Comparison between the ICRU rectal point and modern volumetric parameters in brachytherapy for locally advanced cervical cancer

Abstract

PurposeThe implementation of image-guided brachytherapy in cervical cancer raises the problem of adapting the experience acquired with 2D brachytherapy to this technique. The GEC-ESTRO (Groupe européen de curiethérapie – European Society for Radiotherapy and Oncology) has recommended reporting the dose delivered to the rectum in the maximally exposed 2cm3 volume, but so far, the recommended dose constraints still rely on 2D data. The aim of this study was to evaluate the relationship between the doses evaluated at the ICRU rectal point and modern dosimetric parameters. Material and methodsFor each patient, dosimetric parameters were generated prospectively at the time of dosimetry and were reported. For analysis, they were converted in 2Gy equivalent doses using an α/β ratio of 3 with a half-time of repair of 1.5hours. ResultsThe dosimetric data from 229 consecutive patients treated for locally advanced cervical cancer was analyzed. The mean dose calculated at ICRU point (DICRU) was 55.75Gy±4.15, while it was 59.27Gy±6.16 in the maximally exposed 2 cm3 of the rectum (P=0.0003). The D2cm3 was higher than the DICRU in 78% of the cases. The mean difference between D2cm3 and DICRU was 3.53Gy±4.91. This difference represented 5.41%±7.40 of the total dose delivered to the rectum (EBRT and BT), and 15.49%±24.30 of the dose delivered when considering brachytherapy alone. The two parameters were significantly correlated (P=0.000001), and related by the equation: D2cm3=0.902×DICRU+0.984. The r2 coefficient was 0.369. ConclusionIn this large cohort of patients, the DICRU significantly underestimates the D2 cm3. This difference probably results from the optimization process itself, which consists in increasing dwell times above the ICRU point in the cervix. Considering these findings, caution must be taken while implementing image-guided brachytherapy and dose escalation.