Upper limit of radiation treatment portals in rectal cancer: is it wise to keep using bony landmarks in the present era of 3D conformal treatment?

Background: This study aimed to compare the levels of L5-S1 interspace and the bifurcation of common iliac vessels on simulation images of rectal cancer patients to evaluate the adequacy of superior borders in conventional 2D planning for covering internal iliac vessels.

Materials and methods: Simulation images of 236 rectal cancer patients who received neoadjuvant chemoradiation and surgery were analyzed. The images were retrieved from the radiation treatment database and included delineations of L5-S1 interspace and common iliac vessel bifurcation. Distances between these landmarks were measured.

Results: Among the 236 patients, the majority had the common iliac artery bifurcation positioned above the L5-S1 interspace. Specifically, 78.3% of patients had the right common iliac bifurcation above L5-S1 interspace, with an average distance of 2.02 cm. For the left common iliac artery, 77.11% of patients had the bifurcation above L5-S1 interspace, with an average distance of 1.99 cm. Notably, there were cases where the bifurcations were not at the same level.

Conclusion: Using the L5-S1 junction as the upper border of the treatment portal may result in missing proximal nodes at risk of metastases. However, further research is needed to determine the significance of failures above the L5-S1 interspace for justifying the inclusion of the common iliac artery bifurcation in the treatment portal.