52 speaker MRI-BASED BRACHYTHERAPY IN CERVIX CANCER: WHERE ARE WE NOW?

Abstract

Image guided adaptive brachytherapy (IGABT) of cervix cancer enables individualized irradiation, applying high doses to target volume while respecting organs at risk (OAR) dose constraints. Clear understanding of size and topography of the tumor, OAR and their relations to the applicator is a precondition for IGABT success. Small inconsistencies in definition of these structures may result in significant uncertainties of optimized dose distribution. This may compromise treatment outcome, recording and reporting. One of the most important sources of uncertainties is the choice of imaging modality. Various modalities have been employed in gynaecological IGABT. While US plays an important role in guiding the insertion of BT catheters, it is used only to a limited extent for contouring. As far as CT and MRI are concerned, variations in tissue characteristics that influence MR signal are more pronounced than differences in X-ray attenuation coefficients. MRI therefore exhibits superior soft tissue depiction quality than CT, which is further enhanced by its capability of multiplanar imaging. In the 1990’s, early experience with MRI in BT planning demonstrated its superiority to CT for evaluation of tumor extent, topography and relations to the applicator [Schoeppel, IJROBP 1992;23(1), Tardivon, Radiographics 1996;16(6), Ptter, Selectr Brachyther J 1991;5(3)]. A systematic analysis of potential of MRI in cervix cancer IGABT demonstrated that it enables accurate definition of the regions of interest. [Dimopoulos, IJROBP 2006;64]. While CT is adequate for OAR delinieation, its value in target contouring is limited [Viswanathan, IJROBP 2007;68]. MRI has been recommended for GTV, CTV and OAR contouring and can be considered current gold standard in cervix cancer IGABT [Haie Meder, Radiother Oncol 2005;74, Ptter, Radiother Oncol 2006;78, Nag, IJROBP 2004;60(4)]. Favourable dosimetric reports on this approach are reflected in encouraging clinical outcomes. Results of the ongoing prospective EMBRACE study are awaited with enthusiasm (www.embracestudy.dk). Nevertheless, high cost and limited access to MRI preclude its early and widespread adoption. In a patterns of care study for BT in Europe in 2002, the use of MRI-based planning was reported by 4.5 and 5 % of centres from European countries with high economic wealth and new European countries, respectively. In other countries, MRI-based dosimetry was not used at that time [Guedea, Radiother Oncol 2007;82(1)]. In 2007, we witnessed an increase of the use of 3D imaging, in particular CT, for BT treatment planning in Europe [Guedea, Radiother Oncol 2010;97]. In a survey within the ABS, MRI was used as postinsertion imaging at 2% of centres [Viswanathan, IJROBP 2010;76(1)]. In spite of favourable reports on MRI-based cervix cancer IGABT, its widespread implementation therefore remains impeded by limited resources, especially in the countries of central and eastern Europe and the developing world. Strategies to improve availability of MRI or develop technological solutions and recommendations regarding its combination with lower cost modalities such as CT and US, are therefore essential for the future of cervix cancer IGABT.