Pectoralis major stiffness and stretch response in breast cancer survivors after breast conserving surgery and radiation therapy.

BACKGROUND: Breast cancer survivors frequently experience shoulder dysfunction following radiation therapy, which may be related to a stiffer pectoralis major. However, intrinsic muscle stiffness at slack length and the stretch response across the shoulder abduction range of motion remain unknown. METHODS: We recruited 20 female breast cancer survivors treated with breast-conserving surgery, sentinel node biopsy and radiation therapy 12-60 months prior, alongside 19 cancer-free controls. The sternocostal region of the pectoralis major was imaged using ultrasound shear wave elastography. Shear wave velocities were extracted from the images as measure of muscle stiffness throughout passive shoulder abduction. We determined slack stiffness, slack angle, and elasticity coefficient from the relationship between shear wave velocities and shoulder abduction joint angle. FINDINGS: Slack stiffness was significantly greater in breast cancer survivors compared to controls (p = 0.029), indicating increased intrinsic muscle stiffness. Muscle stiffness remained significantly elevated throughout the entire shoulder abduction range of motion in breast cancer survivors (p < 0.001). However, slack angle (p = 0.917) and elasticity coefficient (p = 0.801) did not differ between groups, suggesting preserved functional lengthening capacity. INTERPRETATION: This study provides the first evidence of increased intrinsic pectoralis major stiffness following breast-conserving surgery and radiation therapy. Despite elevated stiffness at slack length and during lengthening, the muscle's mechanical behaviour during stretch appears preserved. These findings may help explain shoulder dysfunction in breast cancer survivors and support future monitoring strategies.