(P 342) THE EVALUATION AND OUTCOMES OF MESENTERIC AND INTRA-ABDOMINAL DESMOIDS: A TARPSWG STUDY

Objective: The natural history of intra-abdominal desmoids is unknown. Literature on desmoids of the extremities or abdominal wall demonstrated that approximately 25-30% regress, 10-15% rapidly progress, and the majority stay stable. Currently, there are no comparable data on intra-abdominal desmoids and their management. The objective of this study is to characterize their clinical presentation, management, and outcomes.

Methods: Data were collected for eligible patients with intra-abdominal desmoids treated at 38 international sarcoma referral centers from Jan 2008 to Dec 2019. Patients with missing data were excluded. Patients were divided into three cohorts based on their initial management — observation (OBS), medical management (MM), and surgery (SURG) (complete curative vs incomplete palliative resection). Progression-free survival (PFS) was calculated from time of original scan or biopsy (whichever was first) to progression or death for the OBS and MM cohorts. Disease-free survival (DFS) was calculated from date of surgery with complete resection and PFS from the date of incomplete resection to recurrence or progression, respectively, or death for the SURG cohort. For OBS and MM patients, Response Evaluation Criteria in Solid Tumors (RECIST), version 1.1 was used to determine status from the initial to final scan in their respective observation period.

Results: We identified 635 patients (50.2% male). Median age at presentation was 43 years (IQR, 32-57) and median follow-up was 48.3 months (25.5-86.8). 101 (15.9%) patients had FAP. The most frequent anatomic site was mesentery (361, 56.9%), followed by retroperitoneum (112, 17.6%), pelvis (92, 14.5%), intra-peritoneal (54, 8.5%), and unknown (16, 2.5%).

Of the 635 patients, 308 (48.5%) underwent upfront complete resection (R0/R1), of which 256 (83.1%) and 52 (16.9%) were elective and emergent, respectively. Incomplete (R2) resection was the index operation in 10.7% (N=68), with 69.1% being elective and 19.1% being emergent (11.8% unknown). Overall, patients who underwent complete resection had a 5-year DFS of 73.4% (CI, 67.6-79.7%), whereas those that had an R2 resection had a 5-year PFS of 83.1% (73.4-94.0%). For surgical patients, on multivariable Cox model, FAP (p=0.004) and non-mesenteric location (p=0.001) were independent adverse prognostic factors for DFS.

Observation was the initial treatment in 18.7% (N=119) of patients while MM was the initial treatment in 22% (N=140) with a 5-year PFS of 66.7% (57.8-77.0%) and 71.4% (63.9-79.8%), respectively.

Of the 119 OBS patients, at a median follow up of 41.8 months, 8% had complete regression (CR), 18% had partial regression (PR), 45% remained stable (SD), and 29% had progressive disease (PD) [0.8%] unknown). Of the 140 initially MM patients, at a median follow-up of 49.1 months 4% had a CR, 33% had a PR, 44% had SD, 19% had PD. Following first line OBS, 20% eventually had surgery, 21% had medical management and 9% had both treatments. Following first line MM, 28% had surgery. Patients who underwent incomplete/R2 resection after non-surgical management (N=17) had a 5-year PFS of 79.9% (61.6-100%). Across all cohorts, at last follow-up, 5.0% of patients died of desmoids and 2.0% of other causes.

Conclusion: Active surveillance in intra-abdominal desmoids is feasible and yields similar percentages of spontaneous disease stabilization and regression as for desmoid tumors at other anatomical sites. Surgery is a valid option when feasible and could be considered as first line in individual patients or second line after PD on a non-operative approach, especially for non-FAP associated mesenteric tumours. Of note overall approximately 10% of patients required an emergent operation and undertaking an incomplete resection was a valid option in selected cases.