Prognostic role of bone erosion in orbital RMS: a report from the European Pediatric Soft Tissue Sarcoma Study Group (EpSSG)

Background: Orbital rhabdomyosarcoma (RMS) is often limited to the orbital cavity and has a favorable prognosis. In some cases, the tumor can erode the orbital bone and behave as a parameningeal RMS (PM-RMS); thus, it is treated more intensively. However, the current protocols do not provide any guidance on how to consider different grades of bone erosion (BE) that can vary widely, hampering a uniform classification and the subsequent treatment assignment. With the aim of clarifying the role of BE as a risk factor, we analyzed patients with orbital RMS included in the European Pediatric Soft Tissue Sarcoma Study Group (EpSSG) protocol. Methods: We retrospectively analyzed the radiological reports of 199 patients with orbital RMS (PM or not) and defined three grades of BE: minimal (thinning of the bone), moderate (focal bone lysis), and extensive (complete cortical destruction). Results: BE was present in 55 of the 199 (27.6%) patients, which was classified as minimal in 27, moderate in 7, and extensive in 21. Tumors with extensive BE were more frequently large (>5 cm, p = 0.0008) and invasive (T2, p = 0.001). With a median follow-up of 70.4 months (range = 7.1-167.7), a total of 183 patients are alive, with 5-year event-free survival (EFS) and overall survival (OS) rates of 76% (95%CI = 69.2-81.3) and 92% (95%CI = 86.7-94.8), respectively. Patients without any BE had better OS (95% vs. 81%, p = 0.001), but not EFS. Patients with no/minimal/moderate BE had better EFS and OS compared with patients with extensive BE [EFS of 78.1 (95%CI = 71.1-83.5) vs. 57.1 (95%CI = 33.8-74.9), p = 0.0114, respectively, and OS of 94.0 (95%CI = 89.2-96.8) vs. 71.1 (95%CI = 46.6-85.9), p < 0.0001, respectively]. Events and metastatic relapses (in all cases CNS/meningeal) were more frequent in patients with extensive BE. Conclusions: Only those patients with orbital RMS and extensive BE should be considered as PM and should be treated accordingly.