Applicator reconstruction for HDR cervix treatment planning using images from 0.35 T open MR scanner

Magnetic resonance (MR) imaging is widely recognised as the modality of choice for imaging soft tissue such as the target volume and critical structures relevant to high dose rate (HDR) brachytherapy of the cervix. This work sets out to assess some of the issues faced when introducing this technique clinically compared to the more widely used computed tomography (CT). MR can be used either as the sole imaging modality, or in conjunction with CT.

Distortion of the images produced by the MR scanner was assessed with a geometrical phantom. Distortion local to the titanium applicators, introduced by the susceptibility of the applicators themselves, was also measured. The technique used to reconstruct applicators is briefly described. An inter-operator study was performed to assess the variability of applicator reconstruction between operators when MR images are used alone to reconstruct the applicators.

A 14-cm cube within which distortion was less than 2mm at all points was identified. The inter-operator study showed some variability in applicator reconstruction with both MR and CT (median MR/CT 1.3mm/0.9 mm, range 0-3.6mm/0-3.3mm). Inter-operator variation in planning target volume (PTV) V 100% and PTV D 90% for MR/CT was 6.1%/3.0% and 7.4%/6.3%, respectively, and D(2cc) OAR doses varied by up to 1.0 Gy between operators for both MR and CT.

In this study distortion was minimal within a defined volume and inter-observer errors were comparable on MR and CT when used to define applicators and when applied to dose-volume histograms (DVHs). However this does not assure the technique is appropriate with other scanners and applicator sets without further commissioning.