Recommendations from Gynaecological (GYN) GEC-ESTRO Working Group (IV): Basic principles and parameters for MR imaging within the frame of image based adaptive cervix cancer brachytherapy

Abstract

The GYN GEC-ESTRO working group issued three parts of recommendations and highlighted the pivotal role of MRI for the successful implementation of 3D image-based cervical cancer brachytherapy (BT). The main advantage of MRI as an imaging modality is its superior soft tissue depiction quality. To exploit the full potential of MRI for the better ability of the radiation oncologist to make the appropriate choice for the BT application technique and to accurately define the target volumes and the organs at risk, certain MR imaging criteria have to be fulfilled. Technical requirements, patient preparation, as well as image acquisition protocols have to be tailored to the needs of 3D image-based BT. The present recommendation is focused on the general principles of MR imaging for 3D image-based BT. Methods and parameters have been developed and progressively validated from clinical experience from different institutions (IGR, Universities of Vienna, Leuven, Aarhus and Ljubljana) and successfully applied during expert meetings, contouring workshops, as well as within clinical and interobserver studies. It is useful to perform pelvic MRI scanning prior to radiotherapy ("Pre-RT-MRI examination") and at the time of BT ("BT MRI examination") with one MR imager. Both low and high-field imagers, as well as both open and close magnet configurations conform to the requirements of 3D image-based cervical cancer BT. Multiplanar (transversal, sagittal, coronal and oblique image orientation) T2-weighted images obtained with pelvic surface coils are considered as the golden standard for visualisation of the tumour and the critical organs. The use of complementary MRI sequences (e.g. contrast-enhanced T1-weighted or 3D isotropic MRI sequences) is optional. Patient preparation has to be adapted to the needs of BT intervention and MR imaging. It is recommended to visualise and interpret the MR images on dedicated DICOM-viewer workstations, which should also assist the contouring procedure. Choice of imaging parameters and BT equipment is made after taking into account aspects of interaction between imaging and applicator reconstruction, as well as those between imaging, geometry and dose calculation. In a prospective clinical context, to implement 3D image-based cervical cancer brachytherapy and to take advantage of its full potential, it is essential to successfully meet the MR imaging criteria described in the present recommendations of the GYN GEC-ESTRO working group.

Fig. 1

Effect of antispasmodic drug administration on MR image quality: small bowel peristalsis causes movement artefacts in MRI (a). A spasmolytic agent (e.g. N-Butylscopolan or Glucagon chlorhydrate) is therefore commonly administered intravenously to inhibit bowel motion shortly before performing pelvic MRI (b). In (a) the sagittal T2w MR image is blurred due to small bowel motion and due to uterine contraction. In (b) the sagittal T2w MR image of the same patient appears with significant improvement of image quality since it is obtained after injection of Glucagon chlorhydrate.

Fig. 2

MR image plane orientation and coverage of T2w pelvic scanning for assessment of dynamic tumour response during radiochemotherapy of a cervix cancer patient with bulky IIB disease: (a–c) demonstrate the “Pre-RT MRI examination” with para-axial, sagittal and para-coronal slice orientation, respectively. The MR image plane orientation remained the same for “4th week EBRT MRI examination” (d–f) and for “BT MRI examination” (g–i). “4th week EBRT MRI examination”, was performed to illustrate tumour response during EBRT in this particular patient. Such repetitive MRI is not necessarily required for the performance of MR image guided cervix cancer brachytherapy, however clinical repetitive examination is mandatory with documentation on 3D clinical drawing. Para-axial, sagittal and para-coronal slice orientation is selected on sagittal, coronal and sagittal scout views, respectively. Para-axial slices are orientated perpendicular to the long axis of the cervical canal. Sagittal and para-coronal slices are orientated parallel to the long axis of the cervical canal. The coverage which should be obtained is described in detail in the text. The “Pre-RT MRI examination” demonstrates the bulky IIB tumour which is mainly located in the anterior part of the cervix and invades both parametria (right > left). During EBRT the tumour is shrinking significantly, the remnants are remaining mainly located in the anterior part of the cervix and both parametria are still invaded. On “BT MRI examination” which is obtained after applicator insertion additional shrinkage of the tumour is observed and residual gross tumour, as well as grey zones are restricted to the right anterior parts of the cervix and the inner part of the right parametria.

Fig. 3

Differences resulting due to different magnet field strength: comparison between T2w sagittal MR images obtained with a high-field MR scanner (1.5 T (a)) and a low-field MR scanner (0.2 T (b)). Prior to imaging intravaginal contrast (ultrasound gel) was injected in order to distend the vaginal walls and to improve visualisation of vaginal tumour extension. The impact of magnet field strength on signal intensity of tumour and intravaginal contrast is significant. The high field MR images depict the tumour with intermediate-to-high signal intensity and the low-field images of the same patient with high signal intensity.