A Diagnostic Dilemma: When Post-Treatment Scans Mimic Cancer Recurrence
In the ever-evolving field of cancer diagnostics, the ability to distinguish between successful treatment and disease recurrence is paramount. A recent series of case reports has illuminated a significant challenge in this area, specifically concerning a promising new imaging technique for liver cancer. The reports, published in the journal EJNMMI Reports, document instances where a cutting-edge PET/CT scan produced misleading results in patients with hepatocellular carcinoma (HCC) after they underwent stereotactic body radiation therapy (SBRT). In two separate cases, the scans showed areas of intense uptake that initially looked like the cancer had returned. However, further investigation revealed that these bright spots were simply the body’s natural healing response to the radiation—a process known as fibroinflammatory change—rather than a resurgence of the tumor. This finding serves as a crucial warning for oncologists and radiologists, emphasizing that while advanced imaging is powerful, it is not infallible and must be interpreted with a nuanced understanding of the body’s complex response to treatment.
The new technology at the center of this story is [68Ga]Ga-FAPI PET/CT, a molecular imaging technique that has generated considerable excitement in the field of oncology. Traditional PET/CT scans often use a radioactive sugar molecule to highlight metabolically active cells, which works well for some cancers but poorly for others. Hepatocellular carcinoma, however, has a unique biological signature. The tumor itself is surrounded by a dense network of supportive cells, known as the stroma, which contains cancer-associated fibroblasts. These fibroblasts overexpress a specific protein on their surface called Fibroblast Activation Protein (FAP). In healthy adult livers, FAP is virtually absent, but it is found in abundance within the microenvironments of many solid tumors, including HCC. The FAPI tracer is designed to bind to this protein, allowing the PET/CT scan to effectively “light up” the tumor’s supporting framework rather than the cancer cells themselves. This approach has proven to be remarkably sensitive, often detecting liver tumors that are invisible on standard scans. This is because the liver’s normal background activity for this tracer is very low, while the uptake in the tumor’s stroma is exceptionally high, creating a stark and easily identifiable contrast.
However, this very sensitivity is also the source of its vulnerability. FAP is not exclusively expressed by cancer-associated fibroblasts. Any process in the body that involves tissue repair, wound healing, or chronic inflammation will also activate fibroblasts and trigger an increase in FAP expression. This includes the natural healing process that occurs after various medical treatments. When a patient undergoes ablative therapies like radiation, the treatment itself causes localized tissue damage and subsequent inflammation. As part of the body’s repair mechanism, activated fibroblasts rush to the area to lay down scar tissue, or fibrosis. This fibroinflammatory response can be picked up by the FAPI PET/CT scan as a false-positive signal, mimicking the appearance of a recurrent tumor. This is precisely what occurred in the two reported cases, where the radiation-induced scarring in the liver created intense FAPI uptake, prompting an unnecessary scare and a complex diagnostic puzzle for the medical teams involved.
The first case involved a 68-year-old man with a complex medical history who had been treated for HCC that had spread to his bones. His treatment included SBRT to the liver and another bone site, alongside a lengthy course of systemic immunotherapy. A routine follow-up FAPI scan revealed increased activity in the area of the liver that had been previously irradiated. Initial thoughts immediately turned to a possible recurrence of the disease. However, a detailed MRI of the abdomen performed at the same time painted a very different picture. The MRI showed the features of a non-viable scar, with no signs of the blood flow that would typically nourish an active tumor. Despite a slight rise in his serum AFP level—a key tumor marker for liver cancer—the patient remained asymptomatic and clinically stable. The MRI was conclusive in demonstrating that the lesion was fibrotic scar tissue, and the FAPI uptake was ultimately confirmed as a false-positive signal. This case highlights the critical role of correlative imaging: the high-resolution MRI provided the anatomical context needed to correctly interpret the otherwise misleading biochemical signal from the PET scan.
The second case bore a striking resemblance to the first, but with its own unique nuances. A 77-year-old man with a history of alcohol-related liver disease presented with a small, single HCC lesion. He was treated with SBRT with curative intent. Nearly a year later, his tumor marker levels began to climb significantly, raising alarm bells. An MRI, however, showed a positive response to treatment with no visible active tumor. To resolve this discrepancy and check for cancer spread elsewhere in the body, the team ordered a whole-body FAPI PET/CT. The scan revealed an intensely bright, ill-defined area in the exact region that had been irradiated. This finding was highly suspicious for a local recurrence, especially given the patient’s rising tumor marker. Yet, the concurrent MRI again told a different story, showing no diffusion restriction or distinct lesion to confirm a viable tumor. After a multi-disciplinary team review, it was concluded that the imaging findings were once more the result of post-radiation fibroinflammatory change. The decision was made to monitor the patient without further intervention, and at a subsequent follow-up, the lesion had not evolved into a true recurrence, confirming the false-positive nature of the initial scan.
These two cases provide invaluable insights into the limitations of FAPI PET/CT in the post-treatment setting. They demonstrate that SBRT-induced hepatic fibrosis can lead to FAPI uptake that is almost indistinguishable from that of a viable, recurring tumor. This adds a new layer of complexity to post-treatment surveillance, where the primary goal is to detect residual disease early. The standard criteria used to assess treatment response, such as mRECIST, are designed for conventional imaging like CT and MRI and are not perfectly suited for this new molecular tracer. Moreover, the radiologic response to SBRT is often slow to develop, further complicating early interpretation. The authors of the report strongly suggest that while FAPI PET/CT is a game-changer in the initial diagnosis and staging of HCC, it should be used as a complementary tool in the post-treatment setting, rather than a replacement for standard-of-care imaging like contrast-enhanced MRI. The high-resolution anatomical detail provided by MRI is essential for characterizing the structural features of a lesion, which is crucial for differentiating between a scar and a tumor.
The true power of this study lies not just in its findings, but in the clinical message it underscores for daily practice. A single positive FAPI PET/CT signal in a previously treated area is not definitive proof of recurrence. In such complex scenarios, a multi-disciplinary approach is vital. Oncologists, radiologists, and nuclear medicine physicians must work together, pooling all available data—including anatomical imaging like MRI, biochemical trends in tumor markers, and the patient’s overall clinical picture—to arrive at an accurate diagnosis. The false-positive interpretation in these cases was not simply a matter of chance; it was the result of a deliberate and careful analysis that prioritized concordant evidence over a single, albeit striking, imaging finding. The authors rightfully call for larger, prospective studies to better define the role of FAPI PET/CT in this specific clinical context, to establish clear guidelines and criteria for interpreting uptake in irradiated tissue. As our tools become more sophisticated, our interpretative frameworks must evolve in tandem to prevent unnecessary anxiety, avoid invasive procedures, and ensure that patients receive the most appropriate care based on a complete and accurate assessment of their disease status.