## 1 Introduction  
Breast-conserving surgical local excision, followed by radiotherapy (RT) to improve local control and survival, is a successful treatment for early breast cancer. Approximately 31,000 women receive adjuvant breast RT in the UK per year with local relapse rates as low as \(\sim\)6% at 10 years [1]. Given that many women are now surviving decades beyond their breast cancer treatment, a priority now is to reduce the long-term side effects of radiotherapy to the breast. The overall cosmetic outcome has been shown to be an important factor influencing patient psychosocial morbidity after treatment [2]. Evaluation of novel RT regimens must weigh the positive outcomes (including local control, disease-free survival, and overall survival) against treatment-related toxicity and the impact on the patient's quality of life. Currently, morbidity-related endpoints are primarily derived from subjective measures of function, patient comfort and cosmesis, as evaluated by clinicians or patients, which can be subject to variation due to psychological and social factors [3, 4]. Such variation may preclude the pooling of results from multiple clinical studies of radiotherapy [5], required to support studies of genetic predisposition to radiation toxicity [6]. Quantitative, reproducible measures of morbidity could remove these sources of variation allowing more objective assessment of treatment toxicity and facilitating exploration of personalised treatment. Shear-wave elastography (SWE) has the potential to quantify tissue stiffness for the purpose of disease diagnosis, prognosis or staging, and to monitor the response of tissues to treatment [7; 8; 9; 10; 11; 12]. SWE is expected to be able to quantify properties related to breast tissue stiffness using shear-wave speed \(c_{s}\), which under a number of assumptions is related to Young's modulus (E) of the tissue using E = 3\({pc_{s}}^{2}\), where \(\rho\) is tissue density [10]. Absolute or relative measures of \(c_{s}\) or E may be useful. One such relative measure could be the spatial distribution of \(c_{s}\), which can be compared with the radiation dose distribution delivered to the patient, potentially helping to further increase understanding of the relationship between dose and radiation-induced morbidity. Two-dimensional SWE (2D-SWE) which provides the spatial distribution, or map, of \(c_{s}\) may be used for this purpose. Here, we adopt the convention for describing different forms of elastography outlined in [PERSON] et al. [13]. In the context of large multi-centre clinical trials, required to provide the clinical evidence to change practice [1], measures of morbidity must be reproducible between centres. There is evidence to suggest that \(c_{s}\) estimates vary between machines provided by different vendors, transducer types and imaging depth [14; 15; 16]. This may detrimentally affect the application of SWE to the assessment of normal tissue toxicity because it is unlikely that the same SWE equipment will be available across different clinical centres. Investigations that compare \(c_{s}\) estimates made with various machines, transducers and system settings have been largely limited to _in vitro_ studies [17; 18; 19], albeit with some in the liver _in vivo_[20; 21]. No study has compared \(c_{s}\) estimates between machines in normal breast tissues. The purpose of the current study was to compare absolute and relative measures of breast \(c_{s}\) between 2D-SWE machines _in vivo_ to determine if these could be used as reliable metrics in planned cross-centre and cross-machine clinical studies of radiation-induced fibrosis. Previous work has shown that the skin thickens post radiotherapy [22; 23] and other work has indicated that women with greater breast size are more likely to experience breast hardening [24; 25]. Larger breast size is associated with a greater proportion of adipose tissue. It may therefore be important to measure hardening in component breast tissues, i.e., skin, adipose and breast parenchyma.