Multiplex DNA Short Tandem Repeat Analysis
A tiny fragment of high-grade carcinoma was found in histologic sections and in the paraffin block of a benign cervical polyp from a patient with no clinical evidence of malignancy. Thus, it raised the suspicion of block contamination. No malignant tumor was processed the same day as the polyp; however, a similar tumor had been processed 6 days earlier. Multiplex DNA short tandem repeat analysis was applied to paraffin-extracted tissue samples obtained from the polyp, the suspected contaminant, the patient's additional cervical biopsy specimen, and the putative source of contamination.

The results demonstrated that the suspected contaminant and the patient's cervical tissue could not have come from the same patient and that the suspected contaminant derived from the tumor processed earlier, without reasonable doubt. We hypothesize that this friable tumor escaped from cassettes into the processor and contaminated the polyp specimen. Multiplex DNA short tandem repeat analysis can be applied to determine the provenance of minute tissue samples in surgical pathology.

Tissue contamination, or the presence of extraneous tissue in paraffin blocks or histologic sections, is a well-recognized problem in surgical pathology. The frequency at which it occurs is reported to vary from 0.6% in prospective to 2.9% in retrospective analyses or from 0% to 8.8%, including both prospective and retrospective cases. On occasion, it presents a challenge to the pathologist and might result in inappropriate therapy if not recognized or at least in additional diagnostic procedures to resolve the uncertainty.

Since the description of a hypervariable region of DNA composed of variable numbers of tandem repeats and the subsequent description of DNA "fingerprints," our knowledge of the human genome has led to the development of new DNA-based methods for identity testing that require minimal amounts of tissue or body fluid for analysis. DNA regions with repeat units that are 2 to 6 base pairs (bp) long are called microsatellites, simple sequence repeats, or short tandem repeats (STRs). STRs are used widely by forensic scientists because they survive well in poorly preserved tissue, they are amplified easily by the polymerase chain reaction (PCR), and the number of repeats in STR markers is highly variable between individuals, which makes them effective for human identification purposes. Although these molecular methods are used mostly for identity testing in criminal investigations, they also can be used for confirming the presence of a contaminant in paraffin-embedded tissue and for tracing its source. This report illustrates such a case, in which a tiny tissue fragment, suspected to be a contaminant in a paraffin block, was proven not to belong to the patient whose tissue constituted the rest of the block and ultimately was traced to its source.