition and must be systematically excluded 1, 5

When Both Conditions May Coexist

In patients with COPD who received recent thoracic radiation:

• Obtain sputum culture if purulent sputum is present, suggesting infectious COPD exacerbation component 1
• Review radiation dosimetry and correlate imaging findings with radiation fields 4
• Consider that infection can trigger exacerbation in a patient with subclinical radiation injury 1, 4
• Treatment may require addressing both processes: bronchodilators and antibiotics for COPD exacerbation plus corticosteroids for radiation pneumonitis 6, 2, 4