(P 425) CHARACTERIZATION OF RHABDOMYOSARCOMA WITH PAX7::FOXO1 FUSIONS: A REPORT FROM THE SOFT TISSUE COMMITTEE OF THE CHILDREN'S ONCOLOGY GROUP

Objective: Alveolar rhabdomyosarcoma (ARMS) is characterized by specific morphologic features, including round cell morphology arranged in sheets or in nests separated by delicate fibrous septa, as well as molecular features, including the presence of a PAX3 or PAX7::FOXO1 gene fusion in 80-90% of cases. The pathologic characteristics ARMS with PAX7 alterations are not well established, however. ARST1431, the Children’s Oncology Group intermediate risk rhabdomyosarcoma (RMS) clinical trial (supported by Pfizer), included central pathology review as well as identification of FOXO1 fusion partner for all patients with FOXO1 positive RMS, allowing detailed analysis of the morphology for PAX7::FOXO1 positive RMS.

Methods: Central pathology review was conducted by five pathologists, with consensus agreement on histologic subtype and immunohistochemical staining pattern including myogenin expression. Diagnoses were made using the 5th edition World Health Organization Classification for Soft Tissue and Bone Tumours, with the exception of mixed RMS which was defined as discrete, circumscribed regions of both alveolar and embryonal morphology within a single tumor. Institutional confirmation of FOXO1 fusion status was required for all patients enrolled on ARST1431, and all FOXO1 fusion positive cases underwent centralized testing for PAX3 or PAX7 rearrangements using fluorescent in-situ hybridization (FISH).

Results: Central pathology review was conducted for 306 patients enrolled on ARST1431. Of these, 58.5% had embryonal histology, 28.8% were histologic ARMS, 7.5% were spindle cell/sclerosing RMS and 1.6% were mixed embryonal/alveolar morphology, with the remaining 3.6% being insufficient for histologic classification. There were 27.1% fusion positive RMS, of which 16% (n=13) harbored a PAX7::FOXO1 gene fusion. Nine patients were female and four male, ages ranging from 11 months to 22 years (Median: 3 years). These tumors occurred in the extremity (n = 5), trunk (n = 4) and head/neck (n = 4) locations. Eight tumors (69%) demonstrated alveolar morphology (7 classic and 1 solid pattern) and four (31%) had a mixed alveolar and embryonal morphology. One case had insufficient tissue for histologic classification. Notably, only one other mixed RMS was identified in the ARST1431 cohort; this tumor harbored a FOXO1 rearrangement with amplification of the FOXO1 gene but a fusion partner was unable to be confirmed on central FISH analysis. Rhabdomyoblastic differentiation was noted in 77% (10/13) of RMS with PAX7::FOXO1 in comparison to 30% (19/64) of RMS with PAX3::FOXO1 fusions. Myogenin expression was strong and diffuse in all tumors with alveolar morphology, although decreased expression was seen in areas of embryonal morphology in mixed RMS cases.

Conclusion: RMS with PAX7 alterations predominantly demonstrate alveolar morphology; however, a subset of tumors demonstrate true mixed alveolar/embryonal morphology. Mixed embryonal and alveolar morphology is a potential morphologic surrogate for suspecting an RMS with PAX7 alterations.