A system configured for photodynamic or photothermal therapy and/or photodynamic diagnosis of a tissue, said system in combination comprising an assembly of: a hollow catheter and a proximal end adapter for said hollow catheter; said proximal end adaptor comprising an adapter body, having proximal and distal ends, defining an axial lumen extending between the proximal and distal ends, and a catheter mounting element at the distal end of the adapter body where said adapter body is releasably mountable to said hollow catheter, whereby said catheter mounting element is releasably connectable to said hollow catheter and said hollow catheter extends from the distal end of the adaptor body when mounted thereto; said assembly further comprising: a first light guide having a proximal end and a distal end, wherein said first light guide is sealingly attached to said proximal end adapter in said axial lumen and defines an axial optical path extending between the proximal and distal ends of the adapter body towards said tissue and into close proximity thereof or interstitially into said tissue, wherein the proximal end of said first light guide is configured to be positioned within the adapter body, and said distal end of said first light guide extends out of said distal end of said adapter body and into said catheter, an optical connector and an optical connector mounting element for mounting said optical connector at the proximal end of said adaptor body, and wherein said optical connector comprises a second light guide, and is coupled to a light source via said second light guide, such that light from said light source is transmittable from said light source via said second light guide to the distal end thereof and further via said optical connector to the proximal end of said first light guide and via said proximal end adapter towards said tissue through said hollow catheter for said therapy and/or diagnosis of said tissue. a hollow catheter and a proximal end adapter for said hollow catheter; said proximal end adaptor comprising an adapter body, having proximal and distal ends, defining an axial lumen extending between the proximal and distal ends, and a catheter mounting element at the distal end of the adapter body where said adapter body is releasably mountable to said hollow catheter, whereby said catheter mounting element is releasably connectable to said hollow catheter and said hollow catheter extends from the distal end of the adaptor body when mounted thereto; a first light guide having a proximal end and a distal end, wherein said first light guide is sealingly attached to said proximal end adapter in said axial lumen and defines an axial optical path extending between the proximal and distal ends of the adapter body towards said tissue and into close proximity thereof or interstitially into said tissue, wherein the proximal end of said first light guide is configured to be positioned within the adapter body, and said distal end of said first light guide extends out of said distal end of said adapter body and into said catheter, an optical connector and an optical connector mounting element for mounting said optical connector at the proximal end of said adaptor body, and wherein said optical connector comprises a second light guide, and is coupled to a light source via said second light guide, such that light from said light source is transmittable from said light source via said second light guide to the distal end thereof and further via said optical connector to the proximal end of said first light guide and via said proximal end adapter towards said tissue through said hollow catheter for said therapy and/or diagnosis of said tissue.