The Ballpoint Case

A woman is found dead on the floor in her Leiden kitchenette. The circumstances are unclear until X-rays are taken. Her head is found to contain a BIC biros. Did she fall with a pen in her hand or was she murdered? People assume the former, until a psychologist comes forward. Her son is said to have confessed to shooting the pen at his mother with a crossbow during a therapy session. He denies. Is it an accident or the perfect murder?