- A theoretical basis and practical algorithm are described for selecting optimal fiber source locations in multifiber interstitial laser photocoagulation (ILP). By analyzing the shape of overlapping coagulation patterns, the optimal separation of adjacent point heat sources inside a flat target volume boundary is shown to be d* = 2 square root of 2rc, where rc is the coagulation radius of a single source. Against a curved boundary, the algorithm specifies how d* should be altered. To assess the validity of the theory, ILP was conducted in bovine muscle by delivering 1064 nm laser energy through two or four plane-cut optical fibers simultaneously. Delivered power, exposure duration, and source separation were varied. The observed coagulation patterns matched the theory-predicted patterns at delivered powers of 1.60W and 1.85W, but not at 2.40W. Also, the experiments indicate that reciprocity of delivered power and exposure duration is invalid for ILP.