A catheter device for use in the photodynamic treatment of a body cavity or hollow organ of the body, the catheter device comprising: a distal end portion having a longitudinal axis and being for insertion into the body cavity or hollow organ, the distal end portion including: an expansion and positioning balloon, a light source located on the distal end portion so as to be within the body cavity or hollow organ when the catheter device is in use; wherein the light source comprises a plurality of light emitting elements (22, 24, 26) arranged to: project light forward in a distal direction along the direction of the longitudinal axis from a first region of the distal end, project light outwardly of the longitudinal axis from a second region of the distal end, the second region being at least partly within the expansion and positioning balloon, and project light around the point of entry of the catheter device into the body cavity or hollow organ from a third region of the distal end, the third region being closer to the point of entry of the catheter device into the body cavity or hollow organ than the second region; and the distal end portion also including a second balloon for retaining the distal end within the body cavity or hollow organ, the second balloon being spaced apart from the expansion and positioning balloon and centred on a location closer to a proximal end of the catheter device than a centre of the expansion and positioning balloon; characterised in that the expansion and positioning balloon is for correct positioning of the device within the body cavity or hollow organ and a suitable distance between body tissue and the light emitting elements, wherein the expansion and positioning balloon is arranged to expand to a spheroidal or toroidal shape with an expanded diameter and/or height of between 40 mm and 100 mm within the body cavity or hollow organ to thereby distend an outer wall of the body cavity or hollow organ, and wherein expansion and positioning balloon is arranged to centre the device in relation to the distended shape of the body cavity or hollow organ such that there can be a required minimum light dose for all parts of the interior of the body cavity or hollow organ. an expansion and positioning balloon, a light source located on the distal end portion so as to be within the body cavity or hollow organ when the catheter device is in use; wherein the light source comprises a plurality of light emitting elements (22, 24, 26) arranged to: project light forward in a distal direction along the direction of the longitudinal axis from a first region of the distal end, project light outwardly of the longitudinal axis from a second region of the distal end, the second region being at least partly within the expansion and positioning balloon, and project light around the point of entry of the catheter device into the body cavity or hollow organ from a third region of the distal end, the third region being closer to the point of entry of the catheter device into the body cavity or hollow organ than the second region; and project light forward in a distal direction along the direction of the longitudinal axis from a first region of the distal end, project light outwardly of the longitudinal axis from a second region of the distal end, the second region being at least partly within the expansion and positioning balloon, and project light around the point of entry of the catheter device into the body cavity or hollow organ from a third region of the distal end, the third region being closer to the point of entry of the catheter device into the body cavity or hollow organ than the second region; and the distal end portion also including a second balloon for retaining the distal end within the body cavity or hollow organ, the second balloon being spaced apart from the expansion and positioning balloon and centred on a location closer to a proximal end of the catheter device than a centre of the expansion and positioning balloon; characterised in that the expansion and positioning balloon is for correct positioning of the device within the body cavity or hollow organ and a suitable distance between body tissue and the light emitting elements, wherein the expansion and positioning balloon is arranged to expand to a spheroidal or toroidal shape with an expanded diameter and/or height of between 40 mm and 100 mm within the body cavity or hollow organ to thereby distend an outer wall of the body cavity or hollow organ, and wherein expansion and positioning balloon is arranged to centre the device in relation to the distended shape of the body cavity or hollow organ such that there can be a required minimum light dose for all parts of the interior of the body cavity or hollow organ.