The use of dual energy computed tomography (DECT) is increasingly considered in particle therapy (PT) to reduce the range of uncertainties attributed to the conversion of X-ray linear attenuation coefficients into relative stopping power (RSP). DECT scanners equipped with clinically available image conversion software can now be found in PT centers. In this work, RSP calculated on the basis of clinical DECT scanner software (syngo. via) was compared to a validated published procedure (Hudobivnik) for calibration and pediatric head phantoms. Based on material inserts, the average difference between RSP values calculated using syngo. via (Hudobivnik) against reference values were 1.0% (0.7%). This difference excludes the lung inserts as the syngo. via method does not provide Z(eff) values for CT numbers <-500 HU. An analysis of the head phantom showed overall a good agreement with all RSP differences within 1% between the syngo. via and Hudobivnik methods. The use of clinically available syngo. via provides equivalent accuracy as a validated RSP calculation method.