American Board of Internal Medicine (ABIM) Certification Practice Exam

What imaging finding is pathognomonic of radiation pneumonitis?

1. Presence of ground-glass opacities

2. A nonanatomic straight line demarcating involved versus uninvolved lung parenchyma

3. Patchy consolidations in the lower lobes

4. Fluid accumulation in the pleural cavity

The correct answer is: A nonanatomic straight line demarcating involved versus uninvolved lung parenchyma

The imaging finding that is pathognomonic of radiation pneumonitis is characterized by a nonanatomic straight line demarcating the involved versus uninvolved lung parenchyma. This distinctive feature arises from the nature of radiation therapy, which targets specific areas of the lung, leading to a well-defined boundary where the lung has been exposed to radiation and where it has not. The nonanatomic appearance results from the abrupt transition between affected and normal lung tissue, reflecting the precise nature of radiation's effects on lung cells, which are limited to the irradiated field. Ground-glass opacities, patchy consolidations, and fluid accumulation in the pleural cavity can all be observed in various lung conditions and are not unique to radiation pneumonitis. For instance, ground-glass opacities might signify a range of inflammatory processes or interstitial lung disease, while patchy consolidations may indicate infection or other types of pneumonitis. Additionally, fluid accumulation typically suggests a pleural effusion which can occur due to a variety of other pathological processes apart from radiation exposure. Thus, the defined linear demarcation distinctly associated with radiation pneumonitis sets it apart from these other findings.