ORIGINAL ARTICLE C. A. Mosse Æ T. Theodossiou Æ T. N. Mills A system to monitor for fibre breakage during photodynamic therapy with diffuser fibres Received: 12 December 2002 / Accepted: 21 July 2003 Ó Springer-Verlag London Limited 2003 Abstract There is concern that the delicate fibres and cylindrical diffuser tips used for intraluminal PDT of blood vessels might break during the treatment. We have developed and tested a simple system to monitor for fibre breakage during PDT. Light from the laser is passed through a beam expander, then a beam splitter, and then is again contracted into the inlet of a diffuser fibre. Light reflected from the distal diffuser is also partially reflected by the beam splitter into a photodi- ode, which monitors changes in the intensity of the reflected light. The system was tested by simulating catastrophic failures in four fibres. In each case the output fell very significantly (DV ¼ 0.44 ) 1.68V, SD ¼ 0.01V). Despite the small statistical sample, the average change in the reflected light intensity is still significant (p < 0.025, paired Student’s t-test). Our conclusion was that this device can monitor the integrity of diffuser fibres during PDT. Keywords Laser Æ Optically diffusing fibres Æ PDT Introduction Optical fibres with diffusing ends are frequently used to deliver photodynamic therapy (PDT) interstitially or inside a hollow organ, such as the oesophagus. Re- cently, there has been considerable interest in using PDT to prevent restenosis following balloon angio- plasty and/or stenting in the femoral or coronary arteries 1,2 . In this application, an extremely flexible and thin cylindrical diffuser (about 0.5 mm diameter for coronary arteries) is fed along the lumen of a balloon angioplasty catheter and then used to illuminate the interior of the artery where it has been stretched. There is always a great concern that these delicate fibres might break inside an artery, with serious conse- quences. Although breakage of cylindrical diffuser fibres is rare, it is not unknown, and indeed our group has experienced fibre breakdowns during treatments with fibres from three different manufacturers. The most common failure mode is a catastrophic overheating at the point where the diffuser tip joins the core of the fibre. When this happens the diffusing tip can become com- pletely detached from the fibre, leaving what is, in effect, a bare tip fibre. Particularly in the coronary arteries, this poses a hazard of both the diffuser floating away and local thermal or photochemical damage caused by the high-intensity light emerging from the bare tip. Figure 1 shows a pig’s heart, with a large blood clot surrounding the left anterior descending artery after a diffuser had broken off from the fibre. This paper describes a technique that can be used to monitor for sudden breakage by measuring the intensity of the light reflected back along the optical fibre that connects the laser to the diffuser. It is common with long telecommunications fibres to estimate the distance of a break from the start of a fibre by measuring the time it takes for a pulse of light to be reflected back from the break (optical time domain reflectometry, OTDR). However, the current technique differs in that it measures the changes in intensity in light reflected from the tip of the fibre that are symptomatic of a diffuser being replaced by a bare tip. We are not aware of this method having been used before for dif- fuser fibres, although in their 1983 patent Doi and Ka- wamura 3 describe a similar system for detecting damage in the tip of a fibre ‘‘with the emergent end face being coated with either a transmission increasing or a reflec- tion increasing coating.’’ Lasers Med Sci (2003) 18: 162–164 DOI 10.1007/s10103-003-0270-x C. A. Mosse (&) Æ T. N. Mills Department of Medical Physics and Bioengineering, University College London, London WC1E 6JA, UK Tel.: +4420 7679 6262 Fax: +4420 7679 6269 E-mail: s.mosse@medphys.ucl.ac.uk T. Theodossiou National Medical Laser Centre, University College London, London W1W 7EJ, UK