BACKGROUND Trans-bronchoscope treatment for early stage small peripheral lung cancer, such as photodynamic therapy (PDT), has been investigated. However, despite the efficacy of PDT, light delivery issues limit its application. A method of administering mineral oil with a high refractive index (RI) was previously proposed to enhance light delivery in branched or bent anatomic structures. Lipiodol has a high RI and an exhaustive history of use as a contrast medium for bronchography. We aimed to determine whether the use of lipiodol, like mineral oil, could enhance the illumination effect and therapeutic range of PDT for peripheral lung tumors. METHODS We injected lipiodol into a pig lung model, guided by a bronchoscope under fluorescent surveillance, to simulate future treatment in humans, and then illuminated with PDT laser fiber to the lipiodol-infused lung to test the technique feasibility in a pig orally administered 20 mg/kg of 5-aminolevulinicc acid (5-ALA) 2 hours before treatment. We also attempted to determine the maximal tolerable light dose in this pilot study for the future studies in human. RESULTS We successfully injected lipiodol into peripheral lungs by this technique. The pig could tolerate up to a total of 40 mL of lipiodol and 800 J of red light, without severe acute fetal injury in a non-cancerous lung. CONCLUSIONS The technique of injecting lipiodol using bronchoscopy under fluorescent guidance was feasible in a pig model. We can apply the guide sheath through bronchoscopy under fluoroscope inspection. Lipiodol can be used as a light diffuser for the peripheral lung tumor PDT model. No severe lethal acute lung injury was caused by this PDT model under careful manipulation. Additional studies evaluating the dose correlation of the photosensitizer and light are needed.