| Summary: | Diffuse large B-cell lymphoma (DLBCL) usually needs to be treated immediately after diagnosis from a single lymph node biopsy. However, several reports in other malignancies have shown substantial spatial heterogeneity within large tumours. Therefore, we collected multiple synchronous biopsies of twelve patients that had diagnostic or therapeutic resections of large lymphoma masses and performed next-generation sequencing of 213 genes known to be important for lymphoma biology. Due to the high tumour cell content in the biopsies, we were able to detect several mutations which were present with a stable allelic frequency across all the biopsies of each patient. However, ten out of twelve patients had spatially discordant mutations and similar results were found by the analysis of copy number variants. The median Jaccard similarity coefficient, a measure of the similarity of a sample set was 0.77 (range 0.47–1), and some of the involved genes such as <i>CARD11</i>, <i>CD79B</i>, <i>TP53</i>, and <i>PTEN</i> have a known prognostic or therapeutic relevance in DLBCL. This shows that single biopsies underestimate the complexity of the disease and might overlook possible mechanisms of resistance and therapeutic targets. In the future, the broader application of liquid biopsies will have to overcome these obstacles.
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