Background: Separation of the mesoderm-derived muscular structures and the endoderm-derived structures of the hindgut and reclassification of their involvement based on their embryological origin may be of clinical importance in providing anatomical support for a more standardized perineal resection during abdominoperineal resection. The aim of this study was to utilize magnetic resonance images and histological studies of fetal and neonatal specimens to redefine the T3/T4 distinction by reassessment of the intersphincteric plane and the pelvic diaphragm as they pertain to cancer infiltration and as part of the embryological development of the pelvic floor muscles and their connective tissue compartments.
Materials and methods: Pelvic floor anatomy was studied in seven newborn children and 120 embryos and fetuses. Anatomical data were completed by magnetic resonance imaging in 82 patients with T3 and T4 rectal cancers (64 T3, 18 T4; 35 women and 47 men) undergoing neoadjuvant chemoradiation for locally advanced (T3 or T4) rectal cancers.
Results: Clear demarcation between mesodermal and endodermal structures of the pelvic floor, which is equally evident in plastinated sections and magnetic resonance images, is already visible in early fetal stages. There is a constitutive overlap between the endoderm- and the ectoderm-derived components of the pelvic floor.
Conclusion: Our data suggest that the current classification of rectal cancer staging is confusing, where the routinely used TNM classification system unnecessarily differentiates between embryologically identical muscular structures. Tumor spread along the musculature of the hindgut beyond the dentate line could possibly explain the occasional involvement of lymph nodes outside the conventional mesorectum.