Hidden Fibroblast Signal Reveals Endometriosis Lesions Invisible to Standard Imaging

Endometriosis affects an estimated ten percent of women of reproductive age worldwide, yet one of its most stubborn clinical problems is not treatment but simply seeing the disease. Superficial endometriotic implants, in particular, are notoriously difficult to detect on magnetic resonance imaging and transvaginal ultrasound, which means many patients endure years of pain before receiving a definitive diagnosis. Now, a team of radiologists, pathologists, and gynecologists at Mayo Clinic has provided a piece of the biological puzzle that could change how the disease is imaged. In a study published in the European Journal of Nuclear Medicine and Molecular Imaging, the researchers report that endometriosis lesions, whether deep or superficial, express substantial levels of a molecular target called fibroblast activation protein, or FAP, including lesions that conventional imaging struggles to visualize at all.

FAP is a membrane protein expressed by activated fibroblasts, the stromal cells that remodel tissue in fibrotic and inflammatory environments. In oncology, FAP has become one of the most successful molecular imaging targets of the past decade, because fibroblast activation protein is largely absent from normal adult tissues but abundant in the cancer-associated stroma of many epithelial tumors. Radiolabeled FAP inhibitors, commonly referred to as FAPI tracers, allow positron emission tomography to light up tumors with striking contrast. More recently, clinicians have reported incidental FAPI uptake in endometriosis lesions and even suggested that FAP-targeted PET could offer diagnostic value beyond MRI in selected patients. What remained unclear, however, was the biological foundation for those observations: how strongly do endometriotic lesions actually express FAP, and does the level of expression depend on whether a lesion is visible on the imaging tests women typically receive?

The Mayo Clinic team, led by corresponding author Hiroaki Takahashi, set out to answer precisely that question. Because no one had previously linked FAP expression in endometriosis to lesion-by-lesion visibility on MRI and transvaginal ultrasound, the investigators designed a retrospective study that combined pathology, immunohistochemistry, and blinded imaging review. They analyzed surgical specimens from patients with histologically confirmed deep endometriosis and superficial endometriosis, all of whom had undergone MRI, transvaginal ultrasound, or both before surgery. The final cohort included thirteen patients with deep endometriosis and nine with superficial disease, providing a matched basis for comparing the two phenotypes at the tissue level.

To quantify FAP expression, the researchers used immunohistochemical staining of the resected specimens and calculated H-scores, a standard pathology metric that incorporates both the intensity of staining and the proportion of positive cells. H-scores range from zero to 300, allowing semi-quantitative comparison across samples. As a benchmark, the team also stained colorectal adenocarcinoma controls, since colorectal cancer is known to be a FAP-expressing malignancy and therefore provides a reference point for interpreting stromal signal intensity. The comparison showed that endometriosis, across both deep and superficial forms, exhibited substantial FAP expression in its fibroinflammatory stroma, although the H-scores were significantly lower than those of the colorectal cancer controls.

A key finding was that FAP expression did not differ significantly between deep and superficial endometriosis. Median H-scores were 170 for deep lesions and 200 for superficial lesions, a difference that was statistically indistinguishable. This is a notable result because superficial endometriosis is precisely the subtype that most often escapes detection on standard imaging. If superficial implants carry FAP expression comparable to deep lesions, then a whole-body molecular imaging technique targeting FAP should, in principle, be able to detect superficial disease that MRI and ultrasound routinely miss. The study thereby provides a histopathological rationale for extending FAP-targeted PET imaging from oncology into benign inflammatory and fibrotic gynecological disease.

The second half of the study addressed the visibility question directly. Two abdominal radiologists independently reviewed the MRI and transvaginal ultrasound studies and graded the visibility of each pathology-confirmed lesion on a study-specific scale from zero to three, with scores of zero and one classified as low visibility and scores of two and three as high visibility. When a lesion’s location fell outside the available field of view, it was assigned a designation of not applicable and excluded from the visibility comparison. This blinded, reader-based design ensured that the imaging assessments were independent of the pathological FAP results, allowing an unbiased test of whether molecular expression tracks with detectability.

