A 64-year-old woman in Portugal learned she had colon cancer after her first-ever colonoscopy. Twelve weeks later, surgeons removed the section of bowel where the tumor had been marked with tattoo ink. The ink was there. The cancer was not.
Her doctors at the Viseu Dão-Lafões Local Health Unit then did something unusual. Before accepting that the tumor had disappeared on its own, they treated the case as a possible medical error and retraced every step, ending with a DNA comparison between her original biopsy and the removed colon. The profiles matched.
The team, led by gastroenterologist Ricardo Cardoso, described the case in GE - Portuguese Journal of Gastroenterology, published online in January 2026. They concluded it was spontaneous regression, the disappearance of a confirmed cancer without treatment capable of fighting it, and a rare event in colorectal cancer.
A Tumor on Camera, Then Gone in the Operating Room
The woman had been referred for colonoscopy because of iron deficiency anemia. She had several chronic conditions, including high blood pressure, congestive heart failure, atrial fibrillation and cryptogenic organizing pneumonia, an inflammatory lung disease. She was not taking corticosteroids at the time of the procedure or afterward.
The endoscopist found an ulcerated lesion covering about a quarter of the wall of the ascending colon. It was photographed and biopsied, and the colon was tattooed in three spots just beyond it so surgeons could find it later. The biopsies confirmed adenocarcinoma, the most common type of colon cancer. A CT scan showed modest thickening of the colon wall and several nodules smaller than 8 millimeters near the base of the intestine's supporting tissue.
She underwent a laparoscopic right hemicolectomy without complications. But the removed tissue showed no visible tumor, even though the tattoo marks were clearly present. Pathologists found no cancer, no abnormal scarring, and no cancer in the removed lymph nodes.
How Doctors Ruled Out a Mix-Up
The authors wrote that the simplest explanation for a vanishing tumor is human error, so they went looking for one. The search was complicated because a mistake could have happened in any of several places: the outpatient endoscopy unit, two outside pathology labs or the hospital that treated her.
Pathologists first reexamined the original biopsy slides and confirmed the cancer diagnosis. A colonoscopy eight weeks after surgery found no lesion in the rest of her colon, which ruled out the possibility that surgeons had removed the wrong segment.
The most troubling possibility was patient misidentification. If samples had been swapped, another person with cancer could have gone untreated while this woman underwent needless surgery. The team reviewed the entire colonoscopy list from the day of her diagnosis, including the timestamps on the endoscopic photos. Hers was the only suspected cancer and the only tattooed colon that day. She also happened to be the last patient on the list.
Finally, DNA was extracted from the original biopsy and from the surgical specimen. The genetic profiles, based on several short tandem repeat markers, matched. The authors did not formally calculate the odds of a chance match but judged that explanation extremely unlikely.
They closed with the line Sherlock Holmes delivers in The Sign of Four: "when you have eliminated the impossible, whatever remains, however improbable, must be the truth."
Why Colon Cancers Rarely Disappear on Their Own
Spontaneous regression is best documented in cancers such as kidney cancer, melanoma, neuroblastoma, lymphomas and leukemias. Colorectal cancer accounts for fewer than 2% of such cases, according to a review the Portuguese authors cite, a figure repeated in a recent report on regressing colon cancer. A conference abstract in the American Journal of Gastroenterology counted only 33 documented colorectal cases between 1900 and 2020.
Patterns are emerging. A review of Japanese cases cited in the Portuguese report found that regression was more common in right-sided tumors up to about 30 millimeters, in cases with a long wait before surgery, and in tumors that had not grown past the colon's muscle layer. The average time to regression was about two months.
Many recent reports also involve a tumor feature called deficient mismatch repair, a defect in DNA error correction that can make cancers more visible to the immune system. In a Nagoya City University report, surgeons described a 76-year-old woman whose transverse colon cancer shrank within two months and was gone at surgery. Her tumor showed that defect, and the authors speculated that a strong immune response, possibly triggered by biopsy, was responsible. They had also checked for a patient mix-up. The Portuguese report does not say whether her tumor was tested for the feature.
Researchers have proposed several other explanations, including inflammation or reduced blood supply triggered by handling the tumor during biopsy, and high fevers from severe infections. The Portuguese authors noted that their patient had no acute illness or intervention beyond biopsies and tattooing. They speculated that her inflammatory lung disease may have primed an immune response after the biopsy, but stressed that this is a hypothesis. Surgeons in Manchester, England, described two similar cases of biopsy-proven colon cancers that could not be found at surgery.
A Rare Outcome That Is Not a Cure
The authors warned that most cancers that regress spontaneously eventually come back, so regression should not be considered a cure. Their patient remains on the surveillance schedule recommended after colon cancer surgery, and no recurrence has been found so far.
Their larger lesson concerns safety. In endoscopy, patient misidentification is considered a never event, a serious and preventable error. The authors walked through the safeguards used in her case, including identity checks on arrival, before sedation, before the procedure, and again when biopsy samples were labeled.
For patients, the case is not a reason to delay recommended treatment in hopes of a disappearance. It is a reminder that when results do not add up, a careful investigation protects both the patient in front of the doctor and anyone who might have been confused with them.
Key Questions Answered
What happened to the patient's cancer?
A biopsy confirmed colon adenocarcinoma, but when surgeons removed that part of the colon 12 weeks later, no tumor was found. Doctors concluded it had regressed spontaneously.
How did doctors rule out an error?
They reexamined the biopsy slides, repeated the colonoscopy, reviewed every patient seen that day, and compared DNA from the biopsy and the surgical specimen. The DNA matched.
How rare is spontaneous regression of colon cancer?
Extremely rare. Colorectal cancer makes up fewer than 2% of reported spontaneous regressions, and one count found 33 documented colorectal cases from 1900 to 2020.
Does regression mean the cancer is cured?
No. The authors noted that most spontaneously regressing cancers eventually recur, so close follow-up is still needed.
Should patients wait to see if a cancer disappears?
No. These cases are exceptionally rare, and patients should follow the treatment plan recommended by their care team.
Published by Medicaldaily.com
