The use of MRI in planning radiotherapy for gynaecological tumours

Parameters that significantly influence results in radiation treatment of gynaecological malignancies are mainly related to the tumour characteristics and the radiotherapy technique used. High-dose radiotherapy requires accurate localisation of the tumour volume and its relationship to surrounding normal tissues. For many years the standard technique used for irradiation of the pelvic area was the four-field box technique which offered the potential benefit of the lateral fields to shield the rectum and small bowel. However, this conventional technique was designed according to bony landmarks and offered limited information regarding the topography of the tumour and the flexion of the uterus which are influenced by the tumour burden and bladder and rectal filling. CT and MRI enable the visualisation of the cervix, uterus, vagina, iliac vessels and organs at risk, but MRI allows tumour depiction in all planes. In the early 1990s, several studies reported on the value of pelvic MRI in designing the lateral fields of the box technique. They demonstrated that conventional lateral portals would have resulted in a marginal tumour miss and incomplete coverage of the uterine fundus in more than 50% of cases, thus leading to the conclusion that if a box technique is used its design should be based on sagittal MRI. CT-based 3D planning systems are now routinely used in the vast majority of radiotherapy departments. Target volumes and organs at risk are delineated by the physician on each CT slice in order to conform the radiotherapy fields to the tumour volume. For several reasons, such as distortion and lack of electron density which is essential for dose calculation, the implementation of MRI into radiation treatment planning has its limitations. However, MRI can still be used if planning systems integrate tools for CT/MR image registration. There is little experience in the literature for gynaecological malignancies demonstrating that image fusion allows an improvement of the definition of the target and the organ at risk compared to CT alone. Only a few papers in the literature report on the use of CT/MR image registration in planning the external irradiation of gynaecological tumours. Most demonstrate feasibility, but they fail to quantify the improvement for volume definition compared to the use of CT alone. Finally, recent possibilities offered by MRI technology are promising in the area of brachytherapy planning as the full potential of individually defining and evaluating GTV and CTV based on tumour extent and anatomical structures is exploited.