Stereolithographic modelling as an aid to orbital brachytherapy

Purpose: This paper describes the technique of stereolithographic biomodell ing and its application to a patient who was treated using orbital brachyth erapy. Methods and Materials: The process uses a moving laser beam, directed by a computer, to draw cross-sections of the model onto the surface of photo-cur able liquid plastic. Using a stereolithographic apparatus (SLA), solid or s urface data is sliced by software into very thin cross-sections. A helium c admium (HeCd) laser then generates a small intense spot of ultraviolet (UV) light that is moved across the top of a vat of liquid photo monomer by a c omputerised optical scanning system. The laser polymerises the liquid into a solid where it touches, precisely printing each cross-section. A vertical elevator lowers the newly formed layer, and a recoating and levelling syst em establishes the next layer's thickness. Successive cross-sections (0.25 mm thick), each one adhering to the one below, are built one on top of the other, to form the part from the bottom up. The biomodel allowed the implan t to be planned in detail prior to the surgery, The accurate placement of b rachytherapy catheters was assured, and the dosimetry could be determined a nd optimised prior to the definitive procedure. Conclusions: Stereolithography is a useful technique in the area of orbital brachytherapy, It allows the implant to to be carried out with greater acc uracy and confidence. For the patient, it minimises the risk to the eye and provides them with a greater understanding of the procedure. (C) 1999 Else vier Science Inc.