Is it necessary to treat one vertebral body above and below for palliation of spinal metastases?  

No. The reasons to go one above & below were to avoid the dreaded miss from the dreaded days of bone scans, plain films & port films. That is much, much less likely these days with IGRT, CBCT, MRI (PET, etc.). We know treatment volume size correlates with toxicity. You can get some dysphagia...

Based on SBRT literature, the failure rate in adjacent vertebrae is very low because the evaluation is all based on MRI. The same principle will apply for external beam RT.Gerszten et al. 2007- 0 of 500 tumors failed in adjacent vertebraeRyu et al. 2004- 3 of 61 tumors failed in adjacent vertebraeNe...