Why can lung cancer cause blood-stained sputum?

The presence of blood in the sputum, known as hemoptysis, is a significant clinical sign that can indicate the presence of a tumour in the lungs. In the context of lung cancer, blood-stained sputum occurs because the malignancy can cause inflammation or direct damage to the capillaries within the respiratory tract.1

It is important to note that while hemoptysis is a classic indicator of serious conditions like lung cancer or tuberculosis, it can also result from other factors, such as the exacerbation of an existing infection. Because hemoptysis may signal underlying malignancy or capillary damage, it is a symptom that should never be disregarded and requires professional medical evaluation.1

While blood-stained sputum is also associated with mitral stenosis due to increased pressure in the pulmonary circulation, in the specific case of lung cancer, the primary mechanism is the physical impact of the tumour on the lung tissue and its associated blood vessels.21