Optimal Lymph Node Harvest in Rectal Cancer (UICC Stages II and III) after Preoperative 5-FU-based Radiochemotherapy. Acetone Compression is a New and Highly Efficient Method

Introduction: Preoperative 5-fluorouracil-based radiochemotherapy (RCT), followed by total mesorectal excision, is accepted as standard therapy in rectal cancers (UICC stages II and III). The accurate evaluation of ypN status after RCT with valuable lymph node (LN) harvest is essential for postoperative risk-adapted treatment decisions. Actual numbers of assessed LNs and validity of ypN status vary extensively depending on the methods used. Material and Methods: This prospective study validates the acetone compression (AC), whole mesorectal compartment embedding (WME), and fat clearance (FC) methods for LN retrieval in n = 257 rectal cancer specimens obtained from 2 high-volume surgical centers. For optimal LN retrieval, the AC method (n = 161 specimens: 52 cases with RCT, 109 cases without RCT) was compared with the WME (n = 64 cases, with RCT) and FC methods (n = 32 cases: 17 cases with RCT, 15 cases without RCT). The efficacy of LN retrieval, costs involved, and molecular diagnostics were measured. Results: Using the AC method, 41 LNs (mean; range 14 to 86 LNs) were detectable in total mesorectal excision specimens after RCT and 44 LNs (mean; range 9 to 78 LNs) in cases without RCT. The LN yield after RCT obtained by using the AC method was equivalent to that of the WME method (mean 32 LNs/specimen; range 12 to 81 LNs) but demonstrated a better time and cost-efficacy. In addition, the AC method facilitated assessment of any tumor deposits, including perineural invasion, and did not hamper molecular analyses. The AC method increased LN retrieval 4-to-6-fold as compared with the literature and 2-fold compared with manual dissection after the FC method. Discussion: The AC method is the method of choice for accurate LN staging in locally advanced rectal cancer, especially after preoperative RCT, and is well suited for routine gastrointestinal pathology workup.