Prevention of Symptomatic Skeletal Events With Denosumab Administered Every 4 Weeks Versus Every 12 Weeks

Inclusion Criteria:

* Patient has given written informed consent.
* Histologically confirmed diagnosis of breast or prostate cancer before randomization.
* Patient has metastatic breast cancer (stage IV, all subtypes allowed) or prostate cancer (stage IV) and bone metastases and is planned to receive or is receiving antineoplastic treatment.
* Patients with prostate cancer must have evidence of disease progression on continuous androgen deprivation therapy (CRPC).
* Patients must have ≥ 3 bone metastases (lytic or blastic or mixed). The lesions must be documented by radiological evaluation within 12 weeks before randomization (by X-Ray, CT scan, PET-CT, MRI scan or bone scintigraphy).
* WHO performance status 0-2
* Age ≥ 18 years.
* Corrected serum calcium ≥ 2 mmol/l and ≤ 3 mmol/l (medical treatments to obtain serum calcium levels in the normal range are allowed, as far as no denosumab is used. Maximally 1 dose of bisphosphonates in the case of hypercalcemia is allowed, if the bisphosphonate was applied at least 3 weeks before the first dose of denosumab).
* Liver transaminases not more than 1.5 x ULN or not more than 3 x ULN with liver metastases. Serum total bilirubin ≤ 1.5 x ULN (≤ 2.0 x ULN in case of known Gilbert's disease)
* Women are not breastfeeding. Women with child-bearing potential are using effective contraception, are not pregnant and agree not to become pregnant during participation in the trial and during the 12 months thereafter. A negative pregnancy test before inclusion (within 7 days) into the trial is required for all women with child-bearing potential.
* Men agree not to father a child during participation in the trial and during 12 months thereafter.

Exclusion Criteria:

* Definite contraindication for denosumab (e.g. hypocalcaemia \[Albumin-corrected serum calcium \< 2.0 mmol/l\]).
* History or current evidence of osteonecrosis of the jaw.
* Non-healed mucosa in oral cavity (by surgery or as a side effect of any other treatment).
* Jaw or dental conditions that require oral surgery or if surgery or invasive dental procedures are planned.
* Prior use of denosumab for bone metastases (dose 120 mg every 4 weeks) or bisphosphonates to treat bone metastases. Patients treated with denosumab or bisphosphonates against osteopenia or osteoporosis are allowed to enter the trial if the last dose was more than 28 days before randomization.
* Patients with known osteoporosis (T-score ≤ -2.5) at study entry (since fractures from osteoporosis are difficult to be discriminated from fractures through bone metastases).
* Radiotherapy or surgery to the bone within the last two weeks before randomization or planned within 6 weeks after randomization.
* Presence or history of CNS metastases or leptomeningeal disease. A MRI evaluation within 12 weeks before randomization must be performed in case of suspicious symptoms.
* Psychiatric disorder precluding understanding of information on trial related topics, giving informed consent, filling out QoL forms.
* Concurrent treatment in a clinical trial with SSE or SRE as primary endpoint.
* Known hypersensitivity to trial drug or hypersensitivity to any other component of the trial drug (e.g. fructose).
* Any concomitant drugs contraindicated for use with the trial drugs according to the approved product information.
* Any psychological, familial, sociological or geographical condition potentially hampering compliance with the trial protocol.