Katie, an 11-year-old, previously healthy Caucasian girl presents to her general practitioner (GP) with a 2-month history of being generally unwell with lethargy, loss of appetite, anorexia and aching legs. Past medical history reveals febrile convulsions as a young child but nothing else of note. Katie was born at term by normal delivery, is fully immunised and is on no current medication. Examination is unremarkable save for some small, shotty 1 cm diameter cervical and inguinal lymph nodes. Weight is 45 kg (75–90th centile) and height is 155 cm (90–95th centile).

The GP is concerned about a possible diagnosis of malignancy, particularly given the history. She therefore decides that further investigation is warranted and requests a full blood count (FBC).

FBC reveals haemoglobin 10.3 g/dl, white cell count (WCC) 45.5×10⁹/l and platelets 128×10⁹/l. Blood film reveals the presence of circulating lymphoblasts, suggestive of acute lymphoblastic leukaemia (ALL). Katie is referred to the tertiary paediatric oncology centre, where the diagnosis is confirmed by bone marrow examination and induction chemotherapy commenced according to the current ALL protocol. A central venous catheter is inserted for administering chemotherapy, other medication and blood sampling. She receives co-trimoxazole (septrin) routinely for Pneumocystis carinii pneumonia (PCP) prophylaxis. At the end of the 28-day induction phase, repeat bone marrow examination confirms that Katie is in morphological remission.

Katie tolerates the subsequent blocks of chemotherapy well (consolidation followed by two alternating courses of interim maintenance and delayed intensification). Shared care is provided by her local district general hospital and she starts maintenance chemotherapy at week 46 of treatment, as per the protocol. Her central venous catheter is removed at this stage as it is only required infrequently for chemotherapy now and is considered to be an ongoing infection risk.

At week 90, Katie develops intermittent …