The impact of sectional imaging on dose escalation in endocavitary HDR-brachytherapy of cervical cancer: results of a prospective comparative trial

Purpose: The purpose of this comparative prospective study was to assess the effect of CT and MR based individualisation and adaptation on the dose distribution in the target volume and organs at risk compared to a radiography based procedure. Material and methods: In 15 patients MR scans, in 10 patients additional axial CT-scans with compatible tube-ring applicator in situ were performed and digitally transferred to the PLATO[reg] planning system. Considering clinical examination and MR-scan before radiotherapy individual 3-D dose distribution was calculated and adapted based on (1) two orthogonal radiographs; (2) isodoses superimposed on the CT images; and (3) isodoses superimposed on the MR images. Adaptation was strictly limited by the dose level at 2 cm3 bladder or rectum volume (D2) to allow comparison of CT and MR plans. All three individualised dose distributions were superimposed on the MR images and cumulative dose-volume histograms were calculated for comparison. Results: 3-D individualisation based on sectional imaging enabled higher dose to the target volume (isodose enclosing 95% of the CTV=CTV95) compared to individualised treatment plans based on orthogonal radiographs by a mean factor of 1.2 (1-1.7). The dose to bladder and/or rectum wall was at the same time not increased beyond the prescribed tolerance level (71% of the prescribed target dose). In a subgroup of 10 patients MRI based treatment plans were superior to CT based treatment plans allowing for a higher dose (138% vs. 124%). Conclusion: Sectional imaging based treatment planning, in particular using MR, was superior to radiography allowing for a clinical meaningful dose escalation without increasing the dose to bladder and rectum beyond the tolerance level