Comment on: Intercontinental collaborative experience with abdominal, retroperitoneal and pelvic schwannomas

Correspondence to: Lauren Taylor (email: lauren.taylor6@nhs.net)

Plastic Surgery Department

Wythenshawe Hospital

Southmoor Road

Manchester M23 9LT

UK

BJS, https://doi.org/10.1002/bjs.11376, published 01 March 2020

We read with interest the paper by the Transatlantic Australasian Retroperitoneal Sarcoma Working Group reporting the outcomes of their intercontinental collaborative experience with abdominal, retroperitoneal and pelvic schwannomas1. Assessment of volume has been of interest to us in the context of the measurement of schwannomas and neurofibromas, especially where multiple lesions are being monitored over. The gold standard would be to have automatic readings within radiological software capable of performing volumetric analyses. In the meantime, techniques such as three axis measurement, two axis measurement and subsequent estimation of the third axis, and mathematical formulae for the volume of a triaxial ellipsoid can be used. We take issue however, with the formula that has been used in this study as it overestimates the volume of the schwannomas described by a factor of eight.

The formula used to calculate the schwannoma volume in this study is stated as V=4/3π(xyz), where x, y and z have been used to represent the measurements of the axial, coronal and sagittal axes respectively. The correct mathematical formula for the volume of a triaxial ellipsoid, however, is V=4/3π(abc), where a, b and c represent the measurements of the semi-axes rather than the axes. A semi-axis is half the length of an axis. Using the variables stated in this paper, the formula should therefore read V=4/3π(½x½y½z), which resolves to π/6(xyz). Compared to V=4/3π(xyz), we can see an overestimation of the volume by a factor of eight and would ask the group to publish a correction so that this error is not propagated in future related publications.

Other authors2 have suggested simplifying the volumetric equation further by estimating π as 3, such that the formula for the volume of a triaxial ellipsoid is V=π/6(abc)=½(abc), where a, b and c correctly represent the lengths of the semi-axes.