Author(s): WOLFRAM, S., LIPPS, D.B., Institution: UNIVERSITY OF MICHIGAN, Country: UNITED STATES, Abstract-ID: 750

INTRODUCTION:
Radiotherapy (RT) for breast cancer has been associated with shoulder co-morbidities like reduced strength and range of motion (ROM) (1). These impairments may be related to stiffening of shoulder muscles, like the pectoralis major, after RT (2). Increased pectoralis major muscle stiffness likely causes difficulties with shoulder abduction. However, it is unknown how this muscle responds to stretch during this movement. The purpose of this study was to examine the passive stretch response of the pectoralis major during shoulder abduction after RT for breast cancer.
METHODS:
The sternocostal region of the pectoralis major was examined in 18 breast cancer survivors 12-60 months after completing RT (mean ± SD age 57.4 years, height 163.6 ± 7.1 cm, 77.3 ± 10.2 kg) and 18 age-matched cancer-free controls (age 59.3 ± 9.1, height 163.4 ± 6.0 cm, 60.7 ± 10.4 kg). Ultrasound shear wave elastography images were obtained in increments of 5° shoulder abduction as participants remained relaxed. Shear wave velocities (SWV) were extracted from these images as a measure of muscle stiffness. Slack angle, SWV at slack (SWV0) and elasticity coefficient were determined from a piece-wise exponential model fitted to the SWV-joint angle curve (3). Differences between the groups for these parameters were examined with an independent two-sided t-test. Joint angles were normalised to 100%ROM, starting at the slack angle. SWVs were extracted for joint angles at intervals of 10%ROM using non-linear interpolation. A repeated-measures ANOVA tested if SWVs differed between the two groups (between-subjects factor) and joint angles (within-subjects factor).
RESULTS:
Slack angle was similar (p = 0.84) between groups (breast cancer group: 49.4 ± 16.0°; control group: 50.7± 20.5°). SWV0 was significantly greater (p = 0.016) for the breast cancer group (2.41 ± 0.46 m/s) than the control group (2.1 ± 0.26 m/s). There was no difference (p = 0.36) in the elasticity coefficient between the breast cancer (0.0073 ± 0.0026) and the control group (0.0065 ± 0.002). SWVs increased significantly with an increase in joint angle from slack angle onwards for both groups (p < 0.001), with greater SWVs for the breast cancer group than the control group (p = 0.02).
CONCLUSION:
Higher SWVs in the breast cancer group throughout the examined abduction ROM indicate increased stiffness of the pectoralis major after RT for breast cancer, which is likely due to radiation-induced muscle fibrosis. However, slack angle and elasticity coefficient did not differ between groups suggesting that the stretch response of the pectoralis major is not altered. Therefore, higher pectoralis major muscle stiffness after RT may not contribute to limitations in abduction ROM commonly observed in breast cancer survivors.

References
(1) Johansen et al. Acta Oncol, 2014.
(2) Lipps et al. Sci Rep, 2019.
(3) Wolfram & Lipps. J Biomech, 2023.