(P 312) RARE CASE OF INTRAOSSEOUS RHABDOMYOSARCOMA IN A YOUNG CHILD

Objective: The intraosseous Rhabdomyosracoma (IRMS) is a very rare clinical entity originating from bone. The most common location of this tumor is gnatic bones, predominantly mandible. There have been reported only several case reports or small series of such patients (pts). The majority of pts are adults with only several cases reported in children. Recently, a characteristic genetic alterations involving TFCP2 fusion with either EWSR1 or FUS or less commonly MEIS and NCOA2 fusions were described. Prognosis is less favorable than in pts with classical RMS. We report a case of a very young pt with IRMS illustrating the complexity of management of IRMS in children

Methods: 4-year female presented with painlful swelling of lower part of her face after minor trauma. Imaging studies (x-ray, CT, MRI) showed tumorous mass originating from right mandible. Biopsy revealed RMS, Embryonal type, composed of small and spindle cells, few of them with eosinophilic round or ribbon-like cytoplasm. Tumor cells were positive for desmin, myogenin, INI1 preserved. Immunohistochemical stain for MyoD1 was positive in most cells. Pankeratin, p40, chromogranin, synaptophysin, SMA, SATB2 and CLA were negative. Some tumor cells were positive for CD99.
FISH for FOXO1 fusions showed negative result. NGS did not reveal any molecular abnormalities besides NRAS 1p13 mutation.
Metastatic work-up was negative (PET-CT-FDG, Bone marrow biopsy and bilateral cervical l/nodes sampling).
The pt was classified as non-metastatic IRMS, subgroup C, standard risk group. She received therapy according to the EpSSG RMS 2005 protocol based on IVA chemotherapy (CTX).

Results: After 3 courses of IVA CTX repeated imaging showed only minimal rentgenologic response. The pt underwent complete macroscopic resection of tumor with fibular free flap reconstruction. Post-op pathology revealed minimal response to neoadjuvant CTX showing mostly viable tumor with mild to moderate treatment effect (fibrosis), involving 40% of tumor volume. Post-op period was uneventful. Because of lack of satisfactory rentgenologic and pathologic response to given CTX the pt was switched to second line CTX based on Topotecan, Carboplatine, Cyclophosphamide, and Etoposide according to the recommendations of above-mentioned protocol. Concurrently, she received radiotherapy with protons in total dose of 54 Gy. Presently, the pt is off therapy, in CR, and functionally normal.

Conclusion: We present the rare case of a child with IRMS. To our knowledge, this is the youngest reported to date pt with this pathology. The rarity of the location coupled with resistence to chemotherapy makes this case unique and informative. It is probable that pediatric IRMS presents a distinctive clinical entity in terms of therapy responsiveness and clinical prognosis that should demand more aggressive therapeutic approach. More data should be accumulated for optimisation of treatment approach for this unique group of patients.