The use of MRI in planning radiotherapy for gynaecological tumours.

Intracavitary brachytherapy plays a crucial role in the management of invasive cervical cancer or in the management of inoperable endometrial carcinoma. Recently, 3D treatment planning systems have been increasingly used in brachytherapy facilities. CT and/or MRI compatible applicators allow a sectional image-based approach with a better assessment of GTV and definition and delineation of CTV compared to traditional approaches ( Fig. 5 ). CT-based methods accurately localise intracavitary applicators and demonstrate the 3D anatomical relationship between the applicators and neighbouring structures, thereby obtaining the dose delivered to the tumour volume and surrounding organs. But again, CT images have significant limitations when visualising the tumour volume, especially with regard to the applicator in the vagina. First reports on the use of MR images for brachytherapy treatment planning of cervical cancer were published in 1992 when Schoeppel et al. discussed the problem of tumour delineation on MR images after external irradiation . While tumours of the cervix typically demonstrate increased signal intensity on T2-weighted images this can be changed by tumour necrosis, increased oedema and tumour shrinking. MRI, which became available for treatment planning in 1998, was at that time systematically introduced into daily clinical practice in the brachytherapy department of the university hospital of Vienna. From their considerable experience in this area we have learned that MRI provides information for precise topographic definition and delineation of patho-anatomical structures and organs at risk in relation to the applicator in more than 90%. We have also learned that with MRI we are able to visualise the tumour volume covered insufficiently by the dose distribution much better than with CT . During the last 5 years, concepts and parameters have been prospectively developed for brachytherapy treatment planning. Because MRI provides superior soft tissue resolution compared to CT and because electron density does not play a role in dose calculation in brachytherapy, the Gynaecological (GYN) GEC-ESTRO Working Group and the American Image-Guided Brachytherapy Working Group have proposed that T2-weighted MRI be used for imaging using a pelvic surface coil with image-compatible brachytherapy applicators in place for cervical implants. Since 2000, these two groups have been working on recommendations for recording and reporting 3D image-based brachytherapy (BT) for cervical cancer . The recommendations on definition and delineation of GTV and CTV are based on clinical experience and different dosimetric concepts. Development of 3D image-based treatment planning includes a comprehensive approach with systematic description of GTV and topography at diagnosis and at time of BT, taking into account its evolution over time. The full potential of individually defining and evaluating CTV based on tumour extent and anatomical structures has been exploited. According to the GYN ESTRO Group the GTV for BT (GTVB) includes macroscopic tumour extension at the time of BT as detected by clinical examination and as visualised on MRI (high signal intensity mass(es) (FSE, T2) in cervix/corpus, parametria, vagina, bladder and rectum). High-risk CTV for BT (HR CTV) carrying a high tumour load includes GTVB, always the whole cervix, and the presumed extracervical tumour extension at the time of BT defined by means of clinical examination (visualisation and palpation) and/or residual grey zones in parametria, uterine corpus, vagina or rectum and bladder on MRI. No safety margins are added. Intermediate-risk CTV for BT (IR CTV) carrying a significant microscopic tumour load encompasses HR CTV with a safety margin of 5–15 mm ( Fig. 6 ). The amount of the safety margin is chosen according to tumour size and location, potential tumour spread, tumour regression and treatment strategy .

In the light of present knowledge, individual 3D imaging-based treatment planning for gynaecological malignancies is necessary to avoid geographical misses. In 2006, there is no getting away from CT planning as CT gives the electronic density of the tissues, which is essential to ensure a precise calculation of dose distribution. Beam’s eye view-based 3D treatment planning for external beam therapy enables a reduction of bowel and bladder volumes receiving more than 70% of the prescribed dose and, additionally, an adequate coverage of the planning target volume. The use of MRI, which provides us with direct and accurate images of tumours, taking into account the real spatial relationship between tumour and normal anatomy, undoubtedly improves the accuracy of the drawing of the CTV. At the very least, diagnostic MRI should be systematically used, but dosimetric MRI in the treatment position adds more accurate information. Therefore, the process of CT-MRI image registration should be more widely used. Recent possibilities offered by MRI technology are promising in the area of brachytherapy planning as the full potential of individually defining and evaluating CTV based on tumour extent and anatomical structures is exploited.

The effectiveness of radiation therapy in the management of cervical cancer of all stages is well established. Radiation therapy usually consists of a combination of external beam therapy and brachytherapy. At present, radiotherapy is also the only adjuvant treatment that is really effective in improving the results of local control of endometrial carcinoma with poor prognosis factors on surgical specimen. Patients non-operable for medical reasons or patients with unresectable cancer are benefiting from radiotherapy alone. Parameters that significantly influence results in radiation treatment, especially for carcinoma of the cervix, are related to tumour size, tumour extension and the radiotherapy technique used. The endpoint of radiation technique is tumour control obtained with the minimum amount of toxicity. For external beam therapy, the four-field box technique has long been a standard technique. Conventional radiation fields were designed according to bony landmarks. Because of lateral ports, the technique offered the possibility to shield the rectum and small bowel and to reduce the dose to organs at risk. However, without knowledge of the precise tumour volume it was potentially dangerous because of the risk of geographical miss of the tumour. Many observations indicating potential inadequate coverage of tumour volume have been substantiated by investigators using lymphangiography, computed tomography (CT) or magnetic resonance imaging (MRI) . The aim of this paper is to describe the impact of MRI on the planning of radiotherapy before and after the availability of 3D imaging-based treatment planning systems.