Analysis of non-coplanar dose reconstruction for evaluating pre-treatment quality assurance in stereotactic treatments

Purpose

Modern stereotactic radiosurgery (SRS) or stereotactic radiotherapy (SRT) treatments often combine coplanar and non-coplanar beams to enhance dose conformity. However, patient-specific quality assurance (PSQA) for non-coplanar setups remains challenging. During data acquisition in the phantom, couch angles associated with non-coplanar beams frequently collapse to 0°, irrespective of their intended spatial configuration. This simplification prevents deflection of the phantom but does not accurately replicate the actual non-coplanar geometry used in treatment. This study aims to evaluate the impact of incorporating non-coplanar geometry in dose reconstruction compared to reconstructions assuming coplanar geometry and to quantify the differences observed in PSQA outcomes.

Material and methods

The Octavius 4D system, integrated with a 1000 SRS detector array, acquired pre-treatment measurement data for ten SRS/SRT treatment verification plans under planar and non-coplanar beam configurations. Measurement data were analyzed using the Octavius system in conjunction with Verisoft software and compared against treatment planning system-generated verification plans. Dose comparison and validation were conducted using gamma evaluation with criteria of 1 mm/2 %, 2 mm/2 %, and 3 mm/3 % applied across axial, coronal, and sagittal planes in two-dimensional (2D) and three-dimensional (3D) volumetric assessments.

Results

Non-coplanar reconstructions showed significantly higher gamma passing rates in coronal and sagittal planes under stringent criteria, with no significant differences in the transverse plane. Global gamma ratios revealed greater variability in the coronal plane, with 3D analyses less consistent than 2D. Correlations between 2D and 3D evaluations weakened as gamma criteria relaxed.

Conclusions

Non-coplanar dose reconstruction demonstrated superior accuracy in 3D verification by preserving the integrity of couch angles, eliminating the need for their collapse during analysis. The findings recommend adopting a 2 %/2 mm gamma evaluation criterion for both planar and non-coplanar measurements, while a more stringent 1 mm/2 % criterion is suggested for pre-treatment verification of non-coplanar setups to ensure higher precision.