A needle assembly (10) for relieving a pneumothorax and/or a hemothorax, the needle assembly (10) comprising a canula (12), the canula (12) comprising: a distal end portion (20) comprising a cutting end (22) for puncturing a thoracic cavity wall (24), a proximal end portion (14) for removing a fluid or gas from a thoracic cavity (28), a first curved portion (18) between the distal end portion (20) and the proximal end portion (14), an intermediate portion (16) between the first curved portion (18) and the proximal end portion (14), and a first fixing bulge (42) arranged between the first curved portion (18) and the intermediate portion (16), wherein an outer diameter of the canula (12) at the first fixing bulge (42) is increased with respect to the diameter (D1) of the intermediate portion (16), said first fixing bulge (42) being arranged such as to be placed at or close to the inner surface (24) of the thoracic cavity wall (24i), when the canula (12) is inserted for relieving the pneumothorax, such as to prevent or to reduce the risk that the canula (12) inadvertently slides out of its inserted position.