Assessment of Calculation Grid Size Impact on Plan Quality and Gamma Evaluation Metrics in Intracranial Stereotactic Radiotherapy
Ravindra Shende, Joyce Alexis, R. S. RahulAbstract
Background:
This study aimed to evaluate the impact of dose calculation grid size on dosimetric accuracy, plan quality indices, and patient-specific quality-assurance (QA) outcomes in intracranial stereotactic radiotherapy (SRT).
Materials and Methods:
Fifteen retrospective intracranial SRT plans treated with HyperArc volumetric modulated arc therapy were re-planned using five dose calculation grid sizes of 1.0, 1.25, 1.5, 1.75, and 2.0 mm in the Eclipse treatment planning system. All plans were made to deliver 30 Gy in 5 fractions using 6 MV flattening-filter-free beams and calculated using the Anisotropic analytical algorithm. Plan quality was evaluated using multiple plan quality metrics, including RTOG quality coverage index (QI), Paddick conformity index, homogeneity index, and gradient index (GI). Dose spillage outside the target was assessed using volumetric dose indices. Patient-specific QA was performed using SRS MapCHECK with gamma-analysis under multiple criteria. Statistical comparisons were conducted using dependent paired
Results:
Increasing the dose calculation grid size resulted in a systematic increase in planning target volume mean dose and a corresponding reduction in D 95% , indicating loss of target coverage attributable to spatial dose averaging. GI showed the strongest dependence on grid size, demonstrating progressive degradation of dose fall-off with coarser grids. Low-dose spillage to the normal brain and brainstem increased significantly with increasing grid size, while distant organs at risk were comparatively less affected. Gamma passing rates declined progressively with increasing grid size, particularly under the most stringent evaluation criteria.
Conclusion:
Study highlights that dose grid size variations significantly influence dosimetric accuracy, plan quality, and QA robustness in intracranial SRT. Coarser calculation grids compromise target coverage and gradient fidelity, while finer grids (≤1.25 mm) preserve near-reference dosimetric accuracy. Adopting a finer calculation grid ensures reliable dose computation, consistent patient-specific QA, and optimal clinical performance in SRT.