18FDG-PET Positivity Post Radiotherapy is a Biomarker for Relapse in Patients With Plasmacytoma

Abstract

Solitary plasmacytomas are characterised by focal proliferation of monoclonal plasma cells in either soft tissue or bone. The management of solitary plasmacytomas is distinct from other plasma cell dyscrasias with radiotherapy forming the mainstay of treatment, with curative intent. Precise diagnosis of a solitary plasmacytoma is therefore essential, to ensure that the patient is treated on the appropriate pathway. Patients with solitary plasmacytoma can also progress with multiple plasmacytomas or Multiple Myeloma. Established risk factors for progression from solitary plasmacytoma to myeloma/multiple plasmacytomas are raised light chain ratio, aberrant plasma cells in marrow and size of plasmacytoma. Imaging biomarkers of relapse remains an active area of investigation. 18F FDG PET-CT has emerged as a useful tool to diagnose plasmacytomas and stage patients, but data is limited on use of 18F FDG PET-CT to measure treatment response. In Plasmacytoma patients, the role of residual PET positivity post therapy as a biomarker for disease burden and correlation with biochemical results, is not yet known. Likewise, the role of 18F FDG PET-CT as a biomarker to estimate the cure fraction and as a marker of disease progression requires further study. To answer these questions we conducted a retrospective analysis of twenty-four patients, referred with a histological diagnosis of plasmacytoma, over a three year period. We recorded patient baseline characteristics, size and location of their plasmacytoma, imaging findings, paraprotein/light chain and bone marrow results. We then compared their 18F FDGPET-CT and skeletal surveys to analyse what proportion of patients were subsequently upgraded to myeloma on imaging alone and whether their treatment pathway had changed. We also recorded the proportion of patients with solitary plasmacytoma who progressed with progression recorded on 18F FDG PET CT scan on follow-up imaging or until the first sign of biochemical relapse. Progression free and overall survivals were then calculated from time of diagnosis of plasmacytoma. Of the 24 patients, 9 patients were imaged with both skeletal survey and FDG PET-CT. On comparing the imaging findings, FDG PET-CT detected additional lesions in 55.6% of cases (5/9 patients), which were missed by conventional radiography and significantly changed treatment pathway. Follow up post treatment PET/CT was performed in 15/24 (62.5%) of patients. During the follow up period we found that 50% (12/24) patients with plasmacytoma progressed to Myeloma. In 66.7% of these cases, (8/12 patients) post therapy FDG PET-CT was positive, and only 1 PET-CT negative patient progressed. Only 50% (6/12 patients) of patients had biochemical evidence of either persistent disease or relapse at the time of clinical progression suggesting that FDG PET-CT is a useful imaging tool to identify plasmacytoma patients at risk of progression to Myeloma.