Physically flexible radiation-emitting probes and associated illumination methods and systems for delivering radiation or light to the interior of a lumen or cavity. Light-emitting devices are immersed in a flowing liquid coolant within a probe to provide high light output power, and convoluted electrical power buss structures provide physical flexibility of a probe about a longitudinal axis. The probes can be configured for delivering light to the interior of any lumen including for performing therapeutic medical procedures at locations in body lumens including the interior of the human gastrointestinal tract.