Water Equivalence of 3D-Printed ABS-Pro for Orthovoltage Phantom Dosimetry: Radiological Characterization and TOPAS Monte Carlo Validation Across 100–300 kVp

Reliable dosimetric phantoms are critical for quality assurance in orthovoltage radiotherapy (100–300 kVp), yet conventional materials such as liquid water and PMMA present well-known limitations in the kilovoltage energy domain, where the photoelectric effect heightens sensitivity to elemental composition. This study provides the first comprehensive radiological and dosimetric characterization of 3D-printed ABS-Pro as a water-equivalent phantom material across the orthovoltage energy range, with direct reference to the Xstrahl 300 therapy unit at 100, 180, and 300 kVp. Mass attenuation coefficients were derived from the NIST XCOM database; orthovoltage spectra were simulated using TOPAS Monte Carlo and cross-validated against SpekPy v2.5.4; and percentage depth dose curves, lateral profiles, and two-dimensional dose maps were compared across ABS-Pro, PMMA, and liquid water. The mass attenuation coefficient (MAC, μ/ρ) of ABS-Pro deviated from that of water by −4.9% at 100 keV, decreasing monotonically to −3.5% at 300 keV across the full orthovoltage window, all within the ±5% clinical acceptance threshold for phantom dosimetry. Monte Carlo PDD analysis yielded mean absolute deviations of 1.4%, 1.1%, and 0.9% at 100, 180, and 300 kVp—all within the ±2% tolerance criterion—with lateral profile and isodose geometry agreement below 1 mm throughout the clinical field. These results establish ABS-Pro as a validated, cost-effective, and geometrically flexible water-equivalent material for orthovoltage dosimetry.