Magnetic resonance lymphography to localise pelvic lymph nodes for intensity-modulated radiotherapy in gynaecological cancer

Abstract

5070 Background: Intensity-modulated radiotherapy (IMRT) for gynaecological tumours requires accurate target volume delineation. Pelvic lymph nodes are poorly visualised on CT and MRI. Blood vessels with a 1–2cm margin are currently used as surrogate targets. A significant volume of small bowel is potentially irradiaited due to uncertainty about the exact lymph node position. Ultrasmall superparamagnetic iron oxide (USPIO), a novel MR contrast agent, localises in macrophages causing normal lymph nodes to appear black on MR images. This study used USPIO to determine the position of pelvic lymph nodes in relation to anatomic structures and develop guidelines for nodal irradiation. Methods: 15 patients with endometrial or cervical cancer had a diagnostic MRI (4mm slices) with subsequent USPIO administration and a repeat scan after 24 hours. All common iliac, presacral, external iliac, internal iliac and obturator nodes were outlined. On 10 scans, vessels were expanded by a margin of 15mm, 10mm, 7mm, 5mm and 3mm. The obturator and upper sacral regions were included with a corresponding margin. Nodal contours were overlaid on each expanded volume and analysed for coverage. Small bowel within each planning target volume (PTV) was measured. The resulting nodal irradiation guidelines were applied to the next 5 patients. Results: 836 nodal contours were evaluated. The mean number per patient was 56 (range 29–106). Contours covered by 15mm, 10mm, 7mm, 5mm and 3mm expansion were 99%, 96%, 90%, 77% and 63% respectively. The mean PTV bowel volume was 284, 204, 159, 130 and 102cm3. From these results, volume guidelines using an adjusted 7mm margin were applied to a further 5 patients resulting in 97% nodal coverage. Conclusions: MR lymphography with USPIO is an innovative technique to locate pelvic lymph nodes. Guidelines have been developed from these data for pelvic nodal irradiation with IMRT and conformal radiotherapy. A modified 7mm margin around vessels is recommended. This reduction in the margin reduces the small bowel volume within the PTV by 22% without compromising tumour coverage. No significant financial relationships to disclose.