Interstitial laser photocoagulation with four cylindrical diffusing fibre tips: Importance of mutual fibre distance

Simultaneous application of multiple fibres could increase the volume of co agulation produced with interstitial laser photocoagulation (ILP) for solid tumours. To take full advantage of the presumed synergistic thermal effect between the fibres, the optimal combination of laser power and distance be tween the fibres was investigated. Four fibres with a cylindrical diffusing tip of 2 cm length were used, coupled to an optical beamsplitter for Nd:YA G light (four channels, maximal variation 9.5%, transmission >85%). The dis tance between the fibres was 1, 1.5, 2, 2.5, 3 or 4 cm with a power output of either 2, 5, 6 or 7 W/fibre; energy per fibre was constant at 1800 J by adjusting exposure time. After laser application, dimensions of the coagula ted lesions were measured. The optimal mutual fibre distance was 2 cm (p<0.01) at all power levels. Th is resulted in lesions with a mean (SD) volume of 44.5 (2.1) cm(3) and a la rgest diameter of 5.1 (0.4) cm at 7 W/fibre. Smaller distances between the fibres resulted in smaller lesions with central carbonisation, whereas larg er distances resulted in four separate zones of coagulation. It was conclud ed that simultaneous application of four interstitial fibres may result in a considerable increase of volume of coagulation. Fibre position and mutual fibre distance determines whether synergism of the coagulative effect occu rs.