Endocrine Abstracts

JOINT1255

Background: Aneurysmal Bone Cysts (ABC) are rare, benign, osteolytic tumours of bone, characterized by locally aggressive and rapid growth, and treatment experience thus scarce. Despite its benign nature, ABC tends to regrow until end-of-growth and medical treatment is therefore suggested to be continued until final height. We present two childhood cases of inoperable ABC and their medical treatment. Case 1:A 6-year-old girl presented with a limp and muscle pain for some months. MRI revealed a large cystic process at the os sacrum, involving S2 and S3, embedding the ischiadic nerve. Biopsy revealed multinucleated giant cells of osteoclast appearance with no signs of malignancy, and by RNA NGS, a CTNNB1-USP6 fusion, confirming ABC. Fluorodeoxyglucose (FDG) PET/CT revealed increased uptake in ABC. Due to considerable risk for nerve damage/instability of the lower spine, surgical removal of ABC was impossible with medical treatment as the only option. Denosumab treatment was initiated, 70 mg/m² weekly, the first four weeks, thereafter 4-weekly. Pain disappeared rapidly. MRI after three months revealed ABC regression and some consolidation. Case 2:An 11-year-old boy presented with a previous history of lower limb weakness and back pain due to a T5 ABC, which had already required 3 surgeries in one year for recurrent symptoms. As further surgeries were with risk of permanent spinal cord damage, denosumab was initiated, 1mg/kg monthly, alongside zoledronate 0. 05 mg/kg 6-monthly to offset risk of rebound hypercalcaemia. ABC has shrinked with no further significant neurological episodes, and no calcium disturbance.

Discussion: Case 1: Continued treatment plan: denosumab 4-weekly for 6 months. If MRI shows continued ABC regression, and normal metabolic activity on FDG PET/CT, tapering of denosumab initially alternating with zoledronate 0. 025 mg/kg followed by extended dosing intervals is planned. As ABC growth is expected to continue until end-of-growth, medical treatment is suspected to continue at intervals permitting ABC control until end-of-growth. Case 2: As treatment has been ongoing for >2 years (planned to continue until end-of-growth), cautious stretching of dose frequency to 6-weekly has commenced, with ongoing biochemical and MRI monitoring. The dose of zoledronate has been reduced due to increased bone density.

Conclusion: As updated treatment advice describing the use of denosumab in ABC in children are warranted, we would like to invite colleagues treating children with ABC to share their experiences aiming to collaborate on the development of a statement publication on the treatment of ABC in children of different ages.