Purpose: To investigate the interfractional dose variation of a small-bowel displacement system (SBDS)-assisted intensity-modulated radiation therapy (IMRT) for the treatment of cervical cancer.
Methods: Four computed tomography (CT) scans were carried out in 10 patients who received radiotherapy for uterine cervical cancer. The initial CT was taken by use of the SBDS, before the beginning of radiotherapy, and 3 additional CT scans with the SBDS were done in subsequent weeks. IMRT was planned by use of the initial CT, and the subsequent images were fused with the initial CT set. Dose-volume histogram (DVH) changes of the targets (planning target volume [PTV] = clinical target volume [CTV] + 1.5 cm) and of the critical organs were evaluated after obtaining the volumes of each organ on 4 CT sets.
Results: No significant differences were found in PTV volumes. Changes on the DVH of the CTVs were not significant, whereas DVH changes of the PTVs at 40% to 100% of the prescription dose level were significant (V(90%); 2nd week: p = 0.0091, 3rd week: p = 0.0029, 4th week: p = 0.0050). The changes in the small-bowel volume included in the treatment field were significant. These were 119.5 cm3 (range, 26.9-251.0 cm3), 126 cm3 (range, 38.3-336 cm3), 161.9 cm3 (range, 37.7-294.6 cm3), and 149.1 cm3 (range, 38.6-277.8 cm3) at the 1st, 2nd, 3rd, and 4th weeks, respectively, and were significantly correlated with the DVH change in the small bowel, which were significant at the 3rd (V80%; p = 0.0230) and 4th (V80%; p = 0.0263) weeks. The bladder-volume change correlated to the large volume change (>20%) of the small-bowel volume.
Conclusions: Significant DVH differences for the small bowel can result because of interfractional position variations, whereas the DVH differences of the CTV were not significant. Strict bladder-filling control and an accurate margin for the PTV, as well as image-guided position verification, are important to achieve the goal of IMRT.