Mapping pelvic lymph nodes: guidelines for delineation in intensity-modulated radiotherapy

Area of Science:

• Radiation Oncology
• Medical Imaging
• Gynecologic Oncology

Background:

• Accurate delineation of pelvic lymph nodes is crucial for effective radiation therapy in gynecologic malignancies.
• Current methods for defining clinical target volumes (CTVs) for pelvic nodal irradiation can be challenging due to anatomical variability.

Purpose of the Study:

• To establish guidelines for delineating the clinical target volume (CTV) for pelvic nodal irradiation.
• To map pelvic lymph node locations relative to pelvic anatomy using magnetic resonance imaging (MRI).
• To identify optimal margins for pelvic nodal irradiation that balance nodal coverage and normal tissue sparing.

Main Methods:

• Twenty patients with gynecologic malignancies underwent MRI with iron oxide particle administration.
• Lymph nodes were outlined, and five CTVs were generated with margins of 3, 5, 7, 10, and 15 mm around iliac vessels.
• Nodal coverage and the volume of normal tissue (bowel) within the planning target volume (PTV) were analyzed for each margin.

Main Results:

• A 7-mm margin around iliac vessels achieved 88% nodal coverage, while 10-mm and 15-mm margins provided 94% and 99% coverage, respectively.
• The mean bowel volume within the PTV increased with larger margins: 146.9 cm³ (7 mm), 190 cm³ (10 mm), and 266 cm³ (15 mm).
• Minor adjustments to a 7-mm margin ensured 99% pelvic node coverage with reduced normal tissue irradiation.

Conclusions:

• A modified 7-mm margin around pelvic blood vessels serves as a reliable surrogate for outlining pelvic lymph nodes.
• Adjusting margins can enhance coverage of specific nodal groups and decrease irradiated normal tissue volume.
• Recommended guidelines for outlining pelvic nodes have been developed based on these findings.