EJNMMI research · 2026
Correlation of apparent diffusion coefficient, FAP expression in IHC and [68Ga]Ga‑FAPI‑46 uptake using integrated PET/MRI in high-grade soft tissue sarcoma: initial exploratory results.
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Hettler Madelaine, Menge Franka, Ludwig Johannes, Nörenberg Dominik, Schönberg Stefan O, Hinterberger Anna, Wängler Björn, Wängler Carmen, Rudolf Henning, Davarci Güllü, Cieslik Patrick, Cotarelo Cristina L, Harbrücker Melissa, Reißfelder Christoph, Jakob Jens, Kunz Wolfgang G, Grawe Freba
Аннотация
BACKGROUND: Fibroblast activation protein inhibitor (FAPI)-targeted positron emission tomography (PET) has emerged as a promising imaging approach in soft tissue sarcomas (STS), whereas magnetic resonance imaging (MRI) remains the diagnostic standard. We evaluated integrated [68Ga]Ga‑FAPI‑46 PET/MRI including maximum standard uptake value (SUVmax), diffusion-weighted imaging, apparent diffusion coefficient (ADC), and contrast-enhanced sequences compared to FAP expression in immunohistochemistry for tumor assessment in high-grade STS.
RESULTS: Thirteen [68Ga]Ga‑FAPI‑46 PET/MRI examinations from 9 patients were included, of which 4 were follow-up scans after therapy. All STS showed distinct FAPI uptake (median SUVmax 8.4; range 2.9-33). Mean SUVmax was 13 (2.9-33) for G2 and 8.4 (7.5-8.4) for G3 tumors. Neither SUVmax (P = 0.196) nor ADC (P = 0.895) differentiated between FNCLCC Grade 2 and Grade 3. A significant inverse correlation was observed between ADCmean and SUVmax (Spearman's ρ = -0.70, 95% CI: -0.93 to - 0.07, P = 0.035). Immunohistochemistry demonstrated positive FAP expression in 4 of 9 patients, with significantly higher SUVmax in FAP-positive vs. negative tumors in immunohistochemistry (17.7 vs. 6.4; P = 0.047). High concordance of [68Ga]Ga-FAPI-46 uptake and contrast enhancement was observed. Treatment responders showed marked reductions in SUVmax (40-70%) and increased ADC without significant changes in tumor size.
CONCLUSION: In this exploratory pilot study, [68Ga]Ga-FAPI-46 PET/MRI provided complementary functional information in high-grade STS. Although no correlation with grading was observed either using SUVmax or ADC, a significant inverse correlation between ADCmean and SUVmax was identified within this cohort. Preliminary observations regarding treatment monitoring suggested that early changes in tracer uptake and ADC may reflect treatment-related changes, however, these findings were derived from a small pilot cohort. Larger prospective studies are required to validate these observations and to further define the potential role of [68Ga]Ga-FAPI-46 PET/MRI in treatment monitoring and response assessment.
RESULTS: Thirteen [68Ga]Ga‑FAPI‑46 PET/MRI examinations from 9 patients were included, of which 4 were follow-up scans after therapy. All STS showed distinct FAPI uptake (median SUVmax 8.4; range 2.9-33). Mean SUVmax was 13 (2.9-33) for G2 and 8.4 (7.5-8.4) for G3 tumors. Neither SUVmax (P = 0.196) nor ADC (P = 0.895) differentiated between FNCLCC Grade 2 and Grade 3. A significant inverse correlation was observed between ADCmean and SUVmax (Spearman's ρ = -0.70, 95% CI: -0.93 to - 0.07, P = 0.035). Immunohistochemistry demonstrated positive FAP expression in 4 of 9 patients, with significantly higher SUVmax in FAP-positive vs. negative tumors in immunohistochemistry (17.7 vs. 6.4; P = 0.047). High concordance of [68Ga]Ga-FAPI-46 uptake and contrast enhancement was observed. Treatment responders showed marked reductions in SUVmax (40-70%) and increased ADC without significant changes in tumor size.
CONCLUSION: In this exploratory pilot study, [68Ga]Ga-FAPI-46 PET/MRI provided complementary functional information in high-grade STS. Although no correlation with grading was observed either using SUVmax or ADC, a significant inverse correlation between ADCmean and SUVmax was identified within this cohort. Preliminary observations regarding treatment monitoring suggested that early changes in tracer uptake and ADC may reflect treatment-related changes, however, these findings were derived from a small pilot cohort. Larger prospective studies are required to validate these observations and to further define the potential role of [68Ga]Ga-FAPI-46 PET/MRI in treatment monitoring and response assessment.