Consensus guidelines for postoperative stereotactic body radiation therapy for spinal metastases: results of an international survey

Abstract

OBJECTIVE Although postoperative stereotactic body radiation therapy (SBRT) for spinal metastases is increasingly performed, few guidelines exist for this application. The purpose of this study is to develop consensus guidelines to promote safe and effective treatment for patients with spinal metastases. METHODS Fifteen radiation oncologists and 5 neurosurgeons, representing 19 centers in 4 countries and having a collective experience of more than 1300 postoperative spine SBRT cases, completed a 19-question survey about postoperative spine SBRT practice. Responses were defined as follows: 1) consensus: selected by ≥ 75% of respondents; 2) predominant: selected by 50% of respondents or more; and 3) controversial: no single response selected by a majority of respondents. RESULTS Consensus treatment indications included: radioresistant primary, 1-2 levels of adjacent disease, and previous radiation therapy. Contraindications included: involvement of more than 3 contiguous vertebral bodies, ASIA Grade A status (complete spinal cord injury without preservation of motor or sensory function), and postoperative Bilsky Grade 3 residual (cord compression without any CSF around the cord). For treatment planning, co-registration of the preoperative MRI and postoperative T1-weighted MRI (with or without gadolinium) and delineation of the cord on the T2-weighted MRI (and/or CT myelogram in cases of significant hardware artifact) were predominant. Consensus GTV (gross tumor volume) was the postoperative residual tumor based on MRI. Predominant CTV (clinical tumor volume) practice was to include the postoperative bed defined as the entire extent of preoperative tumor, the relevant anatomical compartment and any residual disease. Consensus was achieved with respect to not including the surgical hardware and incision in the CTV. PTV (planning tumor volume) expansion was controversial, ranging from 0 to 2 mm. The spinal cord avoidance structure was predominantly the true cord. Circumferential treatment of the epidural space and margin for paraspinal extension was controversial. Prescription doses and spinal cord tolerances based on clinical scenario, neurological compromise, and prior overlapping treatments were controversial, but reasonable ranges are presented. Fifty percent of those surveyed practiced an integrated boost to areas of residual tumor and density override for hardware within the beam path. Acceptable PTV coverage was controversial, but consensus was achieved with respect to compromising coverage to meet cord constraint and fractionation to improve coverage while meeting cord constraint. CONCLUSIONS The consensus by spinal radiosurgery experts suggests that postoperative SBRT is indicated for radioresistant primary lesions, disease confined to 1-2 vertebral levels, and/or prior overlapping radiotherapy. The GTV is the postoperative residual tumor, and the CTV is the postoperative bed defined as the entire extent of preoperative tumor and anatomical compartment plus residual disease. Hardware and scar do not need to be included in CTV. While predominant agreement was reached about treatment planning and definition of organs at risk, future investigation will be critical in better understanding areas of controversy, including whether circumferential treatment of the epidural space is necessary, management of paraspinal extension, and the optimal dose fractionation schedules.

Keywords: ASIA = American Spinal Injury Association; CTV = clinical tumor volume; GTV = gross tumor volume; MESCC = malignant epidural spinal cord compression; PRV = planning risk volume; PTV = planning target volume; RT = radiation therapy; RTOG = Radiation Therapy Oncology Group; SBRT; SBRT = stereotactic body radiation therapy; consensus guidelines; oncology; postoperative spine stereotactic body radiation therapy; spinal metastases.

Fig. 1

Example of a circumferential treatment plan for a patient treated with 3000 cGy in 5 fractions. Circumferential treatment of the epidural space is controversial as it is challenging from the treatment planning perspective and generally requires greater compromise of the epidural space to meet spinal cord constraints. Nonetheless, the most common practice is to treat circumferentially in patients with circumferential disease preoperatively, although some providers do this only when there is gross residual circumferential disease postoperatively, while others avoid it in all patients. Republished with permission of Elsevier Science and Technology Journals, from Postoperative stereotactic body radiation therapy (SBRT) for spine metastases: a critical review to guide practice, Redmond et al., Int J Radiat Oncol Biol Phys 95(5):1414–1428, 2016; permission conveyed through Copyright Clearance Center, Inc. Figure is available in color online only.

Fig. 2

Sample treatment plan for a patient with metastatic renal cell carcinoma and an L-1 metastasis who had undergone surgical decompression and stabilization. A: Preoperative T1-weighted post-gadolinium MR image showing tumor involvement of the vertebral body and pedicles and extension into the epidural space with tumor encompassing the cauda equina. B and C: Postoperative T2-weighted MR image (B) and CT myelogram (C) showing decompression of the thecal sac and placement of surgical instrumentation for stabilization. D: Final postoperative spine SBRT treatment plan. The red line represents the prescription isodose line. The blue contour is the thecal sac without margin which serves as the cord avoidance structure. It is subtracted out from the PTV (shown as the green contour) to meet the spinal cord constraints. Republished with modification (reformatted) with permission of Elsevier Science and Technology Journals, from Postoperative stereotactic body radiation therapy (SBRT) for spine metastases: a critical review to guide practice, Redmond et al., Int J Radiat Oncol Biol Phys 95(5):1414–1428, 2016; permission conveyed through Copyright Clearance Center, Inc. Figure is available in color online only.