Abstract P1-12-18: Mechanical properties of the shoulder and pectoralis major in women undergoing breast conserving therapy with axillary dissection and regional nodal radiotherapy versus sentinel node biopsy and radiotherapy to the breast alone
Authors: Lipps DB , Leonardis JM , Lehmann S , Dess RT , McGinnis G , Strauss JB , Hayman JA , Pierce LJ , Jagsi R 
Published: May 2019

Background: Breast conserving surgery (BCS) and radiotherapy (RT) reduce the risk of breast cancer recurrence, but can cause various functional deficits in breast cancer survivors. Side effects to the shoulder can include pain, stiffness, and restricted mobility, which are difficult to objectively assess in the clinic.

Methods: The mechanical integrity of the shoulder and the pectoralis major (PM) was assessed in patients at least 1 year post-treatment with BCS and RT. Nine patients with node-negative breast cancer were treated with 2 RT fields to the breast alone after BCS and sentinel node biopsy (Group 1). Nine patients with node-positive breast cancer were treated with ≥3 RT fields to the breast and draining lymphatics after BCS and axillary lymph node dissection (ALND) (Group 2). Nodal RT was delivered to the supraclavicular and infraclavicular (level III axillary) nodes in 9/9 patients, the internal mammary nodes in 6/9 patients, and the full axilla (levels I, II, and III) in 1/9 patient. Nine age-matched healthy controls (mean age 54) with no history of breast cancer or shoulder injury were also examined. The mechanical integrity of the shoulder was objectively quantified using robot-assisted biomechanical measures of shoulder stiffness. The shear elastic modulus, or 'stiffness', of the clavicular and sternocostal fiber regions of PM was assessed using ultrasound shear wave elastography. Participants were examined while they were relaxed or actively contracting force with their shoulder muscles. Linear mixed effect models with Bonferroni-corrected multiple comparisons were used to determine if shoulder stiffness or PM shear elastic modulus differed between the two breast cancer groups and controls.

Results: Patients in Groups 1 (mean age 54) and 2 (mean age 57) were an average (SD) 754(111) and 988(163) days since initiating RT (p=0.003). Shoulder stiffness did not differ between the 2 groups and healthy controls (F2,27=0.76, p=0.48), There was a significant group difference in PM shear elastic modulus (F2,27=8.33, p=0.0015), with Group 2 patients exhibiting an average greater stiffness of 14-21% in the sternocostal and 12-28% in the clavicular regions of the PM versus Group 1 patients (p&lt;0.001) and healthy controls (p =0.021). There was no difference between patients treated with Group 1 and controls (p=0.29).

Conclusions: Although power is limited due to small sample size, this study provides the first evidence that the mechanical integrity of the shoulder remains intact in patients who receive ALND combined with a supraclavicular field (generally without full axillary radiotherapy). The observation of altered PM function without subsequent changes to shoulder stiffness in patients treated with ALND and ≥3 RT fields suggests these patients likely develop new neuromuscular strategies to stabilize the shoulder joint to compensate for the PM. Future work is needed to appreciate whether certain muscle strategies are associated with poorer quality of life in breast cancer survivors, and to prospectively monitor the impact of breast cancer treatments on PM mechanical properties.

Citation Format: Lipps DB, Leonardis JM, Lehmann S, Dess RT, McGinnis G, Strauss JB, Hayman JA, Pierce LJ, Jagsi R. Mechanical properties of the shoulder and pectoralis major in women undergoing breast conserving therapy with axillary dissection and regional nodal radiotherapy versus sentinel node biopsy and radiotherapy to the breast alone [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P1-12-18.