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Case Report
Marked preoperative regression of biopsy-diagnosed microsatellite instability-high colon adenocarcinoma with a small residual traditional serrated adenoma: A case report
1 Department of Surgery, Matsuyama Red Cross Hospital, Matsuyama, Ehime, Japan
2 Department of Surgery and Science, Kyushu University, Fukuoka, Fukuoka, Japan
3 Department of Pathology, Matsuyama Red Cross Hospital, Matsuyama, Ehime, Japan
4 Division of Gastroenterology, Matsuyama Red Cross Hospital, Matsuyama, Ehime, Japan
5 Department of Gastroenterology, Fukuoka Red Cross Hospital, Fukuoka, Fukuoka, Japan
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Kazuhito Minami
Department of Surgery, Matsuyama Red Cross Hospital, 1 Bunkyo-cho, Matsuyama, Ehime 790-8524,
Japan
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Article ID: 101546Z01KM2026
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Minami K, Funakoshi H, Oshiro Y, Nozaka Y, Egashira S. Marked preoperative regression of biopsy-diagnosed microsatellite instability-high colon adenocarcinoma with a small residual traditional serrated adenoma: A case report. Int J Case Rep Images 2026;17(2):30–36.ABSTRACT
Introduction: Marked regression of colorectal cancer before surgery is extremely rare. Microsatellite instability-high (MSI-high) tumors are characterized by a high neoantigen burden and pronounced immunogenicity. Biopsy-associated tissue injury may induce local inflammatory and reparative responses; whether these responses can augment antitumor immunity in an immunogenic tumor microenvironment remains uncertain. We report a case of MSI-high colorectal adenocarcinoma that markedly regressed after biopsy, with no residual carcinoma identified in the resected specimen. The temporal and histopathologic findings raise the possibility of an interplay between biopsy-associated tissue injury and the intrinsic immunogenicity of an MSI-high tumor.
Case Report: Initial colonoscopy in a man in his 70s identified an ascending colon lesion consisting of a protruded component and a shallow depressed component. Magnifying endoscopy demonstrated irregular vascular and surface patterns corresponding to Japan NBI Expert Team (JNET) type 2B, together with a type Vi pit pattern, raising suspicion of submucosal invasion. Biopsy specimens obtained from the lesion revealed moderately to well-differentiated adenocarcinoma. Polymerase chain reaction (PCR)-based MSI testing of the biopsy specimen demonstrated MSI-high status. Follow-up colonoscopy performed 36 days later showed regression of the protruded component. Laparoscopic ileocecal resection (LIR) with lymph node dissection was performed 77 days after the initial diagnosis. Macroscopically, only a small protruded lesion measuring approximately 3 mm remained at approximately the same longitudinal level as the tattoo but several centimeters away along the colonic wall. Histopathologic examination revealed a 3-mm traditional serrated adenoma (TSA) with low-grade dysplasia, accompanied by prominent submucosal fibrosis in the presumed tumor bed. No viable adenocarcinoma was identified despite extensive additional sectioning. Moderate lymphocytic infiltration accompanied by lymphoid follicle formation was observed within the presumed tumor bed.
Conclusion: Although the underlying mechanism cannot be definitively established, the interval endoscopic regression observed after biopsy and the absence of residual carcinoma in the resected specimen are consistent with a possible regression process. However, complete mechanical removal of the malignant component during biopsy or changes resulting from procedure-related tissue injury cannot be excluded. The MSI-high status and lymphocytic infiltration with lymphoid follicle formation provide a plausible immunologic context, but the contribution of an immune-mediated mechanism remains speculative.
Keywords: Biopsy, Colorectal cancer, MSI-high, Tumor regression
SUPPORTING INFORMATION
Acknowledgments
We thank the surgical and gastroenterology teams of Matsuyama Red Cross Hospital for their assistance in patient care.
Use of artificial intelligence tools
During the preparation of this manuscript, ChatGPT by OpenAI was used only for English language editing and improvement of readability. No artificial intelligence tool was used to generate clinical data, pathological data, endoscopic images, radiological images, figures, tables, references, or scientific conclusions. All clinical information, interpretations, references, and final manuscript content were reviewed and verified by the authors, who take full responsibility for the integrity and accuracy of the work.
Kazuhito Minami - Substantial contributions to conception and design, Acquisition of data, Analysis of data, Interpretation of data, Drafting the article, Final approval of the version to be published
Hiroki Funakoshi - Acquisition of data, Drafting the article, Revising it critically for important intellectual content, Final approval of the version to be published
Yumi Oshiro - Acquisition of data, Analysis of data, Interpretation of data, Revising it critically for important intellectual content, Final approval of the version to be published
Yoshiaki Nozaka - Acquisition of data, Revising it critically for important intellectual content, Final approval of the version to be published
Shinjiro Egashira - Acquisition of data, Revising it critically for important intellectual content, Final approval of the version to be published
Guarantor of SubmissionThe corresponding author is the guarantor of submission.
Source of SupportNone
Consent StatementWritten informed consent for publication of clinical details and images was obtained from the patient. According to the institutional policy of our hospital, single anonymized case reports are classified as exempt from formal ethics committee review; therefore, additional Institutional Review Board (IRB) approval was not required.
Data AvailabilityAll relevant data are within the paper and its Supporting Information files.
Conflict of InterestAuthors declare no conflict of interest.
Copyright© 2026 Kazuhito Minami et al. This article is distributed under the terms of Creative Commons Attribution License which permits unrestricted use, distribution and reproduction in any medium provided the original author(s) and original publisher are properly credited. Please see the copyright policy on the journal website for more information.
