A multicentre audit of the geometric accuracy and water equivalence of 3D printed thermoplastic objects for use in radiotherapy

Abstract Background and purpose Plastic materials are widely used as water substitutes in radiotherapy; however, the dosimetric properties of thermoplastic polymers used in 3D printing can vary. A multicentre audit was conducted to quantify variations in geometric accuracy, density and water-mimicking properties of 3D-printed test objects. Materials & methods Ten centres printed three polylactic acid (PLA) blocks at varying infills. Block dimensions, including protruding and recessed discs, were measured and the blocks weighed to determine geometric accuracy and mass density. CT scans were used to derive the printed infills and mass densities corresponding to water equivalence. Tissue phantom ratios (TPRs) were measured for 6MV photon beams and compared to vendor-provided reference data for water. Results A mean error of (−0.2 ± 0.3)mm (mean ± standard deviation, SD) was found for all printed dimensions and disc diameters. Measured CT numbers and mass densities varied by up to 200 Hounsfield units(HU) and 0.20 g/cm³ between centres, respectively. Printed infill and mass densities producing water-equivalent CT density were calculated as (94.5 ± 3.5)% and (1.10 ± 0.03) g/cm³, respectively. Calculated water-equivalent thicknesses formed using combinations of the blocks varied by up to 3.0 mm between centres, with measured TPR data varying by up to 1.2%. Measured and calculated data were normally distributed across centres and fell within ± 2 SD of their respective means. TPR data closely emulated reference values for water. Conclusions A multicentre audit was completed to develop understanding of geometric and dosimetric errors associated with 3D printing in radiotherapy.