Body Surface Area Dosing in Oncology

Body surface area dosing scales cytotoxic doses to patient size so that exposure is similar between small and large adults. It is a convention with a weak physiological basis, and it does not account for organ function, genetics or body composition, but it remains the basis for most regimens.

Calculation. Mosteller formula: BSA (m2) = square root of (height in cm x weight in kg / 3600). Use the same formula throughout treatment. Measure height and weight at baseline and weigh before every cycle.

Examples. Height 165 cm and weight 60 kg gives BSA 1.66 m2. Height 175 cm and weight 90 kg gives 2.09 m2.

Applying the dose.

• Dose (mg) = prescribed dose (mg/m2) x BSA (m2). Round to the nearest vial size or within 5% of the calculated dose when the pharmacy agrees.
• Recalculate when weight changes by 10% or more from the weight used for the last calculation.
• Use actual body weight in obese adults; do not cap BSA or reduce the dose for obesity in curative treatment, since this lowers cure rates. A cap at 2.0 m2 is used only where the regimen sheet requires it.
• In cachexia or oedema, use the dry (best estimate) weight and record the reason.

Drugs not dosed by BSA.

• Carboplatin: Calvert formula, dose (mg) = target AUC x (GFR + 25), with GFR capped at 125 mL/min, so the maximum dose is AUC x 150 mg (AUC 5 gives 750 mg). Use measured GFR where possible; if estimated with Cockcroft-Gault, floor creatinine at 60 micromol/L when assays are standardised.
• Weight-based agents: bevacizumab and trastuzumab (mg/kg).
• Fixed doses: vincristine (capped 2 mg), many oral targeted agents, immunotherapy flat doses, and bleomycin (units/m2 with cumulative limit).

Children. BSA is usual from age 1 year; under 10 kg or under 1 year, dose by weight in mg/kg as set by the paediatric regimen sheet.

Kidney and liver impairment. BSA does not adjust for organ function. Reduce doses independently, using the organ function limits for each drug on the regimen sheet.

Older adults and sarcopenia. Low muscle mass lowers clearance. Consider a reduced starting dose of 10 to 20% in frail patients and escalate if tolerated.

Obesity. Full weight-based dosing is usual; monitor for toxicity, as outcomes are not worse when full doses are given.

Dose rounding. Allow rounding within 5% (10% for some monoclonal antibodies) to reduce waste, particularly where drug supply is costly and vials are shared across patients under pharmacy supervision.

Regional. Where scales or height measurement are not reliable, record measured values at every visit and avoid dosing from a patient’s reported weight. Cost-saving vial sharing needs a sterile preparation unit.

Checks at each cycle. Weigh, recalculate BSA and the total dose, and compare with the previous order. Have a second clinician check any difference of more than 5%.

Errors. A calculation error of 10% or more in either direction is a reportable event; review the patient for toxicity and inform the oncologist.

Escalation. Refer to the oncology pharmacist for an unusual BSA, unusual body habitus, amputation, or paediatric dosing in infants.