(P 272) PREGNANCY-ASSOCIATED DESMOID FIBROMATOSIS: A DUTCH MULTI-CENTRE RETROSPECTIVE ANALYSIS

Objective: The development of desmoid fibromatosis (DF) is associated with pregnancy. The current treatment consensus recommends active surveillance (AS). However, data in pregnancy-associated DF is scarce and it is uncertain whether AS is the best management strategy for this DF-subgroup. The aim of this study was to describe demographic, tumor, obstetric, treatment characteristics and treatment outcome in pregnancy-associated DF.

Methods: All female DF patients who were 18 to 50 years old at diagnosis and diagnosed between January 2000 and December 2020 were identified from the prospectively maintained databases at the participating centers (Netherlands Cancer Institute - Antoni van Leeuwenhoek in Amsterdam, Erasmus Medical Centre in Rotterdam and Radboud University Medical Center in Nijmegen). Retrospective reviews of the electronic patient records were carried out to identify women with a recent pregnancy (delivery ≤5 years ago) at the time of diagnosis. Patients with familial adenomatous polyposis (FAP)-related DF were excluded. Specialist soft-tissue pathologists confirmed the pathological diagnosis in all cases.

Results: In total, 62 patients were included with a median age at diagnosis of 33 years (range 23 – 42) (Table 1). Most DF tumors were seen in the abdominal wall (74%), specifically in the rectus abdominal muscle (45%). Localizations outside the abdominal wall were mostly pelvis (10%) and extremities (10%). CTNNB1 mutational analyses was conducted in 31 patients. In those patients, a CTNNB1-mutation was found in almost all tumors (94%), consisting mostly of T41A (58%) and S45P (19%) mutations, and in 7% a S45F mutation. The median time interval between the pregnancy and the DF diagnosis was 19 months (range 1 – 60 months) and significantly longer for non-abdominal wall patients (p=.047).

Active surveillance (AS) was the most used first line strategy (n = 38, 61%) (Table 2). The median follow-up time for these patients was 35 months. Nine (24%) patients experienced spontaneous regression and 16 (43%) had stable disease at last follow-up (Figure 1). The 5-year progression-free survival of all patients with frontline active surveillance was 63% with all progression-events seen in the first 18 months (Figure 2A).

Surgical resection as first line treatment was performed in 23 (37%) patients (Table 2). The median follow-up time for these patients was 61 months. Of these patients, five (23%) experienced recurrence, which could all be conservatively managed with either AS (80%) or vinorelbine (20%) (Figure 1). After surgery, either as first or second line therapy, the 5-year recurrence-free survival was 80% (Figure 2B). A comparison between R0 and R1 resection margins showed no statistically significant difference with a 5-year RFS of 78% for R0 resections and 93% for R1 resections (p=.31).

Only one patient (2%) received systemic treatment as first line treatment (Celecoxib) and eight patients (13%) received systemic treatment as second line treatment.

Conclusion: In this multi-center analysis of pregnancy-associated desmoid fibromatosis, the data shows the indolent behavior of this subgroup of desmoid fibromatosis and that active surveillance should be the frontline management strategy. Surgery is a good secondary treatment option in case active treatment is preferred with good outcomes, even with R1 resection margins. When surgery is undesirable, systemic treatment generally achieves good control of the tumor.

Table 1 - Pregnancy-associated DF - CTOS.docx

Table 2 - Pregnancy-associated DF - CTOS.docx

Figure 1 - Pregnancy-associated DF - CTOS.docx

Figure 2 - Pregnancy-associated DF - CTOS.docx