A Comparative Analysis of Peripheral Dose Measurement Between Apex and Agility Collimators in Linear Accelerators

The aim of this study is to compare and evaluate peripheral dose (PD) distributions from Apex and Agility multileaf collimators (MLCs) in a linear accelerator at varying beam energies, field sizes, and depths for the optimization of radiotherapy safety and precision.

PD values were determined with Semiflex and PinPoint ionization chambers at depths of 2, 4, 8, and 10 cm in a water phantom. Doses were measured at distances of 1 - 5 cm from the field edge, for photon beams of 6 FF, 6 FFF, 10 FF, and 15 FF MV, with field sizes of 5 × 5 cm² and 10 × 10 cm². The measurements were normalized to the central-axis dose.

The PD varied with depth and beam energy but decreased with increasing distance from the edge of the field. The PinPoint chamber consistently reported lower doses near the field edge (maximum of 63.6% and 66.4%) in Apex and Agility compared to the Semiflex chamber (89.7% and 87.9%) respectively. Agility MLC tended to deliver higher peripheral doses than apex, reaching as high as 87.9% at 10 cm depth and 1 cm distance whereas with Agility, the value was 65.9% under the same conditions.

The PD depends on the design of the MLC, photon energy, depth and beam size. The consistent reduction in peripheral dose with Apex and FFF beams is an encouraging finding that supports their continued use in advanced radiotherapy techniques and clinical decision-making.