Measuring the "Unmeasurable" Assessment of Bone Marrow Response to Therapy Using FDG-PET in Patients with Lymphoma

Rationale and Objectives: To determine if anatomically " nonmeasurable" disease in bone marrow (BM) is assessable for response to therapy by [¹⁸F]-2-fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET)/computed tomography (CT). Materials and Methods: FDG PET/CT images of 27 patients with lymphoma, FDG-avid bone marrow (BM) lesions, and ≥1 FDG-avid, tumor-involved lymph node (LN) at baseline were retrospectively reviewed. FDG uptake in target LNs and BM foci was determined pre- and posttherapy using the standardized uptake value corrected for lean body mass (SUL_mean). Size of the same target LNs was measured pre- and posttherapy on CT. Percentage decreases of LN size and LN and BM SUL were calculated. Response was classified according to revised International Workshop Criteria (IWC) with and without modification for metabolic evaluation of BM and correlated to overall survival. Statistical analyses were performed using paired t-tests, Pearson correlation coefficients, and z-tests. Results: LN size, LN SUL_mean, and BM SUL_mean were significantly higher pre- versus posttherapy (2337 mm² ± 1810 vs. 309 mm² ± 323; 6.94 ± 4.96 vs 1.02 ± 1.00; and 6.81 ± 4.58 to 1.84 ± 1.58, all P < 001, respectively). After therapy, significant correlation was found between percentage declines of LN size and SUL_mean of LNs (r = 0.84, P < 001) or BM (r = 0.56, P = .002) and SUL_mean of LN and BM (r = 0.76, P < 001). Including a metabolic assessment of BM correctly altered overall response assessment in 5/27 (19%) patients and better predicted overall survival than revised IWC. Conclusion: Anatomically " unmeasurable" BM infiltration with lymphoma behaves similarly to LN disease after therapy and is " measurable" by FDG PET/CT. FDG PET/CT is valuable for monitoring tumor response in " measurable" disease and BM, which was previously considered " unmeasurable" by anatomical imaging.