System (1) for treatment by photodynamic therapy comprising an illuminating member (6) which comprises: —a core (35) carrying a light emitting surface (37), and —a hollow sheath (10) adapted to receive the core (35) with the light emitting surface (37) arranged within a balloon (11), the balloon (11) comprising a wall (12) which has an inner surface delimiting an internal space, and an outer surface, the wall (12) being flexible, wherein the internal space of the balloon (11) has a variable capacity, the wall (12) of the balloon (11) being elastically extendible and the balloon (11) presenting a plurality of inflated states in each of which the internal space is filled with a volume of light diffusing solution, and wherein the system further comprises a support provided with a transfer function relating the volume of light diffusing solution of each inflated state with at least one of a corresponding distribution of light power at the outer surface of the wall (12) of the balloon (11) and a corresponding time of illumination for providing a determined dose of light energy.