Malignant Psoas Syndrome Demonstrated by PET/CT in the Context of Metastatic Non Small Cell Lung Cancer

The significant cancer pain syndrome of malignant psoas syndrome (MPS), was first reported as a neurooncologic occurrence in 1990. The Syndrome is characterised by malignant infiltration of a psoas muscle, either directly or by involvement with haematogenously disseminated metastatic cancer. Direct involvement has been from primary tumours of the psoas muscle, or by extension from malignant paraaortic lymphadenopathy. Treatment is difficult, complex analgesia is essential, and benefits have been achieved from chemotherapy, radiotherapy and in select cases, surgery. Here, the case of a 60 year old female who presented with three months of moderate dyspnoea, low back and left inguinal region pain/numbness and lassitude, is reported. Chest radiograph and CT showed a large anterior mediastinal mass. FDG PET/CT revealed the mass to be intensely avid, with heterogeneous central areas of photopaenia. Avid lymphadenopathy was also present in two mediastinal nodal stations. Subdiaphragmatically, there was a metabolically FDG avid soft tissue mass in the superior part of the left psoas muscle, presumably accounting for her pain. There was no FDG avidity elsewhere. Core biopsy of the psoas mass revealed adenocarcinoma of probable lung origin. The patient responded symptomatically to intermediate dose radiation therapy. The pathophysiology of MPS is discussed and the range of cancer types associated with the Syndrome is updated.