Novel Modification of HistoGel-Based Cell Block Preparation Method: Improved Sufficiency for Molecular Studies

Context.— Cell block preparation methods vary substantially across institutions and are frequently suboptimal. The growing importance of biomarker testing in the era of targeted therapies makes optimization of cell block preparation critically important. Objective.— To develop an improved cell block preparation method. Design.— Ex vivo fine-needle aspirates and scrapes from surgically resected tumors were used to develop an improved HistoGel (Thermo Fisher Scientific, Waltham, Massachusetts)-based cell block preparation method. Cellularity yield with the new versus the standard method was assessed in ex vivo split samples and in consecutive clinical fine-needle aspirates processed before (n = 100) and after (n = 100) the new method was implemented in our laboratory. Sufficiency of cell block material for potential molecular studies was estimated by manual cell quantitation. Results.— The key modification in the new method was pretreatment of the pelleted cells with 95% ethanol before the addition of HistoGel (HistoGel + ethanol method). In addition, we optimized the melting conditions of HistoGel and added a dark, inorganic marker to the cell pellets to highlight the desired level of sectioning during microtomy. Cell blocks from ex vivo split samples showed that the HistoGel + ethanol method yielded, on average, an 8.3-fold (range, 1–20) greater cellularity compared with the standard HistoGel-only method. After the switch from the standard HistoGel method to the modified method in our clinical practice, sufficiency of positive fine-needle aspirates for some molecular studies increased from 72% to 97% (P = .002). Conclusions.— We describe a simple and readily adoptable modification of the HistoGel method, which results in substantial improvement in cell capture in cell blocks, leading to a significant increase in sufficiency for potential molecular and other ancillary studies.