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case_images: At baseline (November 2021), contrast-enhanced CT showed thickening of the wall of the upper rectum and part of the sigmoid colon with significant narrowing of the intestinal lumen (A), and the lesion was closely associated with the left iliac vessels (B).
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case_images: The next day, the child underwent a whole abdominal contrast-enhanced CT scan, which showed segmental thickening of the superior rectal and sigmoid walls with a corresponding narrowing of the intestinal lumen ( Figure 1A ). The lesion was closely associated with the left internal iliac vessel and its branches ( Figure 1B ).
case_images: (A) Colonoscopy shows a lesion protruding into the intestinal lumen with combined intestinal stricture. (B) Preoperative hematoxylin and eosin staining for the diagnosis of poorly differentiated adenocarcinoma of the rectum.
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case_images: Colonoscopy showed an irregular mass protruding from the rectum at 12 cm from the anus, with a congested and eroded surface, a lot of necrotic tissue, easy bleeding when touched, and a narrowed circumferential intestinal lumen with resistance to enter the scope, suggesting rectal occupation and stenosis ( Figure 2A ). Intraoperative histopathology was taken to diagnose a poorly differentiated adenocarcinoma of the rectum ( Figure 2B ), and immunohistochemical results showed Ki67 (index   70%), TP53 (mutant type), and the genetic test done by the patient at the company showed MSI-H.
case_images: MRI review in March 2021 after neoadjuvant immunotherapy combined with chemotherapy showed a significantly smaller lesion than before (A). MRI review in April 2021 showed a further smaller lesion than before with no significant thickening (B).
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case_images: After the child underwent three cycles of neoadjuvant therapy, the pelvic MRI on March 10, 2021 showed significant improvement in the thickening of the upper rectal and sigmoid walls compared to the previous image on December 7, 2020 ( Figure 3A ). The pelvic MRI scan on April 7, 2021 showed further improvement in the upper rectal and sigmoid walls compared to the previous one ( Figure 3B ).
case_images: (A) Postoperative general. (B) Postoperative hematoxylin and eosin stain, the arrows indicate necrosis, original magnification x 200.
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case_images: On May 25, 2021, he underwent a colostomy and partial rectal resection under general anesthesia ( Figure 4A ). Postoperative pathological diagnosis showed focal degeneration, necrosis and calcium salt deposition ( Figure 4B ), and positive CKp staining of focal necrotic material was seen in the lymph nodes (7/8), which was determined to be pCR with TRG 0 grade after treatment.
case_images: Timeline of the patient's diagnosis and treatments.
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case_images: The timeline of the patient is summarized in  Figure 5 .