For MRI, no lesions were graded as outside the field of view by either reader, and the low- versus high-visibility split was uneven but workable: Reader 1 classified 5 lesions as low visibility and 16 as high visibility, while Reader 2 classified 3 as low and 18 as high. Median H-scores for the low- and high-visibility groups were 220 and 170 for both readers. Although the low-visibility lesions trended toward higher FAP expression, the differences were not statistically significant. In practical terms, MRI-visible and MRI-invisible endometriosis lesions expressed FAP at broadly similar levels, suggesting that poor visibility on MRI reflects the limitations of anatomical imaging rather than any fundamental difference in the biology of the lesion.

The transvaginal ultrasound results told a more complicated story. Because ultrasound has a limited field of view in the pelvis, one lesion for Reader 1 and two lesions for Reader 2 fell outside the imaging field and were designated not applicable. Among the evaluable lesions, the low-visibility groups were small, containing just two lesions for Reader 1 and one for Reader 2, compared with ten high-visibility lesions for each reader. Median H-scores in the low-visibility groups were 48 and 20, markedly lower than the 165 and 160 observed in the high-visibility groups. The authors are careful to note that these small sample sizes preclude firm conclusions. The apparent association between low ultrasound visibility and lower FAP expression may reflect the small number of evaluable lesions rather than a genuine biological pattern, and larger studies will be needed to determine whether FAP expression correlates with sonographic detectability.

Even with those caveats, the overall conclusion of the study is clear and clinically important: both deep and superficial endometriosis show substantial stromal FAP expression, and critically, superficial lesions that are poorly seen on conventional imaging still express the target. The findings arrive amid a growing literature on FAPI PET beyond cancer. FAP-targeted imaging has been explored in Crohn’s disease, rheumatoid arthritis, and other fibroinflammatory conditions, and several recent case reports and small series have documented FAPI uptake in endometriotic implants, including focal uptake at the rectouterine pouch and subtype-specific uptake patterns that added diagnostic information beyond MRI. Earlier laboratory work had already shown that FAP-positive activated fibroblasts are detectable in the endometriotic microenvironment, where they correlate with stroma composition and with infiltrating CD8-positive and CD68-positive immune cells. The new Mayo Clinic study adds the missing translational link between that biology and the day-to-day imaging visibility of the disease.

The implications extend to diagnosis, surgical planning, and potentially therapy. Delayed diagnosis of endometriosis remains a global problem, with patients commonly waiting years from symptom onset to confirmation, partly because superficial implants are difficult to detect with anatomical imaging and often require laparoscopy for diagnosis. A molecular imaging approach that targets the fibroinflammatory stroma shared by deep and superficial lesions could complement MRI and ultrasound, particularly in mapping disease extent before surgery and in identifying active, fibrotically active lesions among otherwise inconspicuous findings. FAP is also being investigated as a therapeutic target, so confirming its expression in endometriotic tissue opens a second avenue beyond diagnosis. The authors note that their study is retrospective and modest in size, with only twenty-two patients, and that the ultrasound visibility analysis was limited by small numbers. Prospective studies with larger cohorts will be needed to establish how FAPI PET performs against MRI and transvaginal ultrasound in head-to-head diagnostic comparisons. Nevertheless, by demonstrating that even imaging-invisible superficial endometriosis carries the molecular signature that FAP-targeted tracers exploit, the study lays the histopathological groundwork for what could become the first molecular imaging strategy purpose-built for the full phenotypic spectrum of endometriosis.

Subject of Research: Fibroblast activation protein expression in deep and superficial endometriosis and its relationship to MRI and transvaginal ultrasound visibility

Article Title: FAP expression in deep and superficial endometriosis: Association with MRI and transvaginal ultrasound visibility

Article References: FAP expression in deep and superficial endometriosis: Association with MRI and transvaginal ultrasound visibility. (n.d.). https://doi.org/10.1007/s00259-026-08189-3

Image Credits: AI Generated

DOI: 10.1007/s00259-026-08189-3

Keywords: endometriosis, fibroblast activation protein, FAPI PET, molecular imaging, deep endometriosis, superficial endometriosis, MRI, transvaginal ultrasound, immunohistochemistry, H-score, radiology, diagnostic imaging