If a 16-year-old man has a hard lump inside his nipple does that mean he has cancer?

No: It is normal to feel a small lump of breast tissue behind the nipple in a teenaged boy. This can become enlarged during or just after puberty and is called gynecomastia. Breast cancer is unheard of in teenaged boys.

Not necessarily: Hormonal changes in this age group can cause gynecomastia which is swelling of breast tissue. See your doctor for an examination to confirm.

Transient change: About a half of males will undergo a transient nipple/breast enlargement at the onset of puberty. Normally it will resolve slowly as other hormonal changes kick in over about a year. Less often it can persist over 3 years and surgery may be needed for cosmetic improvement. Carrying around extra weight can aggravate the issue as fat can masquerade as breast tissue.