Radius of curvature divided by tube diameter.

To get the radius of curvature, imaging the bend in the tube is a segment of a circle, the radius of curvature is the radius of that circle.

a curving or bending

The radius of curvature and the focal length mean the same so the radius of curvature is also 15 cm.

An exaggerated curvature of the thoracic (upper back). The curvature is outward which gives the appearance of a hump or rounded upper back.

They're probably refering to lateral curvature, which would be scoliosis.