Detection of early changes in the muscle properties of the pectoralis major in breast cancer patients treated with radiotherapy using a handheld myotonometer

Objective: The primary aim was to investigate serial changes in the mechanical properties of the pectoralis major (PM), upper trapezius (UT), and sternoclavicular mastoid muscle (SCM) in breast cancer patients undergoing radiotherapy (RT) using a hand-held myotonometer. The secondary aims were to determine changes in subjective symptoms and to identify correlation with subjective results.

Design: A total of 42 breast cancer patients were enrolled in this longitudinal prospective study. Muscle properties of the PM, UT, and SCM were evaluated before RT, immediately after RT, and 4 months post-RT. Subjective symptom scales of pain and stiffness at rest/stretch of each muscle were evaluated.

Results: The PM showed significant side-to-side differences; the affected PM showed increased tone, stiffness, and decreased elasticity compared with the unaffected PM. The affected PM and UT showed significant time-dependent interactions. Stiffness of the affected PM at stretching was significantly higher 4 months post-RT than baseline. Only the tone and elasticity of the affected PM were correlated with subjective symptoms.

Conclusion: In breast cancer patients who received RT after surgery, increased tone, stiffness, and decreased elasticity were observed in the affected PM compared with the unaffected side, which sustained four months post-RT. Change in muscle properties immediately after RT preceded subjective stiffness, which worsened significantly 4 months post-RT compared with baseline.

Keywords: Breast cancer; Myotonometer; Pectoralis muscle; Radiation therapy; Trapezius muscle.