I recently gave a talk in shul on measures that are utilized in halacha and medicine. We are all familiar with the use of the amah or cubit (tip of the finger to the elbow) and hand breadth (tefach), measurements that were not used solely in Jewish law, but these were measures that were popular and utilized throughout the Middle East in biblical and post-biblical times.

Obviously, these are natural measures to use. Similarly, natural measures developed in Rome and the West, such as the foot and the stone. The first “scientific” system for measurement was the metric system, developed in the 1790s by the French Academy of Sciences as an outgrowth of the French Revolution, which resulted in the development of the meter as a fraction of the size of the earth, eventuating in the metric system.

In the prescription and administration of chemotherapy drugs, most are given as a function of patient size. Most easy to comprehend is the use of weight, measured in kilograms, for this purpose. The drug Vectibix (panitumumab), for example, is prescribed at 6mg/kg reflecting this approach. Other drugs also utilize the kilogram as their base.

However, the metric (no pun intended) that is used the most in chemotherapy prescription and administration is the body surface area (BSA). As the name implies, it represents the surface area of the individual. It is calculated as a function of the individual’s mass and height, but since we do not typically know an individual’s mass, we utilize the weight and height to calculate the BSA. Multiple different algorithms and formulas are available to estimate an individual’s BSA using his/her weight and height, so typically the healthcare professional plugs these values into one of these formulas to derive the BSA. They do vary a bit for pediatric versus adult populations. One such commonly used formula is the Du Bois algorithm:

BSA (m2) = 0.007184 x W0.425 x H0.725

In this formula, weight (W) is expressed in kilograms and height (H) in centimeters.

Another probably more commonly utilized and somewhat simpler formulation is the Mosteller formula:

BSA (m2) = square root of (height (cm) x weight (kg)/3600)

In one study published about 15 years ago, the average BSA for 3,000 cancer patients was estimated to be 2.06 m2 for males aged 20-79 years and 1.83 m2 for females of the same age range. Nowadays, it is estimated that the average BSA for U.S. men is about 1.9 m2 while it is 1.6 m2 for U.S. women.

Why is the BSA preferred over the simpler weight? It is thought to have less dependence on the degree of adipose tissue in the body. The proportion of adipose tissue may have a profound effect on drug clearance and therefore would significantly affect drug doses and drug metabolism. As a result, the BSA is used for chemotherapy drug dosing. It is also used for the calculation of the glomerular filtration rates in renal disease. A third use of the BSA is the calculation of the cardiac index, an important measure of cardiac function.

There are multiple calculators available online for the reader interested in calculating his/her own BSA and which will also convert pounds and inches into the appropriate metric system measurements for this purpose. The measurement with which to avoid confusion is the BMI or body mass index:

BMI = weight/ height2

with weight in kilograms and height in meters.

As I mentioned above, there are as many as 25 different formulas utilized for BSA calculation and they may vary by as much as 0.5 m2 at the extremes. Thus, it is important for healthcare professionals to stick with one of those commonly utilized formulas in order to be consistent. As I indicated above, the Mosteller or du Bois formulas are the most frequently used and seem the safest to stick with.

Alfred I. Neugut, MD, PhD, is a medical oncologist and cancer epidemiologist at Columbia University Irving Medical Center/New York Presbyterian and Mailman School of Public Health in New York. Email: [email protected].

This article is for educational purposes only and is not intended to be a substitute for professional medical advice, diagnosis, or treatment, and does not constitute medical or other professional advice. Always seek the advice of your qualified health provider with any questions you may have regarding a medical condition or treatment.