Purpose: In prophylactic irradiation of infradiaphragmatic lymphatic nodes (LN), the width of the paraaortic and pelvic field is given by a line joining the tips of the transverse processes of the 11th thoracic to the 4th lumbar vertebrae. Then the field boundary follows a straight line to the most lateral point of the acetabulum seen on the simulation film. Another method of setting the field border is to project the lateral contours of the large abdominal vessels from T(1)-weighted coronal MR images of the abdomen onto the simulator radiographs and add a 2-cm margin along the so delineated vessels. In our study, we compared both methods as to full enclosure of paraaortic and pelvic lymphatics or nodal miss.
Material and methods: Abdominal CT scans of 81 patients with involvement of paraaortic lymph node regions with LN sizes not exceeding 2.5 cm were examined. The distance from the center of the appropriate vertebra to the center of the most lateral lymph node was referred to the transverse process as well as to the outside contour of the aorta on the left and the vena cava on the right side. Respectively, the LN were measured referenced to the iliac vessels from the 5th lumbar through to the 2nd sacral vertebra. At the level of the hip joint the distance was measured from the midline as determined by a line through the center of the sacrum, perpendicular to a line connecting center of both femoral heads.
Results: Our measurements showed that lymph nodes do occur (1) lateral to the transverse processes of the thoracic and lumbar vertebrae as well as (2) outside the 2-cm safety margin from the lateral contour of the large abdominal vessels.
Conclusion: These data clearly show that the traditional fields for radiation of infradiaphragmatic lymphatic nodes have not been large enough to enclose almost all retroperitoneal and pelvic lymph nodes with certainty. We recommend an expansion of the fields.