System for treatment by photodynamic therapy of a cavity of a patient's body, the cavity being delimited by tissues comprising cells having a photosensitizer compound absorbed therein, the system comprising an illuminating device intended for illuminating the cavity to be treated, wherein the illuminating device comprises an illuminating member extending along a central axis between opposed proximal and distal ends, the illuminating member comprising: - a core carrying a light emitting surface for emitting a light adapted to activate the photosensitizer compound, the light emitting surface being arranged at the distal end of the illuminating member, - a hollow sheath having a single balloon arranged at the distal end of the illuminating member, the hollow sheath being adapted to receive the core with the light emitting surface arranged within the balloon, the balloon comprising a wall which has an inner surface delimiting an internal space, and an outer surface, the wall being flexible and adapted to allow diffusion of the light emitted by the light emitting surface, the balloon presenting at least one inflated state in which the wall has a symmetry of revolution about the central axis and the internal space is filled with a light diffusing solution so as to diffuse the light emitted by the light emitting surface, and a deflated state in which the internal space is empty, wherein the internal space of the balloon has a variable capacity, the wall of the balloon being elastically extendible and the balloon presenting a plurality of inflated states in each of which the internal space is filled with a volume of light diffusing solution, 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 of the balloon and a corresponding time of illumination for providing a determined dose of light energy, and an electronic unit connected to the illumination device and controlling the illumination device as a function of the volume of light diffusing solution, the support comprising a memory of the electronic unit storing the transfer function, wherein the electronic unit is configured to, during the photodynamic treatment, control the time of illumination on the basis of the transfer function stored in the memory, wherein said transfer function is obtained, prior to the photodynamic treatment, by measuring at least one of the corresponding distributions of light power at the outer surface of the wall of the balloon and the corresponding time of illumination at the plurality of inflated states of the single balloon, for providing the determined dose of light energy at the plurality of inflated states of the single balloon. - a core carrying a light emitting surface for emitting a light adapted to activate the photosensitizer compound, the light emitting surface being arranged at the distal end of the illuminating member, - a hollow sheath having a single balloon arranged at the distal end of the illuminating member, the hollow sheath being adapted to receive the core with the light emitting surface arranged within the balloon, the balloon comprising a wall which has an inner surface delimiting an internal space, and an outer surface, the wall being flexible and adapted to allow diffusion of the light emitted by the light emitting surface, the balloon presenting at least one inflated state in which the wall has a symmetry of revolution about the central axis and the internal space is filled with a light diffusing solution so as to diffuse the light emitted by the light emitting surface, and a deflated state in which the internal space is empty, wherein the internal space of the balloon has a variable capacity, the wall of the balloon being elastically extendible and the balloon presenting a plurality of inflated states in each of which the internal space is filled with a volume of light diffusing solution, 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 of the balloon and a corresponding time of illumination for providing a determined dose of light energy, and an electronic unit connected to the illumination device and controlling the illumination device as a function of the volume of light diffusing solution, the support comprising a memory of the electronic unit storing the transfer function, wherein the electronic unit is configured to, during the photodynamic treatment, control the time of illumination on the basis of the transfer function stored in the memory, wherein said transfer function is obtained, prior to the photodynamic treatment, by measuring at least one of the corresponding distributions of light power at the outer surface of the wall of the balloon and the corresponding time of illumination at the plurality of inflated states of the single balloon, for providing the determined dose of light energy at the plurality of inflated states of the single balloon.