(P 380) MULTIDISCIPLINARY MANAGEMENT OF INTRA-ABDOMINAL DESMOID-TYPE FIBROMATOSIS: A 30-YEARS EXPERIENCE

Objective: Intra-abdominal desmoid-type fibromatosis (DF) is a rare condition, comprising approximately 10% of all DF cases. Locally aggressive disease can lead to severe complications, including bowel obstructions, perforations, and ischemia so that active observation is not uniformly safe. Due to the rarity of intra-abdominal DF, comprehensive data on its incidence, management, and outcomes are limited. This study aims to define outcomes in DF patients with primary, intra-abdominal disease.

Methods: DF patients with primary, intra-abdominal disease treated between January 1993 and December 2023 were identified from a single-institution, prospectively maintained database and retrospectively from pathology database query. Demographics, tumor location, presentation type, size, and initial treatment were compared between treatment groups using chi-squared analyses and Cox-proportional hazards regression. Progression (measured from time between initiation of first and second line treatment regimens) and recurrence free survival (PFS/RFS) were analyzed using Kaplan Meier and log-rank methods.

Results: Median follow-up for 155 newly diagnosed DF patients was 61 months. Primary treatment included surgical resection (69%), systemic treatment (21%) and active surveillance (9.7%). Choice of treatment was associated with tumor site and size, patient age, and underlying diagnosis of FAP (Table 1). Among patient undergoing complete microscopic resection, 5-year RFS was 81%. Size >10cm and age < 45yo tended to be associated with high rates of local recurrence (HR 1.7; p=0.3 and HR 2.2; p=0.1, respectively). Of those patients who recurred after R0/R1 resection (n=12), 4 patients underwent reoperation (4 alive with no disease), 5 received systemic therapy and 3 were managed with active observation (Figure 1); none died from disease. Patients undergoing R2 resection (n=14, 13% of surgical patients) most commonly had tumors localized to the jejunal mesentery. In 3 patients, initial presentation was complicated with perforation, obstruction or fistulization; one patient eventually died of disease.
First-line systemic therapy included liposomal doxorubicin (61%), methotrexate-based regimens (6.1 %) and tyrosine kinase inhibitors (18%). 5-year PFS was 67% in patients receiving liposomal doxorubicin and 70% of patients had decrease in tumor size. Of 14 patients with tumor regression after treatment with liposomal doxorubicin only 2 patients were noted to have regrowth (11 months and 36 months after therapy cessation); both were subsequently managed with active observation. Of patients who progressed after systemic therapy (n=13, 39% of patients receiving medical treatment), 6 (46%) were managed with surgery and 7 (54%) with second line-therapy; no patients died from disease. Two (13%) of the carefully selected cohort (n=15) managed with active surveillance progressed (13%) and required systemic therapy.

Conclusion: Anatomic constraints and risk or intra-abdominal complications associated with growth necessitates variable intervention strategies for mesenteric DF. This long-term series demonstrates that surgery is associated with relatively low-risk of recurrence in patients able to undergo R0/R1 resection. In unresectable disease, liposomal doxorubicin is associated with durable response in a significant proportion of patients. This study underscores the importance of a tailored, multidisciplinary approach in managing intra-abdominal DF.