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However, baseline staging CT performed a few weeks postpartum unexpectedly demonstrated marked spontaneous regression. Because trial enrolment required Response Evaluation Criteria in Solid Tumors-defined radiographic progression, she was deemed ineligible, and no systemic therapy was initiated. Surveillance imaging over the following 2 years confirmed sustained regression and stability of residual disease in the absence of treatment. This case highlights the potential influence of pregnancy-to-postpartum physiology on tumour behaviour through hormonal, immune and angiogenic mechanisms. It also emphasises the importance of short-interval reassessment after delivery before initiation of systemic therapy in reproductive-age patients with metastatic ASPS.<\/jats:p>","DOI":"10.1136\/bcr-2025-270772","type":"journal-article","created":{"date-parts":[[2026,5,28]],"date-time":"2026-05-28T12:30:29Z","timestamp":1779971429000},"page":"e270772","update-policy":"https:\/\/doi.org\/10.1136\/crossmarkpolicy","source":"Crossref","is-referenced-by-count":0,"title":["Pregnancy-associated progression with spontaneous postpartum regression in metastatic alveolar soft part sarcoma"],"prefix":"10.1136","volume":"19","author":[{"ORCID":"https:\/\/orcid.org\/0009-0006-2168-9275","authenticated-orcid":false,"given":"Angel Luis","family":"Orosco-Ttamina","sequence":"first","affiliation":[{"name":"Division of Hematology and Oncology, Department of Internal Medicine, Mayo Clinic, Phoenix, Arizona, 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