6.7 Medical Calculations Using Body Surface Area (BSA)
Learning Objectives
By the end of this chapter, learners will be able to:
- Calculate the BSA of a patient using the Mosteller formula or a nomogram.
- Calculate the dose of a chemotherapy drug based on the patient’s BSA and the standard dose per m2.
- Adjust the dose of a chemotherapy drug based on the patient’s renal or hepatic function, if needed.
Calculate the BSA of a Patient Using the Mosteller Formula or a Nomogram.
- BSA-based dosing is a method of prescribing chemotherapy drugs that normalizes the dose according to the patient’s body size. This is done to reduce the variability in drug exposure and toxicity among different patients and to optimize the efficacy and safety of the treatment.
- BSA is the external surface area of the human body measured in square meters (m2). It can be estimated using various formulas that are based on the patient’s height and weight. The Mosteller formula is one of the simplest and most commonly used formulas. It is given by:
[latex]BSA (m^2) = \sqrt{\frac{height (cm) \times weight (kg)}{3600}}[/latex]
where:
- BSA = Body Surface Area in square meters (m2)
- Height = height of the patient in centimeters (cm)
- Weight = weight of the patient in kilograms (kg)
Calculate the Dose of a Chemotherapy Drug Based on the Patient’s BSA and the Standard Dose per m2.
Once the BSA is calculated, the dose of a chemotherapy drug can be determined by multiplying the BSA by the standard dose per m2. For example, if the standard dose of a drug is 100 mg/m2, and the patient’s BSA is 1.5 m2, then the dose of the drug is:
[latex]\begin{aligned} Dose &= BSA \times StandardDose \\ &= 1.5 \times 100 \\ &= 150 mg \end{aligned}[/latex]
Adjust the Dose of a Chemotherapy Drug Based on the Patient’s Renal or Hepatic Function, if Needed.
However, some chemotherapy drugs may need to be adjusted based on the patient’s renal or hepatic function, as these organs are responsible for metabolizing and eliminating the drugs from the body. If the renal or hepatic function is impaired, drug clearance may be reduced, leading to higher drug levels and increased risk of toxicity. Therefore, some formulas or guidelines have been developed to adjust the dose of certain drugs based on the patient’s creatinine clearance (CrCl) or liver enzymes. These formulas or guidelines should be consulted before prescribing the drugs, and the dose should be modified accordingly.
Example 1
Calculate the BSA of a patient who is 175 cm tall and weighs 70 kg using the Mosteller formula.
Solution:
Using the Mosteller formula, the BSA is:
[latex]\begin{aligned} BSA &= \sqrt{\frac{Height \times Weight}{3600}} \\ &= \sqrt{\frac{175 \times 70}{3600}} \\ &= 1.84 m^2 \end{aligned}[/latex]
Example 2
Calculate the dose of doxorubicin for a patient who has a BSA of 1.8m2 and a CrCl of 40 mL/min. The standard dose of doxorubicin is 60 mg/m2, and the dose should be reduced by 50% if the CrCl is less than 50 mL/min.
Solution:
Using the standard dose and the BSA, the dose of doxorubicin is:
[latex]\begin{aligned} Dose &= BSA \times StandardDose \\ &= 1.8 \times 60 \\ &= 108 mg \end{aligned}[/latex]
However, since the patient has a CrCl of 40 mL/min, which is less than 50 mL/min, the dose should be reduced by 50%. Therefore, the adjusted dose of doxorubicin is:
[latex]\begin{aligned} AdjustedDose &= Dose \times 0.5 \\ &= 108 \times 0.5 \\ &= 54 mg \end{aligned}[/latex]
Example 3
Calculate the BSA of a patient who is 5 feet 8 inches tall and weighs 150 lbs using the Mosteller formula.
