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Body Surface Area (BSA) is a critical biometric parameter in pediatric oncology because it correlates more accurately with physiologic functions such as cardiac output, renal function, and metabolic rate than body weight alone. For many chemotherapeutic agents, including methotrexate, dosing based on BSA (mg/m²) is the standard of care to optimize therapeutic efficacy while minimizing toxicity. This is particularly important in children, where variations in body size and composition are pronounced and can significantly impact drug pharmacokinetics .
To calculate the dose, the Mosteller formula is used as provided in the question:
BSA (m²) = √[(height in cm × weight in kg) ÷ 3600]
Substituting the child's measurements into the formula:
BSA = √[(90 cm × 15 kg) ÷ 3600]
BSA = √[1350 ÷ 3600]
BSA = √[0.375]
BSA ≈ 0.612 m²
Once the BSA is accurately determined, the total dose of methotrexate is calculated by multiplying the BSA by the prescribed dose per square meter:
Dose = BSA × Prescribed dose (mg/m²)
Dose = 0.612 m² × 12 mg/m²
Dose = 7.35 mg
The calculated dose is approximately 7.35 mg, which is the correct answer. In clinical practice, calculated doses may require rounding to a measurable volume based on the drug concentration, institutional policy, and an independent verification process. The other options (12 mg, 15.6 mg, and 21.6 mg) result from using an incorrect BSA or treating the prescribed mg/m² dose as the total dose. Accurate BSA calculation is a foundational nursing responsibility for safe chemotherapy administration, as dosing errors can lead to significant toxicities, including severe myelosuppression and organ damage, particularly in agents like methotrexate where dosing adjustments are critical in the presence of organ dysfunction .
Calculate BSA using the Mosteller formula: √[(height cm × weight kg) ÷ 3600]. For this child: √[(90 × 15) ÷ 3600] = √0.375 ≈ 0.612 m².
Multiply BSA by the prescribed dose: 0.612 m² × 12 mg/m² = 7.35 mg. Round to a safe, measurable dose based on available drug concentration.
Never administer the mg/m² dose as the total dose. Always verify the final calculated dose with a second independent check, especially in pediatric oncology, to prevent life-threatening overdose.
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