Smooth muscular layer: A new helpful criterion to reclassify tumor deposits into metastatic lymph nodes in patients with colo-rectal adenocarcinoma.

Abstract Context The origin of tumor deposit in colorectal cancer is still unknown, and currently there is no single morphological feature to distinguish a metastatic lymph node from a tumor deposit. Histologically, the normal lymph node capsule and trabeculae contain a smooth muscular layer, which when present in extramural deposits would strongly suggest their lymph node origin. Objective We analyze the value of the smooth muscular layer criterion in reclassifying tumor deposit into metastatic lymph node. Design A total of 458 colo-rectal carcinomas surgical specimens treated or not by neoadjuvant (radio)chemotherapy were retrospectively included. Harvested tumor deposits were analyzed by Hematoxylin and Eosin and elastin staining on 10 consecutive serial sections and by α- smooth muscle actin immunostaining. Results A total of 129 tumor deposits were identified. 77 (60%) tumor deposits were reclassified into metastatic lymph node, of which 63 (49%) presented a smooth muscular layer on the initial Hematein Eosin staining and/or after serial tissue sections, confirmed by positive α-smooth muscle actin immunostaining in 43 out of 45 cases (90%). Fourteen (18%) additional tumor deposits were reclassified into metastatic lymph node by the appearance of lymphoid tissue after serial sections. Conclusions The presence of a smooth muscular layer in a presumable tumor deposit is helpful in pointing out its lymph node origin in patients with colo-rectal carcinomas. This criterion could improve the inter-observer agreement of tumor deposit identification, allowing accurate nodal staging and better assessment of patient's prognosis.