A comparison of the biological effective dose of 50-kV electronic brachytherapy with 192Ir high-dose-rate brachytherapy for vaginal cuff irradiation

PURPOSE: Advantages for electronic brachytherapy (EBT) of the vaginal cuff include decreased physical dose to the bladder and rectum. Here we compare Ir-192 with EBT using biological effective dose (BED) to account for the different radiobiological effectiveness (RBE) predicted for low-energy x-rays. METHODS AND MATERIALS: Fifteen data sets from five consecutive postoperative endometrial cancer patients treated with EBT were analyzed. Treatment planning was performed using PLATO software. The dose was prescribed as 21 Gy in three fractions to a depth of 0.5 cm. Physical dose, BED3, and BED10 were evaluated for the mucosa, bladder, and rectum. An RBE value of 1.5 was used for BED calculations. RESULTS: Mucosal physical dose is 28.4% greater with EBT (36.6 vs. 28.5 Gy, p < 0.05). However, the BED10 is increased by 79.1% (55.6 vs. 99.6 Gy, p < 0.05) and the BED3 by 71.5% (118.8 vs. 203.7 Gy, p < 0.05). The physical dose (dose to 50% volume of the organ) to the bladder (9.3 vs. 6.6 Gy, p < 0.05) and rectum (7.2 vs. 4.2 Gy, p < 0.05) are reduced with EBT. BED3 to the rectum and bladder are also reduced but to a lesser extent (13 vs. 8.3 Gy, p < 0.05; 18.9 vs. 14.7 Gy, p = 0.06, respectively). CONCLUSIONS: BED takes into account the higher RBE of low-energy photons generated with EBT and provides a more accurate estimate of the biological effect. When using EBT, physical dose may underestimate the biological effect on the vaginal mucosa and overestimate the benefit for the bladder and rectum. Dose adjustment for EBT based on BED should be considered. (C) 2012 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.