Hi Chiara. Understandable to question this matter.
The ‘traditional’ contra-indication/risk is theoretical / hypothetical only with no convincing evidence one way or the other. You might like to take a look at Laakso and Young (2010) Physical Therapy Reviews, 15(4) 334-343: “EPAs for symptom control in cancer - what is the evidence?”
Based on evidence, my feeling is that it could be used safely for the purpose (foot drop) you’ve described. If you’re particularly concerned, then apply the electrodes closer together then you otherwise might over the tib ant muscle belly to limit the depth to which the current might flow (thus limiting stimulation of bone marrow). Beware not to apply electrodes too close however, as you’ll limit the efficacy of the treatment.
The other thing you might want to reflect on is that if your patients are concurrently undergoing chemo, any theoretical risk of stimulating cancer cells should be obviated during active cancer treatment.
Just beware of skin sensitivity as a result of any whole body irradiation or skin side effects from chemo agents in leukaemias. Ensure exemplary skin and electrode hygiene, give very clear warnings about unwanted sensations, and keep a careful eye during treatments especially with children. Maybe start with minimal motor contraction and short periods to evaluate these factors.
Hope this helps. Can I urge you to keep a photographic log and good Clinical notes and perhaps write this up as a case study for publication?