Impact of Tumor Geometry on Dose Distribution and Delivery Accuracy in Multi-Target Stereotactic Radiosurgery.

Objectives: This study aimed to evaluate the influence of tumor geometry on dose distribution and delivery accuracy, and to assess the impact of the Automatic Lower Dose Objective (ALDO) function on dosimetric performance. Methods: Computed tomography images of a Rando anthropomorphic phantom were used to simulate intracranial multiple metastases. Two contour groups were generated on the same CT dataset, consisting of two spherical targets with diameters of 1 cm and 2 cm, respectively. For each group, target pairs were created with edge-to-edge separation distances ranging from 1 to 6 cm. Automated single-isocenter stereotactic radiosurgery plans were generated using the HyperArc workflow, both with and without the ALDO function. Dosimetric performance was evaluated using the RTOG conformity index, Paddick conformity index, gradient index, and homogeneity index. Patient-specific quality assurance was performed using electronic portal imaging device-based verification and radiochromic film dosimetry. Gamma analysis with multiple criteria was applied to assess the impact of target size and geometric separation on delivery accuracy. Results: The use of ALDO improved dose conformity and gradient performance but resulted in increased dose heterogeneity and higher hot spots. In non-ALDO configurations, the agreement between EPID portal dosimetry and film measurements varied according to target size, gamma criteria, and spatial position. For the 2 cm targets, EPID portal dosimetry generally demonstrated higher gamma passing rates than film measurements, whereas the 1 cm targets showed mixed results depending on measurement position and gamma criteria. These differences likely reflect the distinct detector characteristics and spatial sensitivities of the two QA methodologies. Larger discrepancies were observed under stricter gamma criteria and at off-axis positions, indicating potential influences of target geometry and high-dose gradient regions within the simplified phantom configurations evaluated in this study. Conclusions: Within the simplified two-target phantom configurations evaluated in this study, tumor geometric distribution significantly affects both dosimetric characteristics and QA outcomes in HyperArc SRS. Film measurements provide greater sensitivity, whereas EPID-PD alone may be insufficient for evaluating small-target high-gradient regions under strict gamma criteria.