Calculations
BSA-Based Chemotherapy Dosing for OPRA: Worked Examples and MCQs
Most chemotherapy doses are prescribed per square metre of body surface area, not per kilogram — so before any dosing question can be answered, BSA itself has to be calculated correctly. This OPRA pharmacy calculations guide covers the Mosteller formula, worked BSA-to-dose examples, and the rounding and dose-capping conventions candidates most often get wrong.
Why this topic matters
BSA-based dosing is the standard for most cytotoxic chemotherapy, so an OPRA scenario involving oncology dosing is very likely to require a BSA calculation as the first step before the actual dose can be found. Getting the formula backwards, or missing a rounding/capping convention the question is testing, turns an otherwise correct clinical answer into a wrong numeric one.
Learning objectives
- Calculate body surface area using the Mosteller formula from height and weight
- Apply a calculated BSA to a per-m² chemotherapy dose to find the total dose
- Explain why actual body weight is used for BSA in most adult chemotherapy dosing, and when that's reconsidered
- Recognise why dose-banding and capping conventions exist, and why they must be confirmed against the specific protocol rather than assumed
Core concepts
The formula, at a glance
The two formulas this whole calculation type is built on:
- BSA (m²) = √[ (Height (cm) × Weight (kg)) ÷ 3600 ]
- Chemotherapy dose (mg) = Dose (mg/m²) × BSA (m²)
The Mosteller formula
BSA (m²) = √( (height (cm) × weight (kg)) ÷ 3600 ). This is the BSA equation most commonly used in Australian oncology practice and educational settings because it's simple to calculate and reproduce by hand. An alternative, the Du Bois formula, exists and is used in some contexts, but gives a slightly different result — always use the formula the question specifies, or Mosteller if none is stated, and never mix inputs from one formula's convention into the other.
From BSA to a total dose
Once BSA is known, a per-m² dose is simple multiplication: total dose = dose per m² × calculated BSA. For example, a drug dosed at 75 mg/m² for a patient with a BSA of 1.8 m² gives a total dose of 75 × 1.8 = 135 mg.
Actual body weight — and why it's not automatic
Most modern Australian oncology protocols calculate BSA from actual (measured) body weight, including in most patients with obesity — adjusted or capped weight is the exception, not the default. However, individual protocols may recommend a dose adjustment, capping, or other modification in selected circumstances (e.g. unusual fluid status such as significant oedema or ascites, or specific drugs with their own dosing rules), so the current protocol should always be checked rather than assumed from general principles. Routine BSA capping should never be assumed in an exam question unless the protocol or stem explicitly states it.
Clinical application
Worked example — calculating BSA, step by step
A patient is 170 cm tall and weighs 68 kg. Using the Mosteller formula, one step at a time:
- 170 × 68 = 11,560
- 11,560 ÷ 3600 = 3.211
- √3.211 ≈ 1.79 m²
Worked example — BSA to total dose
Using the BSA calculated above (≈1.79 m²), a chemotherapy agent dosed at 100 mg/m² gives a total dose of 100 × 1.79 ≈ 179 mg.
Worked example — working backwards from a stated total dose
A patient with a BSA of 1.6 m² is prescribed a total dose of 320 mg of an agent normally dosed per m². What is the per-m² dose being used? Rearranging total dose = dose per m² × BSA: dose per m² = 320 ÷ 1.6 = 200 mg/m² — useful for checking whether a written total dose matches the intended per-m² dose for a given patient.
Common mistakes
- Forgetting to take the square root at the end of the Mosteller calculation — this is the single most common arithmetic slip in this calculation type.
- Forgetting to divide by 3600 before taking the square root — the second most common version of the same slip, and one that produces a wildly implausible BSA (often in the thousands) if missed entirely.
- Mixing height in metres with weight in kg without converting height to cm first, which the Mosteller formula as commonly written requires.
- Assuming a dose cap or dose-banding rule applies (or doesn't apply) without checking the specific protocol — these conventions exist precisely because unmodified BSA-based doses can be clinically inappropriate in selected circumstances, but the exact rule varies by drug, institution and patient factors.
- Treating BSA as a fixed patient characteristic rather than recalculating it if weight changes significantly during treatment (e.g. after significant weight loss or fluid shifts).
