Dose distributions produced by a shielded vaginal cylinder using a high-activity iridium-192 source.

Algorithms commonly used to compute the dose distribution around a 192Ir source do not calculate the dose by summing the primary and scatter dose components. Thus if the scatter component is reduced significantly by the insertion of a shield into a vaginal cylinder, the algorithm may overestimate the dose delivered to the tumor volume. This problem was investigated by measuring the relative dose around a 2.5 cm diameter shielded vaginal cylinder in a water phantom by use of a 0.147 cm3 thimble ionization chamber. Measurements were made with the cylinder unshielded and with 0.8 cm thick 90 degrees, 180 degrees, and 270 degrees tungsten shields inserted. A reduction in dose was observed on the unshielded side of the cylinder which increased with distance from the source and with the angle subtended by the shield. This reduction did not exceed 2% within 1 cm from the surface of the cylinder with any shield. However, the reduction in dose increased linearly to 3%, 5%, and 15% at a distance of 10 cm from the source with the 90 degrees, 180 degrees, and 270 degrees shields, respectively. On the shielded side of the cylinder, the dose at the surface is reduced to about 13% of its value without the shield; however, this percentage increases with distance reaching a maximum 2-4 cm from the cylinder. The maximum values obtained with the 90 degrees, 180 degrees, and 270 degrees shields were 23%, 17%, and 16%, respectively.