3366 - Radiotherapy Outcomes in Solitary Plasmacytoma with High Risk Cytogenetic Abnormalities

K. M. Frechette¹, C. N. Day², J. Lucido³, H. Zhang⁴, J. C. Thull⁵, U. Yadav⁶, B. S. Hoppe⁷, J. L. Peterson⁷, W. G. Rule⁸, S. C. Lester³, S. Kumar⁶, F. Buadi⁶, W. I. Gonsalves⁴, and W. Breen³; ¹Mayo Clinic Department of Radiation Oncology, Rochester, MN, ²Department of Biomedical Statistics and Informatics, Mayo Clinic, Rochester, MN, ³Department of Radiation Oncology, Mayo Clinic, Rochester, MN, ⁴Mayo Clinic, Rochester, MN, ⁵Mayo Clinic, Department of Radiation Oncology, Rochester, MN, ⁶Mayo Clinic Department of Hematology, Rochester, MN, ⁷Department of Radiation Oncology, Mayo Clinic, Jacksonville, FL, ⁸Department of Radiation Oncology, Mayo Clinic, Phoenix, AZ

Purpose/Objective(s): High-risk cytogenetic (HRC) features in multiple myeloma (MM) have been associated with worse outcomes. Solitary plasmacytoma (SP), the potential precursor lesion to MM, can also harbor HRC features, which can increase risk of progression to MM. It is unknown whether HRCs impact local control of SP, or whether radiation dose should be personalized based on plasmacytoma genetic or molecular characteristics. We hypothesized that specific HRCs predict higher local failure (LF) and shorter time to progression to multiple myeloma for solitary plasmacytoma treated with definitive radiotherapy. Materials/

Methods: Patients treated with radiation between 2011-2022 for SP who had cytogenetic testing completed were included in this retrospective analysis. FISH testing of cytogenetic abnormalities at the time of diagnosis included t(11;14), t(14;16), t(4;14), t(14;20), del 17p/mono17, gain 1q, amp 1q and trisomies. HRC were defined as presence of any one cytogenetic abnormality noted above, except trisomies or t(11:14) required concurrent del17p or 1q gain/amp to be considered high risk. Patient, disease, and treatment characteristics including dosimetric variables were collected and analyzed for association with LF and progression to MM.
Results: 47 patients were included in this study. Median age was 63 (IQR 50-73) and 62% were male. All patients had solitary bone plasmacytoma, with 72% located in the axial skeleton. Median lesion size was 4.1 cm (range 1.1-14.2). The median radiation dose was 45 Gy given in a median of 25 fractions. The most common cytogenetic abnormalities were trisomies (19%), gain 1q (17%), del 17p/mono17 (15%), and t(11;14) (6%). Seventeen patients (36%) had HRC. There were a total of 5 local failures (n=2 in HRC and n=3 in non-HRC). Cumulative incidence of LF across the entire cohort was 4% and 10% at 2 and 5 years respectively. The cumulative incidence of LF at 2 and 5 years was 6% and 6% in the HRC group and 4% and 13% in the non-HRC group. There was no significant difference in LF between the HRC group and the non-HRC group as well as within any cytogenetic abnormality sub-groups (all p>0.3). Time to progression to MM was significantly shorter for those with HRC features (HR 3.33, p=0.001) and those specifically with t(11;14) abnormalities (HR 7.51, p=0.007) and gain 1q (HR 2.59, p=0.048). Lesion size, minimum dose or maximum dose to gross tumor volume or planned target volume, and lesion location were not predictive of local or distant failure (all p>0.05).
Conclusion: Local control was excellent following RT for SP irrespective of high-risk cytogenetic features. Given the similar rates of local control between groups, it appears reasonable to offer standard definitive RT doses to this high-risk population. High risk cytogenetic features such as t(11;14) and 1q gain did predict for earlier progression from SP to MM, consistent with prior studies. Further validation with larger studies is needed.