Involved Site and Reduced Dose Radiation Therapy for Multiple Myeloma: Should It be the New Standard?

Abstract

Patients with multiple myeloma (MM) often require multiple radiation treatments over the course of their disease. Marrow preservation is a priority in this population, as chemotherapy and stem-cell transplant are the primary curative treatments. However, the optimal dose of palliative radiation is not well defined, as published series report a wide range of effective doses and fractionation with some advocating for doses of 30 Gy or more for palliation. The goal of this study is to assess the relationship between radiation dose and field on re-irradiation of previously treated and adjacent sites at our institution. An institutional review of patients with MM treated with radiation therapy between 1999 and 2017 was performed. Target sites were defined as spinal and non-spinal. BED was calculated assuming α/β of 10. Re-irradiated sites were identified. In 2012 our group transitioned to involved site radiation therapy (ISRT) for treatment of MM spinal lesions. Only involved vertebral bodies were encompassed without including a vertebral body above and below the site of disease as is historical common practice when treating with conventional radiation. Re-irradiation due to failure in an adjacent vertebral body was quantified in this population. We identified 772 patients treated to a total of 1513 sites. Of the 1513 sites, 597 courses were given to spinal sites and 916 to non-spinal sites. 44% (n=340) of patients received radiation therapy to 2 or more sites and 22% (n=166) received radiation therapy to 3 or more sites. The maximum number of sites treated in a single patient was 18. Median follow-up was 24.4 months. Median survival after the first course of radiation therapy was 25.6 months. The most commonly used dose and fractionation patterns were 20-25 Gy in 8-12 fractions (71.2% and 65.2%, spine and non-spine respectively). The majority of patients were treated with five or more fractions (99.1% of spine sites and 98.3% non-spine sites). A total of 39 sites underwent re-irradiation (12 spinal, 27 non-spinal), corresponding to a cumulative re-irradiation rate of 2.6% (2.0% for spinal and 2.9% for non-spinal sites). Median time to re-irradiation was 9.6 months and 10.2 months for spinal and non-spinal sites, respectively. There was no significant association between BED and re-irradiation for spinal targets (odds ratio [OR] .94; p=.20) or non-spinal targets (OR .96; p = 0.21). 296 spine sites were treated with ISRT. Among this population, only one patient required re-irradiation for failure in an adjacent vertebral body. Among MM patients treated with radiation therapy, a dose of 20-25 Gy in 8-12 fractions using involved site radiation is associated with low re-irradiation rate. This dose and fractionation allows for safe salvage radiation therapy in most parts of the body if additional palliation or local control is needed.