Optimisation of dose distribution in single planar iridium implants

Single planar interstitial implants aim to irradiate 10 mm thickness of tissue to uniform dose. Interstitial iridium implants are normally practised with the same linear activity throughout the implant. As a result, the peripheral regions of the implant get lesser dose contribution from the central wires. Our study with the planning system for a 5 x 4 cm2 area of parallel implant geometry shows that the peripheral regions receive only 83% of the prescribed dose and the minimum treated thickness is 7.6 mm only. Increasing either the activity or time weight to the peripheral wires by 25% enables the peripheral regions to receive a minimum of 92% of the prescribed dose and the minimum treated thickness increases to 8.80 mm. The removal time of peripheral wires with time weight option can be calculated manually by a simple formula. (author). 2 refs., 6 figs., 3 tabs