(P 458) BETWEEN LYMPHOMAS AND SARCOMAS: A RETROSPECTIVE, MULTI-INSTITUTIONAL STUDY FROM THE ITALIAN SARCOMA GROUP (ISG) ON HISTIOCYTIC AND DENDRITIC CELL NEOPLASMS (HDCN)

Objective: HDCN are ultra-rare tumors with features of both lymphomas (L) and sarcomas (S), lacking standardized treatments.

Methods: We reviewed consecutive cases of histiocytic S (HS), Langerhans cell S (LS) interdigitating dendritic cell S (IDCS), indeterminate dendritic cell tumour (DCT), follicular dendritic cell S (FDCS), fibroblastic reticular cell tumour (FRCT) diagnosed between 2000-2022 in 7 ISG centers. Response was assessed using RECIST1.1 threshold for complete, CR, or partial response, PR; stable disease, SD; progressive, PD. Survival curves were estimated using Kaplan-Meier.

Results: Sixty-three pts were included, 43 with local (LD) and 20 with metastatic disease (MD; table 1). The median (m-) follow up was 101 mos. The 5-yr OS was 88% in LD, 43% in MD (p=0.002). Systemic symptoms at diagnosis predicted a worse 5-yr OS (36% vs 91%, p< 0.001).
In LD, the 10-yr OS for FDCS and HS was 69% and 86%. Among IDCS, FRCT, LS none recurred. Two pts (LS, FDCS) managed with definitive anthracycline-based (Ab) chemo- (CT)/radiotherapy (RT) are progression-free (PF) at 5 and 3 yrs. All pts recurring locally did not receive adjuvant RT nor those who developed MD received adjuvant AbCT.
In MD, high tumor burden correlated with worse OS (5-yr OS 34% vs 100%, p=0.1). The 2-yr OS in FDC-S was 17% vs 78% in HS. Among 28 pts treated with AbCT, 8 had CR (28%), 10 PR (36%), 5 (18%) SD, 4 (15%) PD. The ORR was 65% (HS:80%; FDCS:55%), the m-PFS 12 mos. Two HS pts, receiving definitive Ab CT/RT, had CR and are PF at 8 yrs; 1 pt with metastatic HS, treated with AbCT, had CR and is PF at 2 yrs.

Conclusion: In our series, the outcome in primary HDCN is favorable. Advanced FDCS is an unmet need with a m-OS < 12mos. Similarly to L, long lasting CR were seen in different HDCN types treated with definitive CT/RT, which can be an option for unresectable cases. AbCT was active with a higher ORR compared to S. These data suggest a role for definitive and perioperative CT and RT, especially in FDCS and HS, warranting prospective confirmation.