Evaluating Optimal Timing of Endocrine Therapy and Radiation Therapy in Early-stage Breast Cancer (REaCT-RETT)

To measure endocrine toxicity. The Functional Assessment of Cancer Therapy - Endocrine Sub-scale (FACT-ES) is a questionnaire specific to patients with endocrine symptoms and measures the side effects and putative benefits of hormonal treatments given (endocrine therapy) in breast cancer. The FACT-ES questionnaire is comprised of 5 sections; physical well-being (7 questions), social/family well-being (7 questions), emotional well-being (6 questions), functional well-being (7 questions) and additional concerns (19 questions). A total of 46 questions are asked that all contain an answer on a scale range of 0-4, with 0 representing the answer 'not at all' and 4 representing the answer 'very much'. Of the 46 questions asked 31/46 have 'very much' representing the worst answer and the remaining 15/46 have 'very much' representing the best answer.

To evaluate the acute and early-moderate late toxicities of radiotherapy in terms of skin toxicity, pneumonitis, and breast pain. This will be recorded using the Common Terminology Criteria for Adverse Events (CTCAE) version 5.0, with breast cosmesis recorded as per the European Organisation for Research and Treatment of Cancer (EORTC) cosmetic rating system for breast cancer. These standardized criteria have been used in a recent Canadian breast radiation randomized trial, RAPID, and the investigators will use a similar template to record any radiation toxicities at different points of time.

Rates of starting endocrine therapy, compliance rates, discontinuation rates, along with the reasons for discontinuation.

To measure the patient quality of life using the validated FACT-B questionnaire. The Functional Assessment of Cancer Therapy for Breast Cancer (FACT-B) is a questionnaire that measures multidimensional quality of life in patients with breast cancer. The FACT-B questionnaire is comprised of 5 sections; physical well-being (7 questions), social/family well-being (7 questions), emotional well-being (6 questions), functional well-being (7 questions) and additional concerns (10 questions). A total of 37 questions are asked that all contain an answer on a scale range of 0-4, with 0 representing the answer 'not at all' and 4 representing the answer 'very much'. Of the 46 questions asked 20/37 have 'very much' representing the worst answer and the remaining 17/37 have 'very much' representing the best answer.

Incremental cost-effectiveness ratios (cost per one quality-adjusted life year (QALY) gained and cost per one endocrine toxicity case averted) will be measured using the EQ-5D-5L questionnaire. The EuroQol 5 Dimension 5 Level (EQ-5D-5L) questionnaire consists of two sections; the descriptive system and the visual analogue scale. The descriptive system comprises 5 dimensions (mobility, self-care, usual activities, pain/discomfort, anxiety/depression) and has 5 levels associated with it (no problems, slight problems, moderate problems, severe problems and extreme problems). The scale range of 1-5 is used for the 5 dimensions where 1 is the best outcome and 5 is the worst outcome. The Visual Analogue scale records the respondent's self-rated health on a vertical, visual scale with endpoints labelled "the best health you can imagine" at the top and "the worst health you can imagine" at the bottom. This ranges 0-100 with 0 being the worst outcome and 100 being the best outcome.

Exploratory analyses will be performed evaluating toxicity in patients receiving different types of radiotherapy (i.e. local versus regional radiotherapy). This will be evaluated using data collected on breast volume, seroma size, dose heterogeneity and the time of stimulation and planning.