Malignant psoas syndrome and bilateral hydronephrosis without evident obstruction in colorectal cancer.

Malignant psoas syndrome is a rare and under-recognised entity with a life expectancy of months. Its presentation is characterised by refractory back pain, which is a red flag that should raise suspicion about the condition. We present a case of a man who presented with refractory back pain and bilateral hydronephrosis without hydroureter who had no evident initial obstruction and showed stent-placement failure. Three months later, a recurrence of colorectal cancer associated with malignant psoas syndrome was diagnosed. We review the current literature on malignant psoas syndrome and hydronephrosis in relation to the presentation of cancer.