The goal of this clinical trial is to individualize the dosage of vincristine, a chemotherapy drug, in children with cancer. The main question it aims to answer is: can vincristine dosage be optimized while carefully monitoring toxicity.

The following will happen:

• Participants will receive vincristine according to the institutional treatment protocol.
• After receiving vincristine, blood samples will be taken at three time points.
• The amount of vincristine in the blood samples will be determined.
• If the amount of vincristine in the blood samples is lower than the reference and the participants do not experience toxicity due to vincristine, the dose of vincristine may be increased.
• Toxicity will be carefully monitored.

Vincristine is among the most widely used and potentially effective chemotherapeutic agents in pediatric oncology patients. However, in black African children it may be sub optimally dosed due to genetic differences in the metabolism of vincristine. This study aims to optimize the dosing regimen of vincristine while carefully monitoring toxicity.

This will be a prospective cohort study consisting of two parts: a feasibility study and the rest of the study. In the feasibility study, 15 children aged 5-14 years who are scheduled to receive at least 2 vincristine administrations can be included. After the administration of vincristine, venous blood samples and finger prick blood samples will be taken to determine the vincristine concentrations. The samples will be shipped to and analyzed in the Netherlands to determine the vincristine concentration in each sample. Based on this, a dose advise will be given for subsequent vincristine administrations. This cycle will be repeated maximum 2 times but maximum 1 dose advice is given. Toxicity will be monitored by determination of the bilirubin, by questionnaires and by physical examination to check for signs of peripheral neuropathy. In the rest of the study, in which 85 children will be included, only finger prick samples will be taken.

Feasibility study:

Inclusion Criteria:

• Black patients aged 5-14 years with a malignancy for which they are scheduled to receive a minimum of two VCR administrations as part of their treatment protocol: acute lymphoblastic leukemia, non-Hodgkin's lymphoma, rhabdomyosarcoma, neuroblastoma, nephroblastoma, retinoblastoma.
• Written informed consent

Exclusion Criteria:

• Severe malnutrition
• Total bilirubin >3 times upper limit of normal
• Pre-existent severe mental retardation e.g. Down syndrome
• Pre-existent peripheral neuropathy (CTCAE constipation, peripheral sensory neuropathy, peripheral motor neuropathy, or neuralgia ≥ 2 or ped-mTNS ≥ 5)

Rest of the study:

Inclusion Criteria:

• Black patients aged 2-14 years with a malignancy for which they are scheduled to receive a minimum of four VCR administrations as part of their treatment protocol: acute lymphoblastic leukemia, non-Hodgkin's lymphoma, rhabdomyosarcoma, neuroblastoma, nephroblastoma, retinoblastoma.
• Written informed consent

Exclusion Criteria:

• Severe malnutrition
• Total bilirubin >3 times upper limit of normal
• Pre-existent severe mental retardation e.g. Down syndrome
• Pre-existent peripheral neuropathy (CTCAE constipation, peripheral sensory neuropathy, peripheral motor neuropathy, or neuralgia ≥ 2 or ped-mTNS ≥ 5)

The data are available upon reasonable request.

The study protocol and ICF is already available upon request. Upon completion of the study, the SAP, CSR and analytic code will be accessible for unlimited amount of time.

The data are available upon reasonable request.