As mentioned, SR is reported in virtually all types of human cancer, with neuroblastoma, renal cell carcinoma, malignant melanoma, and lymphoma/leukemia as the most common. However, SR of CRC is extremely rare, and only seven cases of SR of primary advanced CRC have been reported since 2000 (Table 1).

These reported cases consist of five males and two females, and the median age was 73 (64–80) years. The primary tumor site was mostly on the right side of the colon, and only one case was on the left side. The tumor morphology was mostly type II, and the median tumor size was 27.5 mm. The histological findings varied, and no consistent trend was observed. In addition, the median duration of tumor resolution was 2 months.

One of the reasons for the rarity of SR of CRC is that CRC is usually treated immediately after the diagnosis [5]. Treatment is rarely provided more than a few months after CRC diagnosis. In this study, the patient was treated 2 months after being diagnosed with CRC because some examinations for pancytopenia still had to be performed.

The reported risk factors for SR include sepsis with prolonged fever evolution, invasive treatment (e.g., colostomy), psychological factors, genetic factors, and physical factors [15]. These factors might stimulate several immune reactions or endocrine systems, resulting in SR. Among these factors, physical factors, such as ischemia caused by tumor progression, mechanical stimulation of intestinal peristalsis, circulatory failure caused by distortion or tow by tumor, and tissue biopsy, are involved in the SR of alimentary tract cancer [8]. In this study, preoperatively, the patient did not undergo any treatment except for the tissue biopsy; therefore, tissue biopsy is the only possible factor causing SR of CRC.

Very recently, the relationship between the SR of CRC and dMMR has been pointed out [12, 13]. Mismatch repair is a process involving many proteins in very highly conserved cells to identify and repair mismatched bases. In fact, dMMR, which is found in 15% of patients with CRC (12% sporadic cases and 3% hereditary cases) [16], results in a strong mutant phenotype called microsatellite instability (MSI), which is characterized by an extensively long polymorphism of microsatellite sequences caused by DNA polymerase slippage [17]. By using immunohistochemistry to test the MMR proteins such as MLH1, MutS homolog 2 (MSH2), MSH6, and PMS2, we can indicate the presence or absence of a functional MMR system and thus indirectly, MSI [18].

Microsatellite instability-high (MSI-H)/dMMR tumors correlate with an increase in neoantigens, which are newly formed antigens derived from genetic mutations in cancer cells. In stages II and III CRC, especially stage II, dMMR CRC is a favorable factor for recurrence and prognosis [19]. Furthermore, dMMR CRC has several characteristics, including older age, female sex, right-sided colon, and a high incidence of poorly differentiated adenocarcinoma or mucinous adenocarcinoma. A review of eight cases showed that SR of CRC was consistent with dMMR CRC characteristics, including older age, right-sided colon, and a high incidence of poorly differentiated adenocarcinoma. Finally, we speculated that SR of CRC was caused by a strong immune response induced by stimuli such as tissue biopsy against the background of dMMR. Some dMMR-related tumors are associated with Lynch syndrome, and patients with Lynch syndrome have been reported to show increased mucosal T-cell infiltration even in the absence of cancer [20]. The patient did not wish to be examined and was not examined closely for Lynch syndrome.

When encountering SR of CRC, we need to pay attention to the following points. First, we need to double-check that we are not misidentifying patients. In this case, we confirmed that we never had patient misidentification; we even revalidated our endoscopic system. Next, resection is still required, even if the tumor has disappeared preoperatively. The surgical resection was appropriate because we found no evidence of an absence of cancer cells in the deeper layer compared with that in the submucosal layer. Moreover, tumor regrowth after its remission may be caused by spontaneous decapitation [9]. Nishiura et al. reported a case of SR of advanced transverse colon cancer with remaining lymph node metastasis [13]. Finally, regular follow-up is crucial in SR cases. For these reasons, regular follow-up is necessary, even if SR of CRC occurs. Our patient continues to be followed up until 6 years postoperatively, with no observed recurrence.