Dosimetric Vulnerability of Radiotherapy Techniques Under Non-Rigid Arm-Position-Induced Deformations in Breast Cancer: A Deformable Phantom Study

Accurate positioning is essential in breast radiotherapy; however, rigid phantoms do not reproduce posture-related anatomical deformation. We developed a breast-mimicking rubber and silicone phantom to evaluate dosimetric effects of arm-position-induced deformation in three-dimensional conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), and volumetric-modulated arc therapy (VMAT). The phantom included a central internal marker and five surface optically stimulated luminescence dosimeter attachment points. Treatment plans were generated from computed tomography images to deliver 42.4 Gy in 16 fractions. Surface doses were measured with a linear accelerator in reference and modified arm-shift postures. 3D-CRT showed differences below 1.00% at nearly all points and a maximum deviation of −0.77% laterally. IMRT showed greater sensitivity, reaching 2.53% at the lower point. VMAT showed the largest changes: a 21.19% dose deficit (0.414 Gy) at the lower point and a −5.88% difference laterally. Arm position changes therefore produced localized surface-dose variation, particularly in the lower breast. These findings highlight the importance of careful arm positioning and reproducible setup when highly modulated techniques are used and suggest that deformable phantoms can complement conventional rigid quality-assurance tools. Under these experimental conditions, VMAT was more sensitive than IMRT and 3D-CRT at selected surface measurement points in this phantom model.