Limitations with light microscopy in the detection of colorectal cancer cells

PURPOSE: The failure of light microscopy to predict individual patient surv ival accurately in pStage I and II colorectal carcinoma can hinder planning postoperative therapy and follow-up. This study was designed and conducted in two parts Co assess the influence of relative sensitivity of the light microscope on the pathologist's ability to detect malignant cells in lymph nodes. METHODS: The first part of the study examined the issue of sampling error as a fraction of the number of lymph node sections examined by asking the question, "Does increasing the number of sections (sampling) taken fro m the block increase tumor cell detection in a lymph node?" Three levels of five sections 4 to 5 mu m thick separated by 15 to 20 mu m were obtained f rom each of 494 blocks from 173 cases of pStage I and II colorectal carcino ma. A total of 1,721 lymph nodes were examined. Sections from each level we re stained with hematoxylin and eosin and for the expression of cytokeratin . The second pan: of the study examined the relative sensitivity of the lig ht microscope to detect tumor cells in a lymph node. To simulate lymph node s, cell blocks were made that contained 10(6) or 10(7) mononuclear cells ad mired with increasing numbers of SW480 tumor cells (0, 50, 10(2), 5 x 10(2) , 10(3), and 5 x 10(3) Three pathologists independently examined sections f rom ten control and ten experimental blocks. RESULTS: Results from the firs t part of the study demonstrated cytokeratin-positive cells in 278 lymph no des from 102 of 172 (59 percent) cases. These cells were identified in the first level in 177 (64 percent) as compared with the second or third level or both in 101 (36 percent) of the lymph nodes. Results from the second par t of the study demonstrated an overall sensitivity of light microscopic exa mination of hematoxylin and eosin-stained sections to be approximately 23 p ercent, representing tumor cells correctly detected in 7 sections of the 30 sections containing tumor tells. The overall specificity was 87 percent or 26 sections correctly classified as lacking tumor cells of a possible 30. Immunohistochemical staining for cytokeratin expression improved sensitivit y of the light microscope to detect tumor cells to 18 of 30 (60 percent) an d the specificity to 50 of 30 (100 percent). CONCLUSION: This study demonst rates several sources of variability that can induce errors in pathologic s taging. These include 1) inadequate section, Fe., sampling, of lymph nodes, 2) use of only hematoxylin and eosin-stained sections, 3) samples with tum or cells below the level of detection sensitivity of the light microscope, and 4) observer variability.