Abstract
Histopathologic response and tumor down-staging after neoadjuvant chemoradiation are major determinants of long term outcome in patients with locoregional rectal cancer. Optimal assessment of histopathologic response requires meticulous gross examination, adequate sampling of tumor bed, effective communication between operating surgeon and pathologist, and familiarity with TNM staging and different grading systems for histopathologic response/tumor regression. Several histopathologic response/tumor regression systems are described in last two decades. Almost all of them are based on semi-quantitative or quantitative assessment of fibrosis vs. residual tumor cells in rectal wall and perirectal soft tissue and generally do not include residual tumor in regional lymph nodes. Absence of residual tumor cells (complete pathology response) is associated with more than 90% 5 year recurrence free patients` survival with all grading systems. The variations among different grading systems include; assigning (or not) a numerical value to the amount of residual tumor cells or fibrosis, emphasis given more to residual tumor cells or fibrosis and number of different grades of response and availability of information about reproducibility among pathologists. American Joint Commission on Cancer has accepted 4 point tumor regression grade (TRG) system and is one of the required data point to be included in a surgical pathology report for rectal cancer resection specimen after neoadjuvant therapy.