Background: CT based brachytherapy allows 3-dimensional (3D) assessment of organs at risk (OAR) doses with dose volume histograms (DVHs). The purpose of this study was to compare computed tomography (CT) based volumetric calculations and International Commission on Radiation Units and Measurements (ICRU) reference-point estimates of radiation doses to the bladder and rectum in patients with carcinoma of the cervix treated with high-dose-rate (HDR) intracavitary brachytherapy (ICBT). Materials and
Methods: Between March2011 and May 2012, 20 patients were treated with 55 fractions of brachytherapy using tandem and ovoids and underwent post-implant CT scans. The external beam radiotherapy (EBRT) dose was 48.6Gy in 27 fractions. HDR brachytherapy was delivered to a dose of 21 Gy in three fractions. The ICRU bladder and rectum point doses along with 4 additional rectal points were recorded. The maximum dose (DMax) to rectum was the highest recorded dose at one of these five points. Using the HDRplus 2.6 brachyhtherapy treatment planning system, the bladder and rectum were retrospectively contoured on the 55 CT datasets. The DVHs for rectum and bladder were calculated and the minimum doses to the highest irradiated 2cc area of rectum and bladder were recorded (D2cc) for all individual fractions. The mean D2cc of rectum was compared to the means of ICRU rectal point and rectal DMax using the Student’s t-test. The mean D2cc of bladder was compared with the mean ICRU bladder point using the same statistical test .The total dose, combining EBRT and HDR brachytherapy, were biologically normalized to the conventional 2 Gy/fraction using the linear-quadratic model. (α/β value of 10 Gy for target, 3 Gy for organs at risk).
Results: The total prescribed dose was 77.5 Gyα/β10. The mean dose to the rectum was 4.58±1.22 Gy for D2cc, 3.76±0.65 Gy at DICRU and 4.75±1.01 Gy at DMax. The mean rectal D2cc dose differed significantly from the mean dose calculated at the ICRU reference point (p<0.005); the mean difference was 0.82 Gy (0.48 -1.19Gy). The mean EQD2 was 68.52±7.24 Gyα/β3 for D2cc, 61.71±2.77 Gyα/β3 at DICRU and 69.24±6.02Gyα/β3 at DMax. The mean ratio of D2cc rectum to DICRU rectum was 1.25 and the mean ratio of D2cc rectum to DMax rectum was 0.98 for all individual fractions. The mean dose to the bladder was 6.00±1.90 Gy for D2cc and 5.10±2.03 Gy at DICRU. However, the mean D2cc dose did not differ significantly from the mean dose calculated at the ICRU reference point (p=0.307); the mean difference was 0.90 Gy (0.49-1.25Gy). The mean EQD2 was 81.85±13.03 Gyα/β3 for D2cc and 74.11±19.39 Gyα/β3 at DICRU. The mean ratio of D2cc bladder to DICRU bladder was 1.24. In the majority of applications, the maximum dose point was not the ICRU point. On average, the rectum received 77% and bladder received 92% of the prescribed dose.
Conclusions: OARs doses assessed by DVH criteria were higher than ICRU point doses. Our data suggest that the estimated dose to the ICRU bladder pointmay be a reasonable surrogate for the D2cc and rectal DMax for D2cc. However, the dose to the ICRU rectal point does not appear to be a reasonable surrogate for the D2cc.