Abstract: A radiotherapy spacer is disclosed that has both flexibility for allowing it to gently cover the organs of various shapes along their surfaces and pressure resistance for preventing itself from being crushed under the pressure between organs and thus enabling it to retain its own shape and continuously ensure the therapeutic space. The spacer comprises a fiber assembly formed of entangled biocompatible fibers, which may be bioabsorbable, such as nonwoven fabric, and is characterized in that the density of the fiber assembly is 0.05 g/cm3 to 0.2 g/cm3.