S722
Clinical – Lower GI
ESTRO 2026
contributing to personalized treatment intensification. ERI demonstrated strong predictive value and may serve as a clinically actionable biomarker to guide organ preservation strategies in LARC. Further multicenter validation is warranted. References: Chiloiro G, Cusumano D, Boldrini L, Romano A, Placidi L, Nardini M, Meldolesi E, Barbaro B, Coco C, Crucitti A, Persiani R, Petruzziello L, Ricci R, Salvatore L, Sofo L, Alfieri S, Manfredi R, Valentini V, Gambacorta MA. THUNDER 2: THeragnostic Utilities for Neoplastic DisEases of the Rectum by MRI guided radiotherapy. BMC Cancer. 2022 Jan 15;22(1):67. doi: 10.1186/s12885-021-09158-9. PMID: 35033008; PMCID: PMC8760695. Keywords: rectal cancer, personalized boost, MRI- guided RT Five-year Results of the OPERA Trial: When and How to Assess Tumor Response to Guide Rectal Preservation Syrine Ben Dhia 1 , Jean Pierre Gérard 1 , Tanguy Pace- Loscos 2 , Nicolas Barbet 3 , David Baron 1 , Arthur Sun Myint 4 , Jérôme Doyen 5 1 Radiation oncology, Antoine Lacassagne center, Nice, France. 2 Epidemiology,Biostatistics and Health Data, Antoine Lacassagne center, Nice, France. 3 Radiation oncology, ORLAM center, Lyon, France. 4 Radiation Mini-Oral 3371 oncology, Clatterbridge Center, Liverpool, United Kingdom. 5 Radiation oncology, Antoine Lacassagne center, nice, France
responders (21.9%) (Figure 2). ERI-driven dose escalation therefore increased the expected CR rate in non-responders (historically ~3%) while maintaining acceptable toxicity. NOM was achieved in 14 responders (46.7%) and in 4 non-responders (12.5%); local regrowth occurred in 2 non-responders under NOM, both successfully salvaged. Higher baseline tumor size (>3 cm) and greater circumferential involvement were associated with non-response, but ERI remained the only independent predictor of CR. Treatment was feasible, with 72.6% of patients undergoing at least one adaptive fraction and low rates of Grade ≥ 3 toxicity.
Purpose/Objective: When an organ preservation (OP) strategy is
considered for rectal adenocarcinoma, assessing tumor response after treatment is crucial. The OPERA trial combined an X-ray brachytherapy (CXB) boost with chemo radiotherapy; assessing response at week 24 (W24).This post-hoc analysis evaluates earlier decision (W14) and clinical tumor response evaluation (CTRE) through digital rectal examination (DRE) and rectoscopy. Material/Methods: CTRE data at week 14 (W14) were collected according to RECIST criteria: cCR (no visible/palpable tumor), nCR (no visible/palpable tumor but abnormalities such as ulceration or fibrosis with a firm wall), PR ( ≥ 30% size reduction). “Good” response (ncCR) included both cCR and nCR. MRI was assessed with TRG score: TRG1 (no tumor), TRG2 (fibrosis), TRG3–5 (visible tumor). Decisions included OP in cases of “good” response: ncCR, a second CTRE at week 24, local excision (LE), total mesorectal excision (TME). CTRE was correlated with local relapse and 5-year OP rates.
Conclusion: ERI-guided adaptive MRI-based dose escalation is feasible and can improve complete response rates in patients predicted not to benefit from standard nCRT,
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