The investigators will determine which factors are predictive for the development and severity of everolimus-induced interstitial lung disease and will develop a prediction model based on these risk factors.

In this study the investigators will prospectively investigate pulmonary adverse events during treatment with everolimus. The investigators will distinguish the following everolimus-induced pulmonary adverse events: pulmonary infection, everolimus-induced airway disease and everolimus-induced interstitial lung disease (ILD). The investigators will investigate the predictive value of pneumoproteins, everolimus exposure, pulmonary function tests, four distinct radiological patterns, baseline patient characteristics and the development of skin toxicity or oral mucositis for the development and severity of everolimus-induced ILD.

Inclusion Criteria:

• Adult women with metastatic or locally advanced breast cancer not amenable to curative treatment by surgery or radiotherapy.
• Histological or cytological confirmation of estrogen-receptor positive (ER+) breast cancer
• Postmenopausal women
• Radiological or clinical evidence of recurrence or progression on last systemic therapy prior to enrollment
• Resistance to treatment with a non-steroidal aromatase inhibitor
• Serum platelets ≥ 100x10E9/l
• Everolimus dose adjustment is recommended for patients with hepatic impairment (Child-Pugh A/B/C)
• Performance status ECOG 0 - 2 (Karnofsky index: 60 - 100)

Exclusion Criteria:

• Patients with a HER2-overexpressing tumor
• Known hypersensitivity to mTOR inhibitors, e.g. sirolimus (rapamycin).
• Patients with a known history of HIV seropositivity or hepatitis B or C
• Uncontrolled diabetes mellitus
• Impairment of gastrointestinal function or gastrointestinal disease that may significantly alter the absorption of study drugs (e.g., ulcerative disease, uncontrolled nausea, vomiting, diarrhea, malabsorption syndrome)
• Patients being treated with drugs recognized as being strong inhibitors or inducers of the isoenzyme CYP3A

Postmenopausal women with estrogen receptor (ER) positive advanced breast cancer whose disease is refractory to non steroidal aromatase inhibitors, and are eligible for treatment with exemestane and everolimus.