A cost-effective 3D-printed phantom insert for enhanced small-field patient specific quality assurance for stereotactic radiotherapy plans.

Abstract

Stereotactic radiotherapy on CyberKnife demands high precision in dosimetry due to the involvement of small-field treatments. This study addresses the geometric and dosimetric limitations of conventional phantom setups when using small-volume ionization chambers by designing and validating a 3D-printed polylactic acid (PLA) insert for the Sun Nuclear StereoPhan phantom, optimized for the PTW PinPoint 31022 chamber. The insert was designed to eliminate air gaps and misalignments, ensuring accurate and reproducible point-dose measurements. Radiological suitability was confirmed through CT imaging, with the PLA insert showing a mean Hounsfield Unit (HU) of 137 ± 10 compared to 119 ± 5 for PMMA, demonstrating equivalence. Dosimetric validation on Varian TrueBeam and Accuray CyberKnife systems showed clinically acceptable accuracy, with dose deviations ≤ 1.98% for TrueBeam VMAT/IMRT plans and ≤ 3.502 ± 1.23 for CyberKnife cones as small as 5 mm. In contrast, conventional Farmer chamber setup showed deviations as high as 5.08%. In retrospective CyberKnife patient-specific QA, 87.8% of PLA-PinPoint measurements were within ± 5% and 93.9% within ± 10% of TPS-calculated doses across 33 plans, with a mean absolute deviation of 3.55% with a minimum error of 0.4% and a maximum error of -16%, compared to 11.63% for the SNC125c chamber. These findings demonstrate the feasibility of a cost-effective 3D-printed insert as a promising alternative to conventional phantoms, particularly for small-field CyberKnife QA in resource-limited settings.