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 Gy(alpha beta 3) for bladder, <= 75 Gy(alpha beta 3) for rectum and sigmoid colon) and to increase the HR-CTV dose to D90 >= 85 Gy(alpha beta 10). Results: In the X-ray based plans, D-2cc 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 Gy(alpha beta 10) was acceptable. D90 of the HR-CTV was below 85 Gy(alpha beta 10) in 13 out of 16 patients. After optimisation, the dose constraints in the OAR were not exceeded anymore in any patient. The average D-2cc dose reduction was 7 +/- 6 Gy(alpha beta 3) in the bladder and 7 +/- 4 Gy(alpha beta 3) in the sigmoid colon for those patients in which the dose constraint was initially exceeded. In addition, an average dose increase of 3 Gy(alpha beta 10) 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. (C) 2007 Elsevier Ireland Ltd. All rights reserved.