(P 230) CHARACTERIZING TISSUE-BASED CHANGES DURING AND AFTER RADIOTHERAPY IN MYXOID LIPOSARCOMAS IN THE DOREMY TRIAL

Objective: To assess the tissue-based changes of myxoid liposarcomas over time during and after a modest dose of neoadjuvant radiotherapy through morphological and immunohistochemical analysis.

Methods: We evaluated 23 patients with translocation-confirmed primary myxoid liposarcoma from a prospective, single-arm, phase 2 clinical trial (DOREMY). Treatment consisted of neoadjuvant radiotherapy to 36 Gy in once-daily 2-Gy fractions, followed by definitive surgical resection. Evaluation was performed at four different time points: biopsy pre-treatment, biopsy after 8 fractions, biopsy after 16 fractions, and after definitive surgical resection. Morphological features were assessed on H&E and the percentage of viable tumoral cells, hyalinization/fibrosis, tumoral necrosis, and fatty maturation were estimated on the biopsies and all available sections of the resection specimen. Tissue microarrays were used for immunohistochemistry. Markers for autophagy (LC-3B), anti-apoptosis (Bcl2), activity of mTOR signaling (Phospho-S6) and hypoxia (CAIX) were scored semi-quantitively, by two independent readers, blinded for clinical data, according to intensity (weak, moderate or strong) and percentage of positive cells (0, < 5%; 1+, 5%-25%; 2+, 25%-50%; 3+, 50%-75%; or 4+, 75%-100%). In case of disagreement among the two initial readers, a third reader served as an adjudicator. Markers for apoptosis (cleaved caspase-3), proliferation (Ki67), inflammation (CD45 and CD68) and micro-vessel density (CD34 Chalkley count) were analyzed automatically with digital software for image analysis (QuPath). Descriptive statistics were used to analyze the pathological outcomes and were reported as mean ± standard deviation or median (IQR) where appropriate.

Results: Morphological evaluation revealed a decrease in vital tumor cells during treatment, with a median of 100% (100%-100%) pre-treatment, and a median of 35% (10%-97.5%), 10% (5%-30%) and 5% (5%-15%) after 8 fractions, 16 fractions and in the resection specimen, respectively. Notably, the reduction of vital tumor cells was most pronounced early, before fraction 8. Inversely, the histological treatment effect was observed during treatment, mainly visible as hyalinization and fatty maturation, while tumoral necrosis was less often observed. Three cases had a round cell component that either disappeared or was completely necrotic after 8 fractions. Micro-vessel density remained stable over time, while the amount of immune infiltrate (CD45 and CD68) slightly declined during treatment. Other immunohistochemical markers for apoptosis, anti-apoptosis, activity of mTOR signaling, proliferation, autophagy and hypoxia did not show any significant changes during and after radiotherapy.

Conclusion: A modest dose of neo-adjuvant radiotherapy induces profound tissue changes in myxoid liposarcoma, with the therapeutical effect predominantly occurring during the first 8 fractions, in the conventional low-grade component, as well as in the high-grade round cell component. Furthermore, it alters the tumor micro-environment with less immune infiltrate during therapy. Neither the vasculature nor the proliferation rate, activity of the mTOR pathway, or cellular degradation were affected by radiotherapy. Therefore, it remains to be answered why myxoid liposarcomas are highly radiosensitive.