2819 - Do PET-Avidity Changes after Definitive Radiation Therapy for Solitary Bone Plasmacytoma Predict Progression to Multiple Myeloma?

Purpose/Objective(s):

Positron emission tomography (PET)-based imaging is critical for the diagnosis of solitary bone plasmacytoma (SBP), primarily for exclusion of other myelomatous lesions. Guidelines now suggest repeating baseline imaging after definitive radiation therapy (RT) to monitor for progression to multiple myeloma (MM). The widespread adoption of PET-based imaging has raised questions about how to interpret kinetics of PET avidity at the primary lesion. The purpose of this study was to determine if PET-based biomarkers of response to radiation therapy would predict progression to MM.

Materials/Methods:

A retrospective chart review was performed of all cases of SBP at two academic medical centers from 2015-2025. Cases were included if they were treated with definitive RT and underwent PET-based imaging before and after RT. Cases were excluded if a gross total resection was performed before RT. Clinicodemographic features were extracted from the electronic health record. Standardized uptake value (SUV) max and mean and tumor volume were extracted from fused PET-images to RT treatment plans. Prognostic variables were evaluated in Cox proportional hazards models of MM-free survival, measuring from start of RT to MM or last follow-up.

Results:

Twenty-four patients were included in the analysis. A clonal plasma cell population was present on pre-RT bone marrow biopsy in 8 (33%) cases, and an M-spike was detectable in 18 (75%) cases. Median age was 60.5 years (interquartile range [IQR]: 51-67), and median RT dose was 40 Gy (IQR: 36-45). The median time to post-RT PET was 134 days (IQR: 118-202). The median PET SUV Max and Mean at baseline were 7.9 (IQR: 6.2-15.4) and 2.6 (IQR: 1.8-3.3), respectively. The median decrease in PET SUV Max and PET SUV Mean after RT was 62% (IQR: 32%-79%) and 66% (IQR: 52%-94%), respectively. While tumor diameter was not associated with MM-free survival, higher tumor volume was associated with worse MM-free survival (HR=3.98, 95%CI: 1.71-9.26, for each 100cc increase in volume). Four (17%) patients had an increase in SUV Max after RT, and an increase in PET SUV Max after RT was not associated with worse MM-free survival (HR=0.62, 95%CI: 0.14-2.82). Similarly, 5 (21%) patients had an increase in SUV mean after RT, which was not associated with worse MM-free survival (HR=0.94, 95%CI: 0.21-4.32). When analyzing percent change in SUV Max and Mean as both a continuous variable and binary variable with an optimal cut point, there were no significant associations between change in SUV Max and SUV Mean and MM-free survival (p>0.30 for all analyses). Time between RT and post-RT PET was not associated with a greater change in SUV Max (R=0.18, p=0.45 ) or SUV Mean (R=-0.10, p=0.67).

Conclusion:

While PET-based imaging is critical for re-staging of SBP, changes in SUV at the primary lesion after definitive RT do not appear to predict progression to MM. The prognostic importance of tumor volume requires validation in larger cohorts controlling for established prognostic variables.