Myositis following spine radiosurgery for metastatic disease: a case series

Abstract

OBJECTIVE Spinal stereotactic body radiation therapy (SBRT) has emerged as an attractive method to deliver high doses of radiation to oligometastatic spinal tumors with radioresistant histology. Because SBRT is a palliative therapy, attention to potential radiation toxicities is paramount when counseling patients. The objective of this study was to report radiation-induced myositis after SBRT, a previously undescribed complication. METHODS A total of 667 patients received 891 spine SBRT treatments (either 24 Gy in 1 fraction or 27 Gy in 3 fractions) from 2011 to 2016 and underwent retrospective review. Eleven patients were identified as having radiographic evidence of myositis following SBRT. Clinical and pathologic results were collected, including receipt of anti-vascular endothelial growth factor (VEGF) therapy, radiation dose, equivalent dose in 2-Gy fractions (EQD2), biologically effective dose (BED), and volume of muscle treated. Treatment toxicities were classified according to the Common Terminology Criteria for Adverse Events (CTCAE; version 4.03). Univariate statistical analyses were performed to evaluate the relationships between radiation fractionation schedule and myositis and between anti-VEGF therapy and myositis. RESULTS The cumulative incidence of myositis was 1.9% at 1 year. The median of the mean dose administered to muscle with myositis was 17.5 Gy. The median EQD2 was 55.1 Gy, and the median BED was 82.7 Gy. The median time to the development of clinical symptoms was 1.4 months, while the median time to imaging evidence was 4.7 months. Two patients (18.2%) had CTCAE grade 3 complications. Single-fraction spine SBRT (HR 4.5, 95% CI 1.2-16.9; p = 0.027) was associated with increased risk of developing myositis whereas receipt of anti-VEGF therapy was not (HR 2.2, 95% CI 0.6-7.1; p = 0.2). CONCLUSIONS Radiation myositis following spinal radiosurgery is a rare but important complication. Single-fraction treatment schedules may be associated with increased risk of myositis but should be validated in a larger series.

Keywords: BED = biologically effective dose; CTCAE = Common Terminology Criteria for Adverse Events; EQD2 = equivalent dose in 2-Gy fractions; GTV = gross tumor volume; SBRT; SBRT = stereotactic body radiation therapy; VEGF = vascular endothelial growth factor; complications; myositis; oncology; radiation; spine radiosurgery.

FIG. 1

A: Axial T2-weighted MR image of the L-3 vertebral body and paraspinal muscles obtained at the initial diagnosis of L-3 metastasis. B: MR image of the L-3 vertebral body obtained 6 months after radiation therapy at the onset of myositis symptoms showing T2 hyperintensity of the left psoas muscle and erector spinae muscles. C: Lack of contrast enhancement on a T1-weighted postcontrast scan obtained 6 months after radiation therapy. D: Axial T2-weighted MR image of the L-3 vertebral body obtained 8 months after radiation therapy at the time of the clinical resolution of myositis symptoms. E–G: Axial T2-weighted MR images obtained 1.2 years (E), 1.6 years (F), and 2.5 years (G) after radiation therapy, showing volume loss of the left psoas with vague residual T2 hyperintensity.

FIG. 2

Radiation treatment plans with isodose lines for the prescribed dose of 24 Gy in 1 fraction that was administered to the L-3 vertebral body shown. The axial (A), sagittal (B), and coronal (C) planes are shown. Figure is available in color online only.

FIG. 3

Biopsy specimen of the left psoas muscle demonstrating necrotic myocytes abutting normal myocytes (A) and acute inflammation (B). H & E, original magnification ×20 (A) and ×40 (B). Figure is available in color online only.