Objective To investigate and compare the gross tumor volumes (GTVs) delineated by MRI and CT images in patients with cervical cancer who received 3D intracavitary brachytherapy.

Methods A total of 12 patients, aged 50 –64 years, who were confirmed to have cervical squamous cell carcinoma or adenocarcinoma by biopsy and pathology between September 2012 and June 2016 were selected in the 900 Hospital of the Joint Logistics Team. The tumor was staged during the second and third phases. All the patients underwent pelvic MRI which was administered through external beam radiation followed by a 3D CT-based high dose rate (HDR) intracavitary brachytherapy boost. After radiotherapy CT localization, the CT and MRI images were fused. Next, in the OMP system, three experienced radiologists (radiologists 1, 2, and 3) respectively delineated the GTVs, which included the cervix uterus, corpus uteri, and primary lesion. Thirty-six target outlines were drawn based on the CT images (called GTV_CT), and another 36 target outlines were drawn based on the MRI images (called GTV_MRI). The ratio R of GTV_MRI and GTV_CT were calculated using the paired t-test to compare the difference between the two groups.

Results The average target volume of GTV_CT (93.8±11.8) cm³ was larger than that of GTV_MRI (71.1±3.1) cm³. Some variations were observed among the definition results given by the three radiologists. The differences between the maximum volume and the minimum volume were 41.3 cm³ and 11.3 cm³ for GTV_CT and GTV_MRI, respectively. Compared with Radiologist 3, Radiologists 1 and 2 outlined that the volume of GTV is larger, and the difference between the ratios R is statistically significant (t =−8.644, −5.043, both P = 0.000). The difference in target volumes between the radiologists guided by MRI images was smaller than that by the radiologist guided by CT images and was more slightly affected by individual factors.

Conclusions For patients with cervical cancer who are administered external beam radiation followed by 3D CT-based HDR brachytherapy boost, variations in GTVs were observed among different radiologists for individual factors. The CT/MRI-fusion technology may decrease the variation in target volume between different radiologists.