Scatter and attenuation measurements at distances greater than 10 cm from an Ir-192 source

Most algorithms developed by various investigators for use in brachytherapy treatment planning have typically been designed to calculate the dose with in a 10 cm range of a radiation source. These algorithms predict the dose w ell at distances <10 cm from the source but were not developed and should n ot be utilized to predict the dose at distances >10 cm. On the contrary, tr eatment planning systems and manual calculations will produce erroneous res ults when dose points >10 cm are calculated using these algorithms. The spr ead in the data generated by the above algorithms is 16% at 15 cm and 42% a t 20 cm. Physical measurements were performed at distances between 5 and 50 cm from a high activity Ir-192 source in water. The measured data correlat ed well with the predicted data from 5 to 10 cm, which had a 5% spread. Bey ond 10 cm the measured data fell central to the range of the predicted data , with the spread of the predicted data increasing from 5% to 80% with incr easing distance from the source. The measured data was fitted with a model incorporating a buildup factor and an attenuation factor. The best fit valu es are in reasonable agreement with those obtained by two of the investigat ors, Thomason and Tripathi. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)01001-9].