Exam tips
- • If a stem gives height and weight and asks for a chemotherapy dose, expect BSA to be an explicit intermediate step — calculate and state it before applying the per-m² dose, since a question may award or test it separately.
- • Watch for units carefully: height is typically needed in cm for the Mosteller formula as commonly written, and a stem giving height in metres is testing whether you convert it correctly.
- • A calculated BSA for an average adult usually falls roughly between 1.2 and 2.5 m² — if your answer falls well outside that range, it's rarely because the patient is genuinely that size; check for a units slip (metres vs. cm), a missed square root, or a missed division by 3600 before moving on.
- • Don't assume a dose cap or an adjusted/capped weight applies just because a patient is described as obese or underweight — most modern protocols use actual body weight by default, and a capping convention should only be applied if the protocol or the stem explicitly says so.
Memory tricks
- • "Height and weight, divide by height-and-weight's usual scale (3600), then root it" — BSA = √((cm × kg) ÷ 3600) is the whole Mosteller formula in one line; the most common way to lose the mark is forgetting the final square root (or the division by 3600 before it).
Clinical pearls
- 💡 BSA-based dosing assumes a roughly linear relationship between body size and drug clearance for many cytotoxic agents, but this relationship is imperfect — it's a practical dosing convention refined over decades, not a precise pharmacokinetic measurement for any individual patient.
- 💡 BSA-based dosing remains the default for most cytotoxic chemotherapy largely because that's how the original clinical trials established their doses — newer targeted therapies and immunotherapies are often dosed differently (e.g. fixed dosing, or weight-based mg/kg dosing), so don't assume every oncology drug uses BSA just because chemotherapy conventionally does.
- 💡 Because dose-capping and adjusted-weight conventions are protocol- and drug-specific (and change as evidence and local practice evolve), the safest approach in real practice — and the one OPRA scenarios usually reward — is checking the specific current protocol rather than applying a remembered general rule.
Tables
BSA-based dosing — the calculation in three steps
| Step | What to do |
|---|---|
| 1. Calculate BSA | BSA (m²) = √((height (cm) × weight (kg)) ÷ 3600) |
| 2. Confirm the per-m² dose | Check the prescribed or protocol dose, in mg/m² (or equivalent) |
| 3. Calculate total dose | Total dose = dose per m² × BSA |
Sanity-check reference — approximate BSA by height and weight (not for memorising, for spotting an implausible answer)
| Height | Weight | Approx. BSA |
|---|---|---|
| 160 cm | 60 kg | ≈1.63 m² |
| 170 cm | 70 kg | ≈1.82 m² |
| 180 cm | 80 kg | ≈2.00 m² |
Practice MCQs (100% original)
1. A patient is 160 cm tall and weighs 75 kg. Using the Mosteller formula, what is this patient's approximate BSA?
2. A chemotherapy agent is dosed at 60 mg/m². For a patient with a calculated BSA of 1.9 m², what is the total dose?
3. A patient has lost 12 kg during chemotherapy due to treatment-related toxicity. Which is the most appropriate next step before calculating the next chemotherapy dose?
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Start freeFrequently asked questions
Is the Mosteller formula the only way to calculate BSA?
No — the Du Bois formula is another commonly referenced method and gives a slightly different result. Mosteller is the BSA equation most commonly used in Australian oncology practice and educational settings because of its simplicity, and is the default to use unless a question specifies otherwise.
Is BSA always calculated from a patient's actual body weight?
For most adult patients, yes — most modern oncology protocols use actual body weight, including for many patients with obesity. Adjusted or capped weight is the exception rather than the default, applied only where a specific protocol calls for it (e.g. unusual fluid status or a specific drug's own dosing rules). This should be confirmed against the current treatment protocol rather than assumed.
Does BSA need to be recalculated during a course of chemotherapy?
BSA should generally be recalculated whenever a clinically significant change in body weight occurs, or when required by the treatment protocol — using an outdated figure after such a change risks a dose that no longer matches the patient's current size.
Official references
- eviQ Cancer Treatments Online ↗ — Australian chemotherapy protocols, dosing and dose-modification guidance
- Australian Medicines Handbook ↗ — Drug-specific dosing detail