A Prospective Comparison of Computed Tomography with Transrectal Ultrasonography Assistance and Magnetic Resonance Imaging-Based Target-Volume Definition During Image Guided Adaptive Brachytherapy for Cervical Cancers.

Purpose: Although magnetic resonance imaging (MRI) represents the gold standard for image guided adaptive brachytherapy (IGABT) for cervical cancer, the majority of brachytherapy (BT) continues to be guided by computed tomography (CT). However, CT seems to overestimate the target-volume definition, and the potential of transrectal ultrasonography (TRUS) needs further evaluation. This prospective, comparative study aimed to evaluate CT-based target contouring with the incorporation of TRUS during BT.

Methods And Materials: Patients with locally advanced cervical cancer undergoing magnetic resonance IGABT between January 2013 and March 2014 were included. During the BT procedure, TRUS imaging with central tandem in situ was acquired at 3 representative levels. Reference points/dimensions (D1-D4) of the hypoechoic region on TRUS images with respect to the central tandem were recorded. CT and magnetic resonance BT planning imaging was performed after BT application. The high-risk clinical target volume (HR-CTV) was contoured on CT scans with incorporation of clinical and TRUS imaging findings and was compared with the gold standard MRI-based target approach.

Results: The image sets of 25 patients (International Federation of Gynecology and Obstetrics clinical stage IIB [11; 44%] and IIIB [14; 56%]) were evaluable. The mean (±standard deviation) volumes of HR-CTV on CT and MRI imaging were 39.1 (±20) cm³ and 39 (±19) cm³, respectively (r = 0.92; P < .001) A significant correlation was found between the HR-CTV dimensions (width and thickness) of CT and MRI scans at various levels (r = 0.70-0.80; P < .001) In addition, the absolute differences in target dimensions between CT and MRI were <0.5 cm. A strong correlation was seen between CT and MRI for patients with medial and lateral parametrial invasion (P < .05) compared with no parametrial disease at BT. Furthermore, the mean differences in HR-CTV width between CT and MRI contours at various levels, irrespective of parametrial involvement, was only 0.1 to 0.4 cm.

Conclusions: This study suggests that a CT-based target and organ-at-risk delineation using MRI at diagnosis and real-time TRUS information during BT seems comparable with the gold standard MRI-based approach in IGABT for cervical cancer.