Solution:
First, we need to convert the height and weight to centimeters and kilograms, respectively. To do this, we can use the following conversion factors:
- 1 inch = 2.54 cm
- 1 foot = 12 inches
- 1 lb = 0.4536 kg
Therefore, the height in centimeters is:
[latex]\begin{aligned} Height &= (5 \times 12 + 8) \times 2.54 \\ &= 172.72 cm \end{aligned}[/latex]
The weight in kilograms is:
[latex]\begin{align*} Weight &= 150 \times 0.4536 \\ &= 68.04 kg \end{align*}[/latex]
Then, we can use the Mosteller formula to calculate the BSA:
[latex]\begin{align*} BSA &= \sqrt{\frac{Height \times Weight}{3600}} \\ &= \sqrt{\frac{172.72 \times 68.04}{3600}} \\ &= 1.81 m^2 \end{align*}[/latex]
Example 4
Calculate the dose of methotrexate for a patient who has a BSA of 1.9m2. The standard dose of methotrexate is 12 g/m2.
Solution: Using the standard dose and the BSA, the dose of methotrexate is:
[latex]\begin{align*} Dose &= BSA \times StandardDose \\ &= 1.9 \times 12 \\ &= 22.8 g \end{align*}[/latex]
Example 5
Calculate the dose of doxorubicin for a patient who is 5 feet 6 inches tall and weighs 140 lbs, and has a CrCl of 40 mL/min. The standard dose of doxorubicin is 60 mg/m2, and the dose should be reduced by 50% if the CrCl is less than 50 mL/min.
Solution:
First, we need to convert the height and weight to centimeters and kilograms, respectively. To do this, we can use the following conversion factors:
- 1 inch = 2.54 cm
- 1 foot = 12 inches
- 1 lb = 0.4536 kg
Therefore, the height in centimeters is:
[latex]\begin{align*} Height &= (5 \times 12 + 6) \times 2.54 \\ &= 167.64 cm \end{align*}[/latex]
The weight in kilograms is:
[latex]\begin{align*} Weight &= 140 \times 0.4536 \\ &= 63.5 kg \end{align*}[/latex]
Then, we can use the Mosteller formula to calculate the BSA:
[latex]\begin{align*} BSA &= \sqrt{\frac{Height \times Weight}{3600}} \\ &= \sqrt{\frac{167.64 \times 63.5}{3600}} \\ &= 1.72 m^2 \end{align*}[/latex]
Using the standard dose and the BSA, the dose of doxorubicin is:
[latex]\begin{align*} Dose &= BSA \times StandardDose \\ &= 1.72 \times 60 \\ &= 103.2 mg \end{align*}[/latex]
However, since the patient has a CrCl of 40 mL/min, which is less than 50 mL/min, the dose should be reduced by 50%. Therefore, the adjusted dose of doxorubicin is:
[latex]\begin{align*} AdjustedDose &= Dose \times 0.5 \\ &= 103.2 \times 0.5 \\ &= 51.6 mg \end{align*}[/latex]
So, the dose of doxorubicin for a patient who is 5 feet 6 inches tall and weighs 140 lbs, and has a CrCl of 40 mL/min, is 51.6 mg.
Exercises
1) Calculate the BSA of a patient who is 160 cm tall and weighs 50 kg using the Mosteller formula.
2) Calculate the dose of cyclophosphamide for a patient who has a BSA of 1.6m2. The standard dose of cyclophosphamide is 750 mg/m2.
3) Calculate the dose of cisplatin for a patient who has a BSA of 1.7 m2 and a CrCl of 60 mL/min. The standard dose of cisplatin is 75 mg/m2 , and the dose should be reduced by 25% if the CrCl is between 50 and 60 mL/min.
4) Calculate the BSA of a patient who is 180 cm tall and weighs 80 kg using the Mosteller formula.
5) Calculate the dose of fluorouracil for a patient who has a BSA of 1.9 m2 and a liver enzyme level of 2 times the upper limit of normal (ULN). The standard dose of fluorouracil is 425 mg/m2, and the dose should be reduced by 20% if the liver enzyme level is more than 1.5 times the ULN.
