The invention relates to a variable-focus device (100) for concentrating ionising radiation fluence, which can be adapted to a high-energy electron source of an external radiotherapy device having a linear accelerator, to focus and guide the focus continuously and less invasively, within a determined area, which comprises: a coupling structure that joins the external radiotherapy device having a linear accelerator to an exterior structure (101)； a rotation system joined to the coupling structure using a coupling flange (8), which causes an interior structure (102) to rotate； an electron deflection system (103) in the interior structure (102), the deflection system (103) comprising a first magnetic deflection device (13) and a second magnetic deflection device (14)； elements (104) for correcting an electron beam； a heat-dissipation disc (26) comprising a white ring (19), the electrons from the deflection system (103) hitting the perimeter of the ring, X-rays being generated there； a shield (20)； a focus control system in the electron deflection system (103), which comprises an electronic control system (31) that controls a set of motors (12, 22) that generate coordinated movements of the second magnetic deflection device (14), a correction element and a collimator (25), which allow the position of the focal point to be changed； and at least one laser diode (74) located on the edge of the collimator (25), pointing to the focus, thereby allowing the position of the X-ray beam generated to be determined.Un dispositivo concentrador de fluencia de radiación ionizante (100) de foco variable adaptable a una fuente de electrones de alta energía de un dispositivo de radioterapia externa con acelerador lineal, para focalizar y guiar el foco de manera continua y menos invasiva, dentro de una zona determinada, que comprende: una estructura de acople que une al dispositivo de radioterapia externa con acelerador lineal con una estructura exterior (101)； un sistema de giro que se u
