Minimising Adverse Drug Reactions and Verifying Economic Legitimacy in Children (MARVEL-PIC)

Inclusion Criteria:

* Age \< 18 years
* New cancer diagnosis or patient receiving HSCT or patient has a relapsed cancer diagnosis and is starting treatment after more than 6 months without.
* Starting treatment with a chemotherapeutic agent that is not single agent oral targeted therapy.
* Must also be taking a medication for which there is an established CPIC guideline available.
* Parent or patient is able and willing to give consent for patient to take part and be followed up for at least 12 weeks.
* Patient is amenable to venepuncture and blood draw (5mL ideally with an absolute minimum requirement of 2.5 mL) or has Whole Genome Sequencing available (WGS).
* Patient and/or parent is able and willing to sign an informed consent form.
* Patient and/or parent is able to complete Ped-PRO-CTCAE survey in English, Italian or Chinese.
* Study enrolment limit has not been reached.

Exclusion Criteria:

* Age \> 18 years.
* Patient has a life expectancy estimated to be less than three months by the treating clinical team.
* Duration of the drug of inclusion total treatment length is planned to be less than one week.
* Patient and/or parent is unable to consent to the study.
* Patient and/or parent is unwilling to take part in the study.
* Patient and/or parent is able unable to complete Ped-PRO-CTCAE survey in English, Italian or Chinese.
* Patient has existing impaired hepatic or renal function for which a lower dose or alternate drug selection are already part of current routine care.
* Patient has a glomerular filtration rate of less than 15 mL/min per 1.73m2.
* Patient has advanced liver failure.