Evaluation of Nodal Coverage for Cervical Cancer by In Vivo Localization of Sentinel Lymph Nodes

Abstract

To assess coverage of sentinel nodes, as determined by hybrid SPECT/CT scan followed by sentinel lymph node (SLN) dissection, in carcinoma of the cervix by 2D conventional, CT-based 3D conformal (3D-CRT) and intensity modulated radiation therapy (IMRT) pelvic treatment fields. Twenty patients previously enrolled on a prospective trial assessing the role of SPECT/CT scan and SLN dissection in cervical cancer were evaluated. Operative notes, pathology reports, and SPECT/CT scans were used to locate the SLNs. Identified SLNs were then contoured on a representative patient. The initial nodal area was contoured as a 1 cm circle on an axial slice and then volumetrically expanded 0.5 cm axially and 0.75 cm cranio-caudally to create a sentinel node PTV (snPTV). These contours were hidden and 2D conventional (based on bony landmarks: sup. border, L5-S1 interspace; inf. border, obturator foramina; lat. border, 2 cm lateral to pelvic brim; ant. border, symphysis pubis; post. border, behind sacrum), 3D-CRT (based on vessels contours), and IMRT (per RTOG 0418) treatment fields were designed. Adequacy of the treatment fields was determined by whether the treatment field encompassed the snPTV. Of the 20 patients originally enrolled, 18 had evaluable SPECT/CT images. Forty-four SLN (19 right and 25 left) were identified and evaluated. There were 15 external iliac, 11 obturator, 8 internal iliac, 6 common iliac, 3 presacral, and 1 para-aortic SNL identified. Conventional 2D fields inadequately covered the snPTV in 6 patients (33%), representing 6 of 44 (13.6%) of the SLNs. Specifically, 3 common iliac, 2 internal iliac and 1 para-aortic snPTV regions extended outside the treatment portal. In 3 patients (16.6 %), the snPTV extended beyond the superior border of the AP/PA and LAT fields. In 3 patients (16.6%), the snPTV extended laterally outside the AP/PA treatment portal. With 3D-CRT and IMRT, 2 patients (11.1%) had inadequate coverage, representing 4.5% (2 of 44) of the identified sentinel nodes. Of the two inadequately covered snPTV, one was a high common iliac node that extended 2 mm beyond the treatment field superiorly and the second was a para-aortic node located at the L4 vertebral body level and was entirely outside the treatment field. Overall, the majority of SLNs identified were covered by radiation pelvic fields. However, conventional treatment fields were more likely to leave SLNs uncovered compared to 3D-CRT or IMRT treatment plans, with greater than 95% of snPTV regions adequately covered using 3D-CRT and IMRT volumes. In 2 cases involving high common iliac and para-aortic nodes, the expanded superior border in the IMRT and 3D-CRT plans still placed the snPTV outside of the treated volume.