Hyponatraemia in a woman with a pelvic mass.

admitted with a bulky, right lower abdominal mass. She was known to have end stage renal disease and had received a cadaveric renal transplant eight years previously. Further investigation with imaging confirmed an irregular, soft tissue mass in the right iliac and pelvic region with mass effect compressing on the ureter of the graft kidney. There was significant hypercalcaemia with a serum calcium level of 3.02 mmol/l (normal range 2.20–2.62 mmol/ l). The serum sodium and creatinine levels were 135 mmol/l (normal range 134– 145 mmol/l) and 141 μmol/l (normal range 44–107 μmol/l), respectively. A biopsy of the pelvic mass showed a posttransplant lymphoproliferative disorder. Chemotherapy was started in view of the extensive disease involvement. Treatment consisted of doxorubicin, cyclophosphamide, vincristine, and prednisolone (CHOP) after saline fluid administration. The patient tolerated the chemotherapy well without development of acute tumour lysis. Clinical response was achieved, as indicated by regression of the lymphadenopathy and resolution of the hypercalcaemia. Twelve days after the CHOP chemotherapy, her serum sodium concentration had fallen to 129 mmol/l (normal range 134–145 mmol/l). The patient remained euvolaemic and asymptomatic and no nausea or vomiting had been reported after the chemotherapy. The hospital chart documented that the daily urinary output ranged from 1.5 to 2 litres. The urinary osmolality was 268 to 302 mOsm/kg as compared to her serum osmolality of 255 mOsm/kg. Other laboratory studies revealed a serum potassium level of 4.1 mmol/l (normal range 3.8–5.4 mmol/l), random glucose 8.3 mmol/l, serum creatinine 99 μmol/l, and a urinary sodium concentration of 40 mmol/l. Both thyroid function and adrenal assessment were normal and there were no pulmonary or mediastinal lesions seen on a chest radiograph.