Natural History of Amyloidosis Isolated to Fat and Bone Marrow Aspirate

Background: Light chain Amyloidosis (AL) is characterized by deposition of light chain derived amyloid fibrils in major organs and/or soft tissue. An observational study on natural history and outcome of localized immunoglobulin light-chain amyloidosis without vital organ (liver, heart, kidney, peripheral and autonomic nervous system) involvement has shown extremely low rate of progression (1%) to systemic amyloidosis at a median follow-up of 74.4 months (Mahmood S et al. The Lancet Haematology; 2015; 6:e241-e250). There is, however, limited evidence in published literature on natural history of AL amyloidosis confined to fat and/or bone marrow biopsy, without involvement of vital organs or other soft tissues.

Methods: We retrospectively identified patients with AL amyloidosis limited to fat and/or bone marrow aspirate in a single-institution database. Patients were evaluated for progression to systemic amyloidosis. Statistical analysis was done using JMP 10.0.0 (SAS Institute Inc.).

Results: We identified 117 patients, with a median age of 70 years, who had light chain amyloidosis detected in abdominal fat aspirate and/or bone marrow biopsy, without systemic involvement. Amyloid was seen in fat only in 39%, marrow only in 54% and in both sites in 7%. The median follow up was 45.6 months (95% CI-38.1-57.7) from detection of amyloid. Of these, 64% were alive at the time of analysis. Among 117 patients, 65 were treated for a diagnosis of another plasma cell disorder made prior to or concurrent with detection of amyloid. The remaining 52 patients only had isolated fat or marrow amyloid. Among 65 patients with another diagnosis of plasma cell disorder requiring treatment, 3 progressed to systemic amyloidosis, one each with cardiac, renal and lymph node (LN) involvement detected at 32, 42 and 65 months respectively from the detection of amyloid. The first 2 patients had underlying multiple myeloma, and the third patient with LN involvement had underlying Waldenström macroglobulinemia. Among 52 patients without another diagnosis of a plasma cell disorder requiring treatment, at a median follow up of 32 months, no progression to systemic amyloidosis was observed. Median overall survival (OS) in 117 patients from detection of amyloid, using Kaplan-Meier survival estimates, was 60.2 months (95% CI-48.9-146.1).

Conclusion: Our study highlights the fact that isolated amyloidosis detected in fat and/or bone marrow aspirate, in the absence of another plasma cell dyscrasias that require therapy, is unlikely to progress to systemic amyloidosis. Watchful waiting might be considered in such patients after a thorough evaluation to rule out systemic involvement.