Use of the rectal retractor to reduce the rectal dose in high dose rate intracavitary brachytherapy for a carcinoma of the uterine cervix.

Commercially available rectal retractors can be used in high dose rate intracavitary brachytherapy (HDR ICR) as one of the methods for reducing the rectal dose in radiotherapy for a uterine cervical cancer. However, the extent of the rectal protection achieved using these rectal retractors has not been reported. The aim of the study was to examine the effect of a rectal retractor on reducing the rectal dose in HDR ICR. Thirty patients were treated with HDR ICR using rectal retractors. Tandem and ovoids were applied in 15 patients and ovoids only were used in the other 15 patients. During the simulation, the rectum was filled with barium, and anteroposterior and lateral radiographs were then taken with and without the rectal retractor. Along the anterior rectal wall outlined, 4 to 8 points (median 6) were chosen to calculate the dose for each patient including the rectal point (RP), which is an author-defined rectal point modified from the definition of the rectal reference point in the ICRU report 38. The length of the measured rectum was 3-7 cm (median 5 cm). The bladder point (BP) dose was measured as recommended by the ICRU. The prescription doses to point A varied from 3.5 to 5 Gy (median 4 Gy). Paired comparisons were made on the individual patients by calculating the normalized mean doses of the RP, the maximal point (MP), and the longitudinal average (LA) with and without the rectal retractor. The doses to the bladder points (BP) were also calculated in parallel to the rectal points. The anterior rectal walls were displaced posteriorly after inserting the rectal retractor. In the tandem and ovoids group, the number of patients with a reduced dose in the RP, MP and LA were 14 (93.3%), 12 (80.0%) and 13 (86.7%), respectively. In the ovoids only group, the corresponding figures were 14 (93.3%), 14 (93.3%) and 14 (93.3%). In the tandem and ovoids group, the reduced dose in the RP, MP, and LA dose were 0.52 Gy (13.0%), 0.50 Gy (12.5%), and 0.39 Gy (9.8%), respectively (p < 0.05). In the ovoids only group, the RP, MP, and LA dose were reduced by 0.62 Gy (15.5%), 0.92 Gy (23.0%), and 0.54 Gy(13.5%), respectively (p < 0.05). There was no significant change in the bladder point doses when the rectal retractor was applied, although the mean BP dose were 0.27 Gy and 0.09 Gy lower for the tandem and ovoids group and for ovoids only group, respectively (p > 0.05). The mean RP, MP, and LA dose reduction rates of the patient subgroup where the RP dose was < 70% of the prescription dose were compared with the subgroup where the RP dose was > 70%. The effect of the rectal dose reduction was significant only in the subgroup of patients who received > 70% of the prescription dose (p < 0.05). The use of the rectal retractor was a simple and an effective method for reducing the rectal dose. It was also considered to be a highly reproducible method, which can replace the time-consuming vaginal gauze packing in HDR-ICR.