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.