Body Surface Area Dosing (mg per m²)

Oncology doses by surface area rather than by weight, because many of the physiological quantities that determine how a cytotoxic drug behaves, including cardiac output, glomerular filtration and basal metabolic rate, scale closer to surface area than to mass. A protocol quoting 100 mg/m² for a patient of 1.73 m² means $100 \times 1.73 = 173$ mg. That 1.73 m² is the conventional average adult, which is why so many normalised quantities are quoted per 1.73 m².

The convention has been criticised for decades, and the criticism is worth knowing. Surface area itself is not measured; it comes from a formula, and the Du Bois fit was built on nine subjects in 1916 while Mosteller is a 1987 simplification of the same lineage. Different formulas disagree by a few percent on the same body, and by more at the extremes. Beyond that, several studies have found that BSA explains only a small fraction of the between-patient variation in clearance for many agents, so it reduces dosing variability far less than the practice implies. Carboplatin abandoned it entirely in favour of the Calvert formula, which doses on renal function directly.

Two practical points. Doses are usually capped at a BSA of 2.0 or 2.2 m² for very large patients, because the fits were never validated there and the linear extrapolation gets unsafe. And the calculated number is the start of the conversation, not the end: dose reductions for organ dysfunction, prior toxicity, performance status and drug interactions are applied on top, and choosing a chemotherapy dose is a clinical decision that this multiplication only feeds.