A 192Ir nomogram system for single plane implants.

Nomograms for square planar arrays spanning the range from 3 X 3 cm to 10 X 10 cm were developed. The nomograms are intended to be used for pretreatment planning of implant geometry, so that the therapist may enter the operating room with a plan for the optimal implant in mind. We show that clinically useful implants are those in which the reference isodoses are fully coupled. Decoupling occurs when ribbon spacing exceeds approximately 1.2 cm and leads to undesirable "cold spots" within the treatment volume. Ribbon spacing of 1.0 cm is recommended. This represents a trade-off between adequate ribbon coupling and minimum tissue damage from trocar placement. For clinically useful arrays, the area enclosed by the reference isodose contour (85% of the maximum dose rate) is approximately 50% of the array area. Reference isodose contour and its enclosed area are independent of seed strength for a given array, as long as all seeds within the array are of equal strength.