Imaging-Based Local Control Outcomes Specific To Spine Stereotactic Body Radiotherapy (SBRT) For Prostate Cancer Metastases

Abstract

We report the first dedicated series of spine SBRT specific to prostate cancer metastases. A prospective database was retrospectively reviewed identifying 183 spinal segments in 93 prostate cancer patients treated with SBRT. All patients were followed regularly post-SBRT with both a clinical exam and full spine MRI. The primary endpoint was imaging-based local control rates and secondary outcomes included overall survival (OS) and vertebral compression fracture (VCF). Of the 183 treated spine segments, 130 (71%) had no prior radiation, 18 (10%) were post-operative, and 35 (19%) were re-irradiated after conventional RT. The median follow-up was 16 months (range 2-70 months). 67 patients were oligometastatic (<5 metastases) of which 36 had a solitary spinal metastasis. 44 patients (47%) had castrate resistant disease at the time of SBRT, while 10 of the 49 patients with hormone sensitive disease at the time of SBRT developed castrate resistance during follow up at a median time of 15 months after SBRT (range 7 – 37 months). Of the 183 spinal lesions, 120 (65%) were sclerotic, 32 (18%) were lytic, and 31 (17%) were mixed. The median spinal instability neoplastic score (SINS) was 5 (range 0-13). The majority of spinal segments (75%) were treated with 24-28 Gy/2 fractions while the rest (25%) were treated with 25-30 Gy/4-5 fractions. Actuarial local control (LC) rates at 1 and 2 years were 97% and 92%, respectively. The median OS was 61 months (range 43-77 months) and the 1- and 2-year OS rates were 91% and 81%, respectively. The cumulative risk of VCF was 5% at 1-year and 13% at 2 years. Excellent local control rates were observed in this cohort of patients with spinal metastases from prostate cancer with an acceptable risk of VCF. The high rate of OS in prostate cancer supports the need for excellent LC of spinal disease. Whether similar rates could be observed with conventional palliative radiation remains to be proven. Further work will identify predictive factors for local control and fracture rate after SBRT.