Proximal lumbosacral plexopathy associated with malignant involvement of the psoas major muscle by primary or metastatic cancer is termed the malignant psoas syndrome (MPS). It is characterized by deep somatic nociceptive (muscle inflammation and spasm) and peripheral neuropathic pain (lumbar plexus injury). Its low frequency may reflect a significant under recognition of this cancer pain syndrome. Patients with MPS are usually severely disabled with cancer pain poorly responsive to polymodal analgesic strategies.
Stevens MJ, Atkinson C and Broadbent AM. The malignant psoas syndrome revisited: case report, mechanisms, and current therapeutic options. Journal of Palliative Medicine, 2010. Vol 13, number 2.