The use of MRI deformable image registration for CT-based brachytherapy in locally advanced cervical cancer

Purpose: Incorporation of MRI into image-based brachytherapy (IBBT) is limited by logistics, reimbursement, and workflow demands. Our goal is to determine if deformable image registration (DIR) using a preimplantation MRI is feasible to construct a high-risk target volume during IBBT.

Methods and materials: From 2010 to 2013, 20 patients were treated with high-dose-rate IBBT for cervical cancer. A preimplantation MRI was fused to the planning CT, and DIR was performed using MIM v6.1. The gross tumor volume (GTV) and high-risk clinical target volume were contoured on the MRI (HR-CTV MRI), and a separate high-risk clinical target volume was made from the deformable image registration of the preimplantation MRI to the planning CT (HR-CTV'). The treated target volume from the planning CT without the DIR or fusion (HR-CTV BT) was compared with the HR-CTV'. The geometric means of the GTV, HR-CTV MRI, HR-CTV', and HR-CTV BT were analyzed. Statistical analysis using Wilcoxon rank and analysis of variance were performed.

Results: There was a significant larger difference between the GTV and the HR-CTV MRI, HR-CTV', and HR-CTV BT (p < 0.0001). There was also a significant difference between the HR-CTV MRI vs. the HR-CTV BT (p < 0.040). There was no significant difference between the HR-CTV MRI and HR-CTV'. DIR was advantageous in the setting of residual disease pre-IBBT.

Conclusions: DIR is feasible to define an HR-CTV for MRI-guided, CT IBBT. The HR-CTV MRI predicted a smaller treatment volume in comparison with the HR-CTV BT. DIR is limited by patient anatomy and is most beneficial in patients with gross disease.