Improved commissioning of lung stereotactic body radiotherapy using a customized respiratory motion Phantom: a single- institutional study.

Stereotactic body radiation therapy (SBRT) involves delivering high doses of radiation with geometric precision in a few hypofractionated schedules. In lung SBRT, respiratory motion is an additional concern as it could cause the delivered dose distribution to deviate from the treatment plan. Therefore, it is crucial to conduct accurate commissioning tests on a dynamic phantom. In this study, the QUASAR™ Respiratory Motion Phantom was customized using 3D-printed parts to minimize motion-induced errors in measurements. The customisations included a specialized ion chamber insert designed to move with the tumour and measure the average dose at its centre. A film insert was also developed for secure fixation, enabling precise dose verification on a static plane while minimizing the risk of friction-related damage. The quality assurance (QA) tests were performed on the plans created for phantom studies indicated that ion chamber measurements were within 1.9% of the planned dose, and film gamma analysis demonstrated pass rates over 95% using the 3%/1 mm criteria. A set of SBRT volumetric modulated arc therapy (VMAT) plans were created for a suite of test patients using both flattened and flattening filter free (FFF) 6 MV beams and utilising robust optimization. A standardized patient-specific QA protocol was used to evaluate the treatment plans of 20 test patients, yielding film gamma pass rates above 98.8%. The suggested approach, using the 3D-printed inserts, effectively mitigated dose-blurring, providing a robust tool for lung SBRT commissioning and ensuring the reliability of lung cancer treatment with SBRT.