1060 - The Effect of Palliative Radiotherapy Field Volume on Rates of Significant Hematologic Toxicity in Patients with Multiple Myeloma

S. C. Zhang¹, S. Kim², J. Steers¹, B. Stiehl¹, R. A. Vescio³, D. R. Oveisi³, B. Hakimian¹, K. M. Atkins¹, and L. K. Ballas¹; ¹Department of Radiation Oncology, Cedars-Sinai Medical Center, Los Angeles, CA, ²Department of Biostatistics, Cedars-Sinai Medical Center, Los Angeles, CA, ³Department of Hematology/Oncology, Cedars-Sinai Medical Center, Los Angeles, CA

Purpose/Objective(s): Radiotherapy (RT) is an effective palliative treatment for multiple myeloma (MM). While palliative RT is well tolerated, RT can cause cytopenia at nearly any dose. Volume of bone marrow receiving 10Gy (BMV10) has been associated with hematologic toxicity (HT) in cervical cancer, but no studies have investigated this in MM specifically. We hypothesized that absolute BMV10 is associated with rates of significant HT in MM patients receiving palliative RT. Materials/

Methods: This single institution retrospective analysis evaluated patients with MM who received palliative RT (dose =35Gy) between 2007-2023 and had at least 2 weeks of follow up laboratory data. RT doses were standardized to EqD2. Complete blood count values were recorded pre-RT, post-RT, and at nadir within 90 days (d) of completing RT. Significant HT was defined as new transfusion or growth factor requirement (“new” defined as no transfusion/growth factor within 14d prior to RT), admission for hematologic toxicity (e.g., febrile neutropenia), and/or systemic therapy pause/discontinuation within 90d of RT. BM was manually delineated for each treatment plan (defined as bone volume within RT field). RT dose distribution was standardized into EqD2 on a voxel basis using Varian Velocity. Absolute BM volume receiving EqD2 10Gy (BMV10) was recorded for each treatment. Only the first RT course was analyzed per patient, though some patients received RT to multiple lesions during their first course. For these, BMV10 from all treated lesions was summed into one volume. Logistic regressions were performed with significant HT within 90 days of completing RT as the primary endpoint.
Results: 125 patients were included in our analysis. Median age was 67 years (interquartile range [IQR] 60-74y). 60 (48%) patients were female. 45 (37%) patients had multiple sites treated (mean 2). 105 (84%) patients received concurrent systemic therapy. Median BMV10 was 266cc (IQR 157-501cc). Median RT EqD2 was 26Gy (IQR 23-33Gy). During the 90d window after completing RT, 49 (39%) patients developed grade = 3 anemia (median 29d from RT start), 34 (27.2%) grade = 3 thrombocytopenia (median 32d), 19 (15.6%) grade = 3 neutropenia (median 46d), and 52 (42.6%) grade = 3 lymphopenia (median 28d). 48 patients (38.4%) developed =1 significant HT at median 28 days (IQR 15-50 days) from RT start. On univariable analysis, BMV10 was significantly associated with rate of significant HT (p=0.018) while RT EqD2 was not (p=0.997). On multivariable analysis, BMV10 was significantly associated with rate of significant HT (p=0.049) after adjusting for RT EqD2 dose, single vs multiple lesions treated, lesion location (e.g., spine, pelvis, limb, soft tissue), and type of systemic therapy given with RT.
Conclusion: To our knowledge, this is the first study to identify a relationship between volume of irradiated bone marrow and hematologic toxicity in MM. We believe these findings are of high relevance in minimizing compromise of systemic therapy from palliative RT in MM patients.