Correlation of Biochemical and FDG PET/CT Responses Following Induction Therapy in Newly Diagnosed Multiple Myeloma: A Prospective Observational Study.

Background: Multiple myeloma is a heterogeneous malignancy with patchy bone marrow involvement, often leading to discrepancies between biochemical and imaging-based response assessments. Site-specific bone marrow biopsies may miss focal disease, while FDG PET/CT detects metabolically active lesions, and the two approaches offer complementary prognostic value. Materials and Methods: This prospective study included 44 newly diagnosed multiple myeloma patients. The primary aim was to assess the correlation between biochemical and PET/CT responses at six months post-induction. A secondary objective was to evaluate the impact of PET/CT response on 12-month event-free survival (EFS). Results: The median age was 55.5 years. At baseline, more than 3 focal lesions and extramedullary disease (EMD) were observed in 61.4% and 34.1% of patients, respectively. After six months of induction therapy, 86.3% achieved at least a very good partial response (≥VGPR) biochemically, but 52.3% remained PET/CT-positive. Baseline >3 focal lesions and EMD significantly predicted persistent PET/CT positivity (p = 0.004). Notably, 50% of patients with ≥VGPR still showed PET/CT-positive findings. At 12 months, 75% of patients who experienced clinical events had been PET/CT-positive at six months, compared with 47.2% of those without events (p = 0.245). The 12-month event-free survival was lower in the PET/CT-positive group (73.9% vs. 90.4%, p = 0.182), though this difference was not statistically significant. Conclusion: 18-FDG PET/CT can detect residual disease not captured by biochemical markers, highlighting the value of combined assessment in multiple myeloma. Baseline >3 focal lesions and EMD predicted persistent PET/CT positivity. Although PET/CT positivity at six months showed a trend toward worse 12-month EFS, larger studies are needed to confirm its prognostic significance.