Use of computed tomography and radiation therapy planning software to develop a novel formula for body surface area calculation in dogs

Background: Body surface area (BSA) can reflect metabolic rate that might normalize dosing of chemotherapeutics across widely variable weights within a species. The current BSA formula for dogs lacks height, length, and body condition. Hypothesis: Computed tomography (CT) imaging will allow inclusion of morphometric variables in allometric modeling of BSA in dogs resulting in an improved formula for BSA estimation. Animals: Forty-eight dogs from 4 institutions with whole-body CT images. Methods: Retrospective and prospective case series. Body surface area was contoured using whole-body CT scans and radiation therapy planning software. Body length and height were determined from CT images and also in 9 dogs by physical measurement. Nonlinear regression was used to model the BSA data sets using allometric equations. Goodness-of-fit criteria included average relative deviation, mean standard error, Akaike information criterion, and r ² (derived from the r-value generated by regression models). Results: Contoured BSA differed from the current formula by −9% to +19%. Nonlinear regression on untransformed data yielded BSA = 0.0134 × body weight [kg] ^∧ 0.4746 × length (cm) ^∧ 0.6393 as the best-fit model. Heteroscedasticity (increasing morphometric variability with increasing BSA) was an important finding. Conclusions and Clinical Importance: Computed tomography-derived BSA was used to incorporate body length into a novel BSA formula. This formula can be applied prospectively to determine whether it correlates with adverse events attributed to chemotherapy.