ESTRO 2024 - Abstract Book

S1517

Clinical - Lower GI

ESTRO 2024

1566

Proffered Paper

Intensification of RT dose, rather than chemotherapy, in Total Neoadjuvant Therapy for rectal cancer

Alessandra Castelluccia 1,2 , Gianmarco Grimaldi 2 , Domenico Marchesano 2 , ivan Annessi 2 , Federico Bianciardi 2 , Annamaria Di Palma 2 , Maria Valentino 2 , Laura Verna 2 , Maria Rago 2 , Cristian Borrazzo 2 , Marica Masi 2 , Randa El Gawhary 2 , Maurizio Portaluri 1 , PierCarlo Gentile 2

1 "A. Perrino" Hospital, Radiotherapy, Brindisi, Italy. 2 San Pietro FBF Hospital, Radiotherapy, Rome, Italy

Purpose/Objective:

The total neoadjuvant therapy (TNT) includes multimodal treatment strategies for locally advanced rectal cancer that administer both radiation and systemic therapy before surgical resection. Recent prospective randomized trials show that TNT using full-dose chemotherapy (as opposed to lower radio-sensitizing chemotherapy doses) may improve pathologic complete response (pCR) rates 1-2 . The purpose of this study is to evaluate the tolerability and efficacy of dose-escalated short-course RT using stereotactic MR-guided adaptive technique, included in a TNT protocol for rectal cancer.

Material/Methods:

Patients (pts) with histological proof of newly diagnosed, primary, locally advanced adenocarcinoma of the rectum (cT3/4 cN0-2 cM0), underwent dose-escalated short-course RT using a hybrid MR-Linac (MRIdian ViewRay). The prescribed dose was 25 Gy (5 Gy/fr) to the mesorectum with a simultaneous integrated stereotactic boost to the rectal lesion of 40 Gy (8 Gy/fr) at 80% isodose, corresponding to 60 Gy in 2Gy equivalent dose (EQD2, α/β = 10Gy), delivered on 5 days (3 fr/week). A MR-guided adaptive strategy was applied, consisting in daily on-table recountouring and replanning to control interfractional volume changes, due to rectal and bowel motion, followed by a gating approach combined with real-time cine-MR to manage intrafraction motion. Sequential chemotherapy was administered after RT; type, dose and number of cycles of systemic therapy were prescribed at discretion of oncologist, based on a case by case clinical evaluation, for a maximum of 3 cycles. Finally, total mesorectal excision was performed. Primary endpoint was evaluation of adverse effect of radiotherapy, graded according to the Common Terminology Criteria for Adverse Events (CTCAE v5.0). Secondary endpoint was pCR rate. From October 2020 to October 2023, 39 patients (pts) underwent neoadjuvant stereotactic MR-guided adaptive RT(SMART) for rectal cancer. Median follow-up time was 16 months (range 4-36). Sequential chemotherapy consisted of capecitabine alone for twenty-three (59%) pts; CAPOX or FOLFOX was prescribed for six (15%) pts. Ten pts were unfit for chemotherapy due to age and/or comorbidities. All patients underwent total mesorectal excision and were completely resected (R0). Mean interval between the completion of SMART and surgery was 8 weeks (range 4-30). No grade 3 or higher toxicity was recorded. Tenesmus with mild pain was the most reported symptom. Other gastrointestinal symptoms recorded were: grade 1 rectal pain in 3 pts (8%), mild rectal hemorrhage in 2 (5%) patient, grade 1 and grade 2 proctitis in 8 (20%) and 6 (15%) pts, respectively; grade 1 diarrhea occurred during chemotherapy for 19 (49%) pts. Regarding urinary symptoms, 2 pts (5%) reported grade 1 dysuria. Twelve patients (31%) complained slight fatigue. A tumor and/or nodal downstaging occurred in all resected patients. Twelve (31%) patients achieved complete response (pCR). The pCR occurred with a prolonged Results: