In 2005, the EBCIG demonstrated the efficacy of tamoxifen in improving overall survival in hormone receptor positive breast cancers. However, tamoxifen shows partial estrogen agonist activity, which is responsible for the drug's detrimental effects such as endometrial carcinoma, thromboembolism, and tamoxifen resistance. More recently, aromatase inhibitors have been shown to be superior to tamoxifen in the metastatic and adjuvant settings. The ATAC trial demonstrated improved disease-free survival (DFS) for 5 years of anastrozole compared to 5 years of tamoxifen 3. The BIG 1-98 trial results demonstrated that after a median follow-up of 25.8 months, letrozole improved DFS and distant DFS when compared to tamoxifen. Based on these results, adjuvant hormonal therapy with Aromatase Inhibitors (AI) has become the preferred therapy for post-menopausal woman.

However, AI therapy is also associated with toxicities that merit in-depth studies, one of them being an increase in musculoskeletal pain. In the ATAC trial, at a median follow-up of 5.7 years, arthralgia was significantly higher (35.6% vs. 29.4%) and fractures were also increased (11.0% vs. 7.7%) when anastrozole was administered for 5 years following surgery with or without chemotherapy 3. The incidence of arthralgia was also significantly higher in the MA-17 trial, with 25% of patients receiving letrozole developing arthralgia compared with 21% in the placebo group following 5 to 6 years of tamoxifen 5.

Traditionally in cancer clinical trials, the reporting of musculoskeletal pain has been based on the "Common Terminology Criteria for Adverse Events", which covers a wide range of symptoms and does not facilitate the documentation of a pain syndrome in a specific manner. Therefore, there is a need to design a study that will describe the nature of the pain associated with the administration of AI therapy using tools that have been validated for capturing a multidimensional phenomenon such as pain.