Double Lesion Sign
If you encounter atelectasis of RUL and RLL sparing RML, it is difficult to comprehend a single endobronchial lesion to account for both lesions. There has to be two independent endobronchial lesions, hence it was proposed that it is unlikely to be due to primary bronchogenic cancer. The concept is based on knowledge of Anatomy.
Caution: Do not rule out lung cancer when you see this phenomenon, you just have to find an alternate explanation for the second lesion. The second lesion could be due to aspiration of blood from the first lesion.