Improving Delineation of Gross Tumor Volume Using Magnetic Resonance Imaging for External Beam Radiation Therapy of Locally Advanced Cervical Cancer International Journal of Radiation Oncology, Biology, Physics POSTER VIEWING ABSTRACT| VOLUME 96, ISSUE 2, SUPPLEMENT , E303-E304, OCTOBER 01, 2016
Date
10/01/2016Abstract
Purpose/Objective(s)
To evaluate and compare the gross tumor volumes (GTVs) delineations use various MRI sequences, CT, and PET for the external beam radiation therapy (EBRT) planning for locally advanced cervical cancer.
Materials/Methods
MRI, CT and PET data acquired for 23 patients with locally advanced cervical cancer (staging from FIGO IB2 to IVA), using a 3 T large-bore MRI scanner, a large-bore CT scanner, and a PET/CT scanner, were analyzed. For each patient, the GTVs were delineated manually on CT (GTVCT) and four MRI sequences, including T1, T2, T1 with fat suppression and contrast, and ADC-DWI (GTVT1, GTVT2, GTVT1F+, and GTVADC). In addition, GTV contours on PET images were generated using the threshold of 40% of maximum SUV (GTV40%) as well as SUV of 2.5 (GTV2.5). MRI, CT, and PET were registered based on rigid-body registration, allowing a comparison of various GTVs from different image modalities. The GTVs defined by MRI sequences were compared to GTVT2, and the differences were measured with the overlap ratio (OR) and relative volume ratio (RVR) relative to GTVT2 (GTVT1/GTVT2, GTVADC/GTVT2, and GTVT1F+/GTVT2). The GTV created based on the union of the GTVT2 and GTVADC was referred to as the MRI-based GTV (GTVMRI).
Results
High soft tissue contrast with MRI reduces GTV delineation uncertainty as compared to CT and PET for cervical cancer. However, the GTVs defined by different imaging modalities are different. The average GTV volumes for T1, T2, ADC, and T1F+ were (in cm3) 62.8 ± 62.1, 58.4 ± 61.2, 50.9 ± 59.8, 66.4 ± 64.9, respectively. The differences between T2 and other MRI sequences are significant (P<0.05). The average ORs for GTVT1, GTVT1F+, and GTVADC related to GTVT2 were 86.3%, 81.6% and 61.6% while the corresponding average RVRs were 113.8%, 112.3% and 77.2%, respectively. There is no significant difference between GTVT1 and GTVT1F+ and these two are considered equivalent. GTVT1 is generally larger than GTVT2, which is mainly due to the lower soft tissue contrast at the tumor boundary in T1 compared to T2. In contrast, GTVADC is generally smaller than GTVT2 and encompasses suspicious regions that are sometimes not revealed by T2 because of different imaging mechanisms. The uncovered volume accounts for up to 16% of the GTVT2. In this sense, the MRI-based GTV (GTVMRI, generated from the union of the GTVT2 and GTVADC) may be more appropriate than individual GTVT2 and GTVADC. There was a significant difference between GTVMRI, GTV2.5, GTV40%, and GTVCT in volumes. On average, GTVMRI is 18.4% smaller than GTVCT for the cases studied.
Conclusion
MRI provides improved visualization of disease than CT or PET for cervical cancer. Among the image modalities studied, the combination of T2 and ADC provided a reasonable GTV definition without including excessive surrounding normal tissues. The GTVMRI generated based on the union of T2 and ADC is substantially smaller than conventional CT defined GTVCT. Further studies are required to confirm that the use of GTVMRI for EBRT of cervical cancer is appropriate.
Author List
Y. Song X. Chen B.A. Erickson A. LiAuthor
Beth A. Erickson MD Professor in the Radiation Oncology department at Medical College of WisconsinView Online
