Spontaneous Regression of Soft Tissue Sarcoma Following Biopsy: A Case Report and Systematic Review of the Literature

Spontaneous regression (SR) of malignancy is a rare phenomenon offering unique insights into tumor-host immunology. In sarcomas, the incidence, triggers, and clinical implications of SR are poorly characterized. We aim to synthesize the published literature on SR in sarcoma and report a novel case from our institution where diagnostic biopsy triggered a complete pathological response (pCR). We retrospectively reviewed one institutional case: a 59-year-old female with myxofibrosarcoma (MFS) who demonstrated significant regression following core needle biopsy. We subsequently conducted a descriptive systematic review in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, rather than a formal meta-analysis. PubMed, Embase, and Scopus were searched from inception to July 2025 for reports of SR in pathologically confirmed sarcomas without prior cytotoxic therapy. In our institutional experience, the patient (59-year-old female, MFS) achieved complete clinical resolution within weeks of biopsy. Definitive wide excision revealed no viable tumor cells, confirming pCR. The systematic review identified 32 published studies describing 32 unique cases of SR in sarcoma. Diagnostic biopsy was the leading identified trigger (25%). Biopsy-associated regressions occurred more rapidly (median 0.9 months) compared to infection-associated (median five months) or spontaneous cases. Despite clinical regression, 75% of patients in the biopsy-triggered group underwent definitive surgical resection. SR in sarcoma is frequently preceded by an immune-stimulating event, most notably biopsy. While SR provides in vivo evidence of host anti-tumor immunity, the potential for incomplete or transient response underscores that SR should not preclude definitive oncologic management. Ultimately, SR can be conceptualized as a "neoadjuvant immune boost" rather than a definitive cure. Wide surgical resection based on pre-regression tumor fields remains the standard of care to eradicate microscopic disease and ensure oncologic safety.