6) Calculate the BSA of a patient who is 150 cm tall and weighs 45 kg using the Mosteller formula.
7) Calculate the dose of paclitaxel for a patient who has a BSA of 1.5 m2. The standard dose of paclitaxel is 175 mg/m2.
8) Calculate the dose of gemcitabine for a patient who has a BSA of 1.8 m2 and a CrCl of 30 mL/min. The standard dose of gemcitabine is 1000 mg/m2, and the dose should be reduced by 50% if the CrCl is less than 40 mL/min.
9) Calculate the BSA of a patient who is 165 cm tall and weighs 55 kg using the Mosteller formula.
10) Calculate the dose of etoposide for a patient who has a BSA of 1.7 m2. The standard dose of etoposide is 100 mg/m2.
11) Calculate the dose of irinotecan for a patient who has a BSA of 1.6 m2 and a liver enzyme level of 3 times the ULN. The standard dose of irinotecan is 180 mg/m2, and the dose should be reduced by 33% if the liver enzyme level is more than 2.5 times the ULN.
12) Calculate the BSA of a patient who is 170 cm tall and weighs 60 kg using the Mosteller formula.
13) Calculate the dose of vinorelbine for a patient who has a BSA of 1.8 m2. The standard dose of vinorelbine is 25 mg/m2.
14) Calculate the dose of capecitabine for a patient who has a BSA of 1.9 m2 and a CrCl of 50 mL/min. The standard dose of capecitabine is 1250 mg/m2 twice daily, and the dose should be reduced by 25% if the CrCl is between 30 and 50 mL/min.
15) Calculate the BSA of a patient who is 190 cm tall and weighs 90 kg using the Mosteller formula.
16) Calculate the BSA of a patient who is 6 feet 2 inches tall and weighs 180 lbs using the Mosteller formula.
17) Calculate the dose of bleomycin for a patient who has a BSA of 1.8m2. The standard dose of bleomycin is 15 units/m2.
18) Calculate the dose of oxaliplatin for a patient who has a BSA of 1.7 m2 and a CrCl of 80 mL/min. The standard dose of oxaliplatin is 85 mg/m2, and the dose should be reduced by 25% if the CrCl is between 30 and 80 mL/min.
19) Calculate the BSA of a patient who is 5 feet 4 inches tall and weighs 120 lbs using the Mosteller formula.
20) Calculate the dose of docetaxel for a patient who has a BSA of 1.6 m2 and a liver enzyme level of 4 times the ULN. The standard dose of docetaxel is 75 mg/m2, and the dose should be reduced by 50% if the liver enzyme level is more than 3 times the ULN.
21) Calculate the BSA of a patient who is 5 feet 10 inches tall and weighs 160 lbs using the Mosteller formula.
22) Calculate the dose of dacarbazine for a patient who has a BSA of 1.9 m2. The standard dose of dacarbazine is 250 mg/m2.
23) Calculate the dose of pemetrexed for a patient who has a BSA of 1.8 m2 and a CrCl of 45 mL/min. The standard dose of pemetrexed is 500 mg/m2, and the dose should be reduced by 50% if the CrCl is between 30 and 45 mL/min.
24) Calculate the BSA of a patient who is 5 feet 6 inches tall and weighs 140 lbs using the Mosteller formula.
25) Calculate the dose of topotecan for a patient who is 5 feet 11 inches and 215 lbs and a CrCl of 55 mL/min. The standard dose of topotecan is 1.5 mg/m2 daily for 5 days, and the dose should be reduced by 20% if the CrCl is between 40 and 60 mL/min.
Answers to Odd Exercises
1) 1.49 m2 3) 96 mg 5) 646 mg
7) 263 mg 9) 1.59 m2 11) 193 mg
13) 45 mg 15) 2.18 m2 17) 27 units
19) 1.57 m2 21) 1.89 m2 23) 450 mg
25) 2.65 mg
