MRI-guided treatment-planning optimisation in intracavitary or combined intracavitary/interstitial PDR brachytherapy using tandem ovoid applicators in locally advanced cervical cancer.

Objective: To study the impact of MRI-guided treatment planning on dose/volume parameters in pulsed dose rate (PDR) brachytherapy (BT) for cervical cancer. Additionally, we investigated the potential benefit of an intracavitary/interstitial (IC/IS) modification of the classical tandem ovoid applicator.

Methods: For 24 patients we compared Standard PDR BT plans, Scaled Standard plans and MRI-guided Optimised plans. The total EBRT/BT prescribed dose to Manchester point A or to 90% of the HR-CTV (D90 HR-CTV) expressed in EQD(2) was 80 Gy(alphabeta10) in 17 patients (Period I) and 84 Gy(alphabeta10) in 7 patients (Period II). The constraints to 2 cm(3) of the OAR were 90 Gy(alphabeta3) for bladder and 75 Gy(alphabeta3) for rectum, sigmoid and bowel. Most cases were treated with a traditional intracavitary tandem ovoid applicator. In 6 patients we used a newly designed combined IC/IS modification for the second PDR fraction and investigated the benefit of the interstitial part.

Results: The average gain of MRI-guided optimisation expressed in D90 HR-CTV was 4+/-9 Gy(alphabeta10) (p<0.001) and 10+/-7 Gy(alphabeta10) (p=0.003) in the two periods. The dose to 2 cm(3) of the OAR met the constraints. In the group that was treated with the combined IC/IS approach, we could increase the D90 HR-CTV for the second PDR fraction with 5.4+/-4.2 Gy(alphabeta10) (p=0.005) and the D100 with 4.8+/-3.1 Gy(alphabeta10) (p=0.07).

Conclusions: Three-dimensional MRI-guided treatment planning and optimisation improves the DVH parameters compared to conventional planning strategies. Additional improvement can be achieved by using a combined IC/IS approach.