RECIST Responses to Radiation in Retroperitoneal Soft Tissue Sarcoma: When and How Often Do They Occur?

Retroperitoneal sarcoma (RPS) is a rare mesenchymal tumor where complete resection is often difficult due to size and invasion of adjacent organs. This analysis aimed to explore the association between neoadjuvant radiation and downsizing of RPS prior to surgical resection.

Retrospective review of adult, nonmetastatic patients undergoing curative-intent treatment at a single National Cancer Institute (NCI)-designated center between January 1, 2000, and December 31, 2020, was conducted. Patients with incomplete records were excluded. Imaging obtained preradiation/postradiation was reviewed by study radiologists to quantify the following: (1) absolute and relative change in tumor size and (2) RECIST (Response Evaluation Criteria in Solid Tumors) response prior to resection. Subgroup analyses were performed by RECIST response.

The cohort (n = 22) was 60% female, with a mean age of diagnosis at 60.8 years. Liposarcoma (n = 13, 59.1%) and leiomyosarcoma (N = 6, 27.3%) were most common. Intensity-modulated radiation therapy was most frequently utilized (median dose 5000 cGy). Among tumors that decreased in size (N = 14, 63.6%), the mean relative decrease was 7.1%. There was one partial RECIST response observed, no change in organ abutment posttreatment, and no alterations to planned multivisceral resection. In contrast, 8 tumors (36.4%) remained unchanged or increased in size (mean relative increase 21.9%). By RECIST criteria, the majority exhibited stable disease (N = 17, 77.3%). Tumors demonstrating progressive disease (N = 4, 18.2%) exhibited a mean relative size increase of 38%.

Favorable RECIST responses with neoadjuvant radiation are rare for RPS, with a significant proportion increasing in size on therapy. These trends caution use of neoadjuvant radiation for RPS for the purpose of downsizing or altering the operative plan.