Bottom line: May-Thurner syndrome, or iliac compression syndrome, occurs when a DVT is associated with compression of the left iliac vein by the left iliac artery.
Kim DK, et al. Deep Vein Thrombosis Associated with May-Thurner Syndrome in an Amyotrophic Lateral Sclerosis Patient -A Case Report. Ann Rehabil Med. 2011 June; 35(3): 441–444.