Delineation of a reference target volume for pelvic lymph node irradiation using MR imaging with ultrasmall superparamagnetic iron oxide particles

Abstract

Materials/Methods: 20 patients with gynaecological cancer had a pre-operative MRI with USPIO administration. Common iliac, presacral, external iliac, internal iliac and obturator nodes were measured and outlined. Five clinical target volumes (CTV) were generated for each patient by expanding blood vessels by 3mm, 5mm, 7mm, 10mm and 15mm. Nodal contours were overlaid on every CTV and analysed for coverage. Planning target volumes (PTV) were created by adding a margin of 1cm to the CTV. The volume of bowel, bladder and rectum within each CTV and PTV was measured to aid selection of the appropriate margin for each nodal group. Results: 1216 nodal contours were evaluated. The mean number per patient was 61 (range 30‐101) and the mean short axis diameter was 3.6mm. Nodal coverage was 56%, 76%, 88%, 94% and 99% nodes by 3mm, 5mm, 7mm, 10mm and 15mm vessel expansion respectively. The mean volume of bowel within the PTV was 96cm3 (15.4%) with a 3mm vessel margin, 121cm3 (19.4%) with a 5mm margin, 146.9cm3 (23.7%) with a 7mm margin, 190cm3 (30.8%) with a 10mm margin and 266cm3 (42.9%) with a 15mm margin. Conclusions: MR imaging with USPIO is an innovative technique to localise and stage lymph nodes. It has enabled the production of reference CT images delineating the pelvic lymph node regions. Guidelines have been developed for pelvic nodal irradiation with IMRT using a modified 7mm margin, with proposed adjustments to improve coverage of specific target nodal groups. These guidelines enable a reduction in the volume of bowel within the PTV by 23% without compromising tumour coverage.