Value of Magnetic Resonance Imaging Without or With Applicator in Place for Target Definition in Cervix Cancer Brachytherapy

Purpose: To define, in the setting of cervical cancer, to what extent information from additional pretreatment magnetic resonance imaging (MRI) without the brachytherapy applicator improves conformity of CT-based high-risk clinical target volume (CTV_HR) contours, compared with the MRI for various tumor stages (International Federation of Gynecology and Obstetrics [FIGO] stages I-IVA). Methods and Materials: The CTV_HR was contoured in 39 patients with cervical cancer (FIGO stages I-IVA) (1) on CT images based on clinical information (CTV_HR-CT_Clinical) alone; and (2) using an additional MRI before brachytherapy, without the applicator (CTV_HR-CT_{pre-BT MRI}). The CT contours were compared with reference contours on MRI with the applicator in place (CTV_HR-MRI_ref). Width, height, thickness, volumes, and topography were analyzed. Results: The CT-MRI_ref differences hardly varied in stage I tumors (n=8). In limited-volume stage IIB and IIIB tumors (n=19), CTV_HR-CT_{pre-BT MRI}–MRI_ref volume differences (2.6 cm³ [IIB], 7.3 cm³ [IIIB]) were superior to CTV_HR-CT_Clinical–MRI_ref (11.8 cm³ [IIB], 22.9 cm³ [IIIB]), owing to significant improvement of height and width (P<.05). In advanced disease (n=12), improved agreement with MR volume, width, and height was achieved for CTV_HR-CT_{pre-BT MRI}. In 5 of 12 cases, MRI_ref contours were partly missed on CT. Conclusions: Pre-BT MRI helps to define CTV_HR before BT implantation appropriately, if only CT images with the applicator in place are available for BT planning. Significant improvement is achievable in limited-volume stage IIB and IIIB tumors. In more advanced disease (extensive IIB to IVA), improvement of conformity is possible but may be associated with geographic misses. Limited impact on precision of CTV_HR-CT is expected in stage IB tumors.