A Dosimetric Comparison of Pelvic Lymph Node Volume Delineation Using Magnetic Resonance Imaging Versus Computed Tomography in Gynecological Cancers

Abstract

Definitive external beam radiotherapy (EBRT) targets the primary tumor and at-risk pelvic lymph node groups. Magnetic resonance imaging (MRI), with its superior soft tissue definition compared with computed tomography (CT), is routinely used in cervical cancer EBRT planning to accurately define the primary tumor. MRI can also provide more accurate visualization of lymph nodes, vessels and tissue planes, such that MRI-based nodal clinical target volume (CTVn) delineation may result in smaller volumes and reduced dose to organs at risk (OAR). The aim of this study was to dosimetrically compare EBRT plans in patients with CTVn delineation using MRI fused with CT (CT-MRI) and CT alone. This study was important to examine the advantage of MRI in radiotherapy; prior to the addition of a new MRI simulator and MRI Linac in our center. Nine patients with cervical cancer and two patients with recurrent endometrial cancer were treated using definitive EBRT at our institution with diagnostic MRI scans and were included in this study. The planning CT (pCT) scan for each patient was obtained and fused with T2-weighted MRI scan using bony landmarks and deformable registration. Two separate CTVn volumes were contoured by a radiation oncologist, one based on pCT and a second based on MRI. All other structures including primary CTV (CTVp) and OAR (rectum, sigmoid, bowel, bladder, and femurs) were contoured on the pCT only. Planning target volume (PTV) contours were created by combining CTVn and CTVp with a 1 cm margin. Volumetric modulated arc therapy (VMAT) plans were generated for both pCT-based PTVs and CT-MRI-based PTVs using institutional protocol. Plans were evaluated for PTV volume, coverage (D98) and dose to OAR. Differences between pCT and CT-MRI were analyzed using paired t-tests and results are presented as means and SD (standard deviation). Twenty-two VMAT plans were generated. The mean (SD) PTV volume was 1485.9 cm³ (171.7) for pCT and 1424.5 cm³ (168.0) for the CT-MRI plan (p<0.001). The mean (SD) PTV D98 for pCT and CT-MRI was the same at 43.1 Gy (0.3) (p = 0.677). There was no significant dosimetric difference noted to any of the OAR (Table). This novel method for reducing the CTVn volume by using MRI to avoid anatomical boundaries such as fascial planes and bowel produced an overall reduction in the target volume. However, this did not result in a dosimetric advantage to the surrounding OAR. The most clinically relevant benefit of MRI in gynecological EBRT planning remains in the accurate definition of the primary tumor.Tabled 1Abstract 3080; Table;Organ at RiskpCT, Mean (SD)CT-MRI, Mean (SD)p-valueRectum (V40)56.9% (5.2)49.8% (11.8)0.111Sigmoid (V40)82.2% (20.6)78.7% (28.7)0.588Sigmoid (V45)27.5% (13.9)26.5% (17.8)0.787Bowel (V40)197.8cm³ (97.8)210.8cm³ (103.5)0.605Bowel (V45)73.3cm³ (43.1)67.3cm³ (45.9)0.664Bladder (V45)22.5% (4.1)24.5% (5.9)0.172Right Femur (V30)10.9% (2.3)10.3% (2.9)0.458Left Femur (V30)12.0% (3.1)11.6% (2.2)0.570 Open table in a new tab