Development of a modular-based phantom for quality assurance of single-isocenter multiple-target stereotactic radiosurgery

<i>Background.</i>Single-isocenter multiple-target (SIMT) treatments are a robust beam delivery technique involving modulated MLCs for off-axis targets. SIMT requires quality assurance (QA) for both dosimetry and geometry to ensure accurate beam delivery according to the treatment plan. A primary challenge in SIMT QA is the need for phantoms capable of evaluating both dosimetric and geometric accuracy. Traditionally, separate phantoms are used for each QA aspect, which can be impractical and time-consuming due to the need for multiple setup changes.<i>Materials and methods.</i>A modular phantom for SIMT QA was fabricated using polymethyl methacrylate and designed to perform both dosimetric and geometric evaluations. Geometric QA was assessed using off-axis Winston-Lutz tests, while dosimetric QA was performed using point dose measurements and film dosimetry.<i>Results.</i>Geometric QA results showed that off-axis Winston-Lutz tests achieved sub-millimeter accuracy (<1 mm), comparable to standard isocenter tests with a commercial phantom. Dosimetric QA revealed less than 5% point-dose difference compared to TPS calculations. Film dosimetry yielded gamma passing rates of 94.45 ± 5.58%, 97.09 ± 3.74% and 92.24 ± 6.14% for gamma criteria of 5%/1 mm, 3%/3 mm, and 3%/1 mm, respectively.<i>Conclusion.</i>This study presents the development and validation of a modular phantom for QA in single-isocenter, multi-target SRS. The phantom demonstrated reliable performance in point dose verification, gamma analysis, and Winston-Lutz testing, confirming its effectiveness for both geometric and dosimetric QA. Its modular design supports flexible configuration across multiple target positions while maintaining sub-millimeter precision, even at off-axis locations.