Neoadjuvant chemoradiation therapy (nCRT) is the standard treatment modality for locally advanced rectal cancer (LARC), and patients achieving complete response (CR) generally have improved local control, metastasis-free survival, and overall survival.

The aim of this clinical trial is to investigate the impact of radiotherapy dose escalation in rectal cancer by identifying poor responders during treatment using the Early Tumor Regression Index (ERI). Patients are treated using magnetic resonance-guided radiotherapy (MRgRT). ERI is calculated at fraction 10. Patients with an ERI value below 13.1 continue the standard treatment schedule, whereas patients with an ERI value above 13.1 undergo adaptive replanning with dose escalation up to 60.1 Gy to the residual tumor volume.

Concomitant chemotherapy consists of fluoropyrimidine-based treatment, with oxaliplatin permitted in patients at high risk of recurrence. In selected high-risk patients, consolidation chemotherapy with three cycles of FOLFOX may be administered during the interval before surgery.

Following protocol amendments, the study was expanded to include a total planned enrollment of 179 patients. Additional blood samples for circulating tumor DNA (ctDNA) analysis and stool samples for gut microbiome profiling are collected at predefined time points. These longitudinal multi-omics data are integrated with magnetic resonance imaging-based delta-radiomics features to develop composite biomarker models for prediction of treatment response.

The primary outcome measures are complete response, defined as ypT0N0 after Total Mesorectal Excision (TME), ypT0 ycN0 after Local Excision (LE), or ycT0N0 in patients managed with Watch and Wait, and the prospective validation of the magnetic resonance-guided radiotherapy delta-radiomics predictive model.

Neoadjuvant chemoradiation therapy (nCRT) is the standard treatment modality for locally advanced rectal cancer (LARC). Patients who achieve a complete response (CR) after treatment generally experience improved local control, metastasis-free survival, and overall survival. Because response to radiotherapy is dose-dependent in rectal cancer, dose escalation may increase complete response rates. The ability to identify patients who are unlikely to achieve a complete response before surgery or during treatment is therefore of major clinical importance. An Early Tumor Regression Index (ERI) has been developed to predict pathological complete response after nCRT. Patients with ERI values below 13.1 have been shown to demonstrate a stronger response during treