The influence of location of the crossing needle on dose distributions from planar implants

Three typical planar radium implants with one uncrossed end were investigated for optimization of the crossing needle location. Isodose distributions in a plane 0.5 cm from the plane of the implant were calculated and the sizes and shapes of the areas receiving at least 85% of the maximum dose were evaluated. The merits of each implant geometry are discussed along with certain clinical considerations. As a refinement to the Paterson-Parker rules of geometry, it was found that an optimum distribution is achieved when the crossing needle is located near the physical ends of the parallel needles.