Cancer Treatment Today

Lung cancer with negative chest xray is rare but possible, about 25% an endobroncial cancer that does not show up on the xray. A CT of chest is, therefore, appropriate with persistent cough and negative chest xray.

Sally Stapley,Negative chest X-rays in primary care patients with lung cancer
Br J Gen Pract. Aug 1, 2006; 56(529): 570–573.