Reduced Dose-density of Denosumab for Unresectable GCTB

Inclusion Criteria: * Histologically proven primary or metastatic unresectable GCTB or resectable GCTB but not a candidate for surgery, excluding primary or metastatic GCTB in the jaw. * Evidence of active disease at time of registration based on local investigator's assessment (according to RECIST v1.1) * Age ≥ 18 years old and skeletally mature (ie, radiographic evidence of at least 1 mature long bone (e.g. humerus with closed growth epiphyseal plate) * Patient must have received denosumab before entering this trial: * The duration of treatment with full dose denosumab (120 mg SC ) as per current label must be at least 12 months and patient may have received up to 15 months of denosumab. * And patient must have received at least 12 doses of denosumab 120 mg before entering into this trial. * ECOG/WHO PS 0-2 * Albumin-adjusted serum calcium level ≥ 2.0 mmol/L (8.0 mg/dL) * Representative formalin fixed, paraffin embedded tumor blocks or unstained tissue slides, either from the primary tumor or a metastatic lesion, must be available for histological central review. * Women of child bearing potential (WOCBP) must have a negative serum (or urine) pregnancy test within 7 days prior to the first reduced dose of study treatment. * WOCBP should use adequate birth control measures, as defined by the investigator, during the study treatment period and for at least 5 months after the last treatment cycle. A highly effective method of birth control is defined as a method which results in a low failure rate (i.e. less than 1% per year) when used consistently and correctly. Such methods include: * Combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation (oral, intravaginal, transdermal) * Progestogen-only hormonal contraception associated with inhibition of ovulation (oral, injectable, implantable) * Intrauterine device (IUD) * Intrauterine hormone-releasing system (IUS) * Bilateral tubal occlusion * Vasectomized partner * Sexual abstinence (the reliability of sexual abstinence needs to be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the patient) * Female subjects who are breast feeding should discontinue nursing prior to the first dose of study treatment and until 5 months after the last study treatment. * Before patient registration, written informed consent must be given according to ICH/GCP, and national/local regulations. Exclusion Criteria: * Currently receiving other GCTB specific treatment (eg, radiation, chemotherapy, or embolization) * Concurrent bisphosphonate treatment and calcitonin * Known or suspected current diagnosis of underlying malignancy including high-grade sarcoma, osteosarcoma, fibrosarcoma, malignant giant cell sarcoma * Known diagnosis of second malignancy within the past 5 years (subjects with definitively treated basal cell carcinoma and cervical carcinoma in situ are permitted) * Creatinine clearance \< 30 mL/min * Hemoglobin \< 10.0 g/dL or 6.2 mmol/L * Prior history or current evidence of osteonecrosis/osteomyelitis of the jaw * Active dental or jaw condition which requires oral surgery, including tooth extraction * Non-healed dental/oral surgery * Planned invasive dental procedure for the course of the study * Known hypersensitivity to the active substance or to any of the excipients (glacial acetic acid, sodium hydroxide, sorbitol (E420), polysorbate 20) * Treatment with other investigational device or drug 30 days prior to registration * Known hypersensitivity to products to be administered during the study (calcium and/or vitamin D) * Unstable systemic disease including active and uncontrolled infection, uncontrolled hypertension, unstable angina, congestive heart failure, or myocardial infarction within 6 months before registration * Any psychological, familial, sociological or geographical condition potentially hampering compliance with the study protocol and follow-up schedule.