ESTRO 2021 Abstract Book

S85

ESTRO 2021

microscopy and quantified, both in irradiated and control zAvatars, followed by normalization. When a cut off ratio of ≥ 1.4 was reached, a significant apoptotic effect was assumed, and tumoral cells were classified as radiosensitive. Otherwise they would be considered radioresistant. Tumor size and micrometastasis formation were also assessed. Patients were treated as per institutional protocol with image-guided VMAT RT, either with long-course (LC) chemoradiotherapy with 5-Fluoracil (5FU) or with short-course (SC) RT. Dose- prescription was 25 x 1.8 Gy (with integrated boost of 2.24 per fraction) for LC and 5x5 Gy for SC. Ten weeks after RT, clinical response was assessed with MRI sequences (T2W and DWI) and endoscopy. Tumor regression grades were collected for MRI (mrTRG) and endoscopy (eTRG), and the downsizing effect was measured with MRI. Clinical response was assessed comparing the initial tumor characteristics and their changes after treatment and classified as complete, near-complete, partial or poor by the Tumor Board. The correlation between zAvatars response and the clinical response of the corresponding patient was studied. Results Between February 2019 and November 2020, 8 patients with LARC, mean age 73 years old (SD 17.6) were enrolled in this study. Patients’ patterns of clinical responses are displayed in Table 1. There were 37% (3/8) complete or near-complete clinical responses, 38% (3/8) partial response, and 25% (2/8) poor response to RT. Of the corresponding 8 zAvatars analyses, 6 (75%) were classified as RT response. When correlating the human and the Avatar model, we observed a matched-response to radiosensitivity/radioresistance in 75% of cases (6/8). Patterns of matching responses are shown in Fig1.