Follicular lymphoma, a cancer of the white blood cells, is usually incurable. Patients cycle through periods of therapy that partially shrink their cancers, before the disease progresses again. That’s what appeared to be happening with a 61-year-old man in Italy: Diagnosed with cancer in August of 2019, he promptly began a course of chemotherapy, completing it in February 2020. All that was left to do was monitor the tumor’s growth.

So when a June scan revealed that the patient’s tumor appeared to be growing, Martina Sollini, a professor of nuclear medicine at Humanitas University in Italy, and her colleagues weren’t surprised. Until the biopsy came back negative. Another biopsy and a follow-up scan in September confirmed the original findings: The cancer had gone into complete remission. His medical team was left to figure out how.