Pneumonia carcinomatosa from small cell undifferentiated carcinoma of the lung presenting as "reverse radiation pneumonitis".

We describe a patient with recurrent small cell undifferentiated lung carcinoma after chemotherapy and mediastinal radiation therapy who presented with peripheral pulmonary infiltrates on chest radiograph. At autopsy the patient was found to have carcinomatous pneumonia confined to the radiographically abnormal lung. The descriptive term "reverse radiation pneumonitis" is applied in view of the striking nonsegmental distribution of these pulmonary infiltrates, which occurred only outside the irradiated field. In this patient, radiation therapy successfully controlled disease in the treated lung parenchyma, thus accounting for this unusual radiologic and histologic picture. Pneumonia carcinomatosa, occurring after lung irradiation, can therefore be added to the differential diagnosis of radiographic peripheral pulmonary infiltrates.