Impact of Formalin Fixation on Colonic Polyp Size and Post-Polypectomy Surveillance Thresholds: Evidence from a Prospective Paired Analysis
This prospective paired analysis demonstrates that formalin fixation causes a significant, directional reduction in colonic polyp size—particularly for small and sessile lesions—often shifting them across critical surveillance thresholds, thereby supporting the adoption of fresh ex-vivo measurements to ensure accurate post-polypectomy risk stratification.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The human colon is a long, winding tube that processes food and absorbs water, but it is also a place where small, abnormal growths called polyps can develop. Most of the time, these polyps are harmless, but some can slowly turn into cancer over many years. Because of this risk, doctors perform a procedure called a colonoscopy, where a flexible camera is used to look inside the colon. If a doctor sees a polyp, they remove it immediately. The size of that removed polyp is the most important clue doctors use to decide how often a patient needs to return for another check-up. A polyp smaller than a certain size might mean the patient can wait ten years before the next scan, while a larger one might require a return visit in just three years. Currently, the standard way to measure these removed growths is to place them in a chemical preservative called formalin, which keeps the tissue from rotting so a pathologist can study it under a microscope. The pathologist then writes down the size on a report, and that number becomes the basis for the patient's future medical care.
A team of researchers in Turkey recently asked a simple but critical question: does the process of preserving the tissue in formalin change its size? They suspected that just as a piece of fruit might shrink slightly after being dried out, a soft polyp might shrink after sitting in the chemical solution. If the polyp shrinks, the number on the medical report would be smaller than the actual size of the growth when it was inside the patient. This matters because if a polyp is right on the edge of the size limit, even a tiny amount of shrinkage could make it look small enough to send the patient home for a longer wait, when in reality, they might need closer monitoring. To find the answer, the researchers set up a careful experiment where they measured the polyps twice: once immediately after they were removed from the body, while they were still fresh and wet, and again after they had sat in the preservative for a day.
The study involved 53 patients who had 63 polyps removed during their routine colonoscopies. The researchers, including surgeons and pathologists, worked together to ensure the measurements were precise. As soon as a polyp was snipped out and caught in a net, one of the doctors measured its widest point with a standard ruler. They recorded this "fresh" size before the tissue was put into the jar of formalin. The jars were then left for between eight and twenty-four hours, the standard time for the tissue to be preserved. Afterward, a pathologist, who did not know what the first measurement was, took the same polyps out of the jars and measured them again with an identical ruler. The goal was to see if the numbers changed and, if they did, how much.
The results were clear and consistent. The polyps did indeed shrink. On average, the polyps became about one millimeter smaller after sitting in the preservative. While one millimeter sounds tiny, it represents a reduction of roughly thirteen percent of the original size. Every single polyp that changed got smaller; none of them got bigger or stayed exactly the same size. The shrinkage was not random, either. It happened in a predictable way, affecting the smaller, sessile (flat-based) polyps more than the larger, pedunculated (stalked) ones. In fact, the smaller the polyp was to begin with, the more it seemed to shrink in percentage terms. This is significant because the medical guidelines rely heavily on specific size cutoffs to make decisions.
The most important finding was how this shrinkage affected the classification of the polyps. The researchers looked at the polyps that were just barely over the ten-millimeter limit when they were fresh. They found that four of these polyps shrank enough to fall below the ten-millimeter mark once they were measured after preservation. If a doctor had only seen the final report, they would have thought these patients had small polyps and could wait longer for their next check-up. In reality, those patients had larger polyps that required closer attention. The effect was even more dramatic at the lower end of the scale. The number of polyps classified as "diminutive," meaning very small, more than doubled after the tissue was preserved. Many polyps that were just above the five-millimeter limit when fresh shrank down to five millimeters or less, moving them into a category where different, less invasive rules apply.
The study also compared these precise measurements to what the doctors guessed the size was while they were still looking inside the patient's colon. The doctors' visual guesses were often off by a wide margin, sometimes overestimating and sometimes underestimating the size by several millimeters. In contrast, the shrinkage caused by the preservative was small, steady, and always in the same direction. The researchers concluded that the best way to know the true size of a polyp is to measure it immediately after it is removed, before it is put in the preservative. This is a simple step that costs nothing and takes only a moment. By recording this fresh measurement, doctors can ensure that the decision about when a patient needs to return for another colonoscopy is based on the actual size of the growth, not a shrunken version of it. This small adjustment could prevent patients with borderline-sized polyps from being sent home too soon, ensuring they receive the level of care their health actually requires.
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