Potential of dose optimisation in MRI-based PDR brachytherapy of cervix carcinoma

Background and purpose: In this study on PDR treatment planning of utero-vaginal carcinoma, we analysed the dosimetry of traditional X-ray based plans as it presents on MR images. The potential gain of MRI-based dose optimisation was assessed. Patients and methods: Sixteen patients boosted with PDR brachytherapy after external beam therapy were included. The clinical X-ray based plans were projected on MR images. The GTV, HR-CTV and IR-CTV were retrospectively contoured, as well as the bladder, rectum and sigmoid colon. The dose in the critical organs and target coverage was investigated. In a second phase, the plans were manually optimised using the MR information. The objectives were to lower the dose in the critical organs (≤85 Gyαβ3 for bladder, ≤75 Gyαβ3 for rectum and sigmoid colon) and to increase the HR-CTV dose to D90 ≥ 85 Gyαβ10. Results: In the X-ray based plans, D2cc in bladder and sigmoid colon exceeded the tolerance doses in 10/16 and 7/16 patients, respectively. Coverage of the IR-CTV with the 60 Gyαβ10 was acceptable. D90 of the HR-CTV was below 85 Gyαβ10 in 13 out of 16 patients. After optimisation, the dose constraints in the OAR were not exceeded anymore in any patient. The average D2cc dose reduction was 7 ± 6 Gyαβ3 in the bladder and 7 ± 4 Gyαβ3 in the sigmoid colon for those patients in which the dose constraint was initially exceeded. In addition, an average dose increase of 3 Gyαβ10 was accomplished in the HR-CTV. Conclusions: MRI-based dose optimisation can play an important role to reduce the dose delivered to the critical organs and to improve target coverage