The most likely diagnosis is radiation pneumonitis. Patients with radiation pneumonitis present with cough and/or dyspnea approximately 6 to 12 weeks after the exposure. Patients with a high total dose of radiation, preexisting lung disease (especially COPD), and concomitant chemotherapy and radiation therapy are at increased risk. The disease can be severe, with progression over days to weeks leading to acute respiratory failure. Gemcitabine is associated with radiation pneumonitis and drug-induced pneumonitis. The factor that is most pathognomonic of radiation pneumonitis is the imaging finding of a nonanatomic straight line demarcating involved versus uninvolved lung parenchyma as can be seen in this patient's CT scan (arrows). However, this finding is no longer an absolute diagnostic criterion owing to differing types of radiation delivery, such as stereotactic and conformal regimens. The abnormalities in classic radiation pneumonitis typically resolve within 6 months but can progress to a well-demarcated area of fibrosis with volume loss and bronchiectasis.

Although this patient is at risk for progression of her malignancy, the ground-glass and nonanatomic straight line imaging findings are not typical for non–small cell lung cancer recurrence. Additionally, the clinical and radiographic worsening over days to weeks is a helpful clue to distinguish radiation pneumonitis from recurrent local or metastatic cancer.

Radiation fibrosis is clinically distinct from radiation pneumonitis, although the diseases may have overlapping clinical presentations. Radiation fibrosis can occur in patients with or without a history of radiation pneumonitis. Radiation fibrosis occurs 6 to 24 months after radiation therapy and represents a long-term fibrotic sequela of lung damage, most often within the radiation field. Symptoms are uncommon, but patients with previous marginal lung function may have worsening dyspnea. The fibrotic process is irreversible. This patient's symptoms and time course are not compatible with radiation fibrosis.

Viral pneumonia is also part of the differential diagnosis, but the radiographic finding of a straight line of demarcation makes this an unlikely diagnosis.