(P 432) LONG-LASTING DISEASE CONTROL WITH TRABECTEDIN IN A CASE OF METASTATIC EWSR1::PATZ1 SARCOMA

Objective: EWSR1::PATZ1 translocation associated neoplasms include glioneuronal tumors, cases of kidney neoplasms and head and neck, chest and abdominal wall spindle cell or small round cell sarcomas. Their molecular and clinical features are not fully known. Treatment is not standardized, and the multidisciplinary approach in sarcoma cases is derived from protocols commonly used in Ewing or other soft-tissue sarcomas. We describe a case of a long-lasting metastatic disease controlled by trabectedin in a young female patient with a small round cell EWSR1::PATZ1 sarcoma of the chest wall.

Methods: In June 2015, a 28 years old female was diagnosed with a mesenchymal small round cell neoplasm of the chest wall. Immunohistochemistry: CD99-, vimentin+, desmin+, WT19+, WT180-. Fluorescence In-Situ Hybridization revealed EWSR1 gene translocation. No partner was found at that time. The patient underwent 5 courses of pre-operative chemotherapy with vincristine/doxorubicin/ifosfamide, and one cycle of etoposide/ifosfamide with an initial partial response, which was lost after the fifth cycle. En-bloc complete resection with prosthetic reconstruction, adjuvant chemotherapy with etoposide and ifosfamide for 3 more courses and external-beam radiotherapy (60Gy) followed. After 4 years, the disease relapsed with multiple pleural/chest wall ipsilateral metastases. The relapse was confirmed by biopsy, which revealed the presence of a EWSR1::PATZ1 translocation. The patient underwent complete macroscopic resection of all metastases and remained in follow up for the following 2 years, when another multinodular recurrence was clinically confirmed. Lacking evidence on the management of this histotype, the patient started trabectedin in March 2021.

Results: To date, after 33 cycles over two years, the patient remains on treatment and formally stable according to RECIST. Overall, the treatment is well tolerated and side effects were, as expected, G2 nausea, asthenia, and neutropenia, the latter controlled with q21 to q28 interval prolongation and one-level dose reduction.

Conclusion: Although a formal RECIST response was not observed, we provide evidence on the possible efficacy of trabectedin in a very rare sarcoma histotype, lacking protocols on the proper management. To our knowledge, this is the first case report on trabectedin use in EWSR1::PATZ1 small round cell sarcoma, in the literature. Given its rarity, large, prospective trials on this histotype remain very unlikely to be run. Real-world data collection is encouraged.