Magnetic resonance-guided histopathology for improved accuracy of tumor response evaluation of neoadjuvant treatment in organ-infiltrating rectal cancer.

Background And Purpose: The novel procedure of magnetic resonance-(MR) guided histopathology was applied to determine the false-negative rate of conventional histopathologic tumor response evaluation of neoadjuvant radiation/chemoradiation therapy (RT/CRT) in organ-infiltrating rectal cancer.

Materials And Methods: Ninety-two consecutive patients that had received RT/CRT and proceeded to extended total mesorectal excision for organ-infiltrating rectal cancer were identified from the institutional database. For each patient, the study radiologist and pathologist separately interpreted preoperative MR images and histologic preparations from the surgical specimen, to determine whether tumor down-staging had resulted. In cases of discrepancy (52 patients), histologic sections were jointly reassessed for residual tumor in areas outside the mesorectal fascial compartment, using MR images as guidance for where to inspect.

Results: Following RT/CRT, 67.5% of cases were found to remain ypT4, even though half of the study population had complete (ypT0; 7.6%) or near-complete (sparsely remaining tumor; 43.5%) histomorphologic tumor regression. After MR-guided histologic reassessment of surgical specimens, the false-negative rate of conventional histopathology for detection of ypT4 was determined to be 41.1%. Five-year estimate for locally recurrent disease was 12.7%.

Conclusion: This response data to neoadjuvant RT/CRT in organ-infiltrating rectal cancer indicate that tumor down-staging is over-estimated by conventional evaluation.