Tumour lysis syndrome and lack of efficacy: case reportA 74-year-old woman developed fatal tumour lysis syndrome (TLS) during treatment with letrozole for occult metastatic breast carcinoma. She exhibited lack of efficacy while being treated with rasburicase for TLS and died.The woman with various comorbidities, presented with progressively worsening dyspnoea and nonproductive cough. She had been prescribed furosemide by her general practitioner for presumed cardiac failure as the cause of her dyspnoea; however, her symptoms did not improve. Based on further investigations, she was diagnosed with occult metastatic breast carcinoma. She was then started on oral letrozole 2.5mg per dose. However, on the following day, she was due to be discharged home when she developed a fever of 38.2°C. During this time, she met the clinical criteria for sepsis (differential diagnosis).The woman was then treated with amoxicillin/clavulanic-acid [co-amoxiclav]. Laboratory investigations showed an acute kidney injury. Laboratory investigations showed elevated levels of sodium, potassium, serum creatinine, urea and CRP, and decreased estimated glomerular filtration rate. She received appropriate treatment to manage hyperkalaemia. Additionally, she received IV fluids. Based on the findings, she was diagnosed with TLS. Therefore, she received rasburicase 3mg once daily [route not stated] for 3 days with aggressive IV fluid therapy. Her renal function stabilised for 24 hour; however, at this point, it was clear that she had clinically deteriorated significantly. On day 7 and day 8 post letrozole initiation, her laboratory investigations showed uric acid 0.74 and 0.21 mmol/L, calcium 2.20 and 2.26 mmol/L, phosphorous 1.87 and 1.86 mmol/L, respectively. She was increasingly drowsy throughout the day and less responsive. Her Glasgow Coma Scale (GCS) score fluctuated between 12 and 14 due to confusion, and was often not rousable to pain. Her vital signs were progressively worsening with reducing oxygen saturations and an increasing supplementary oxygen requirement. Blood cultures did not identify a causative organism, and she had a single isolated fever at the time of initiating sepsis management. Thus, it was concluded that she was not necessarily septic at all during the acute illness but that these symptoms had been related to malignant disease and the development of TLS. As she was approaching towards end of life, she was discharged home with hospice cares in place. Two days after discharge, she died due to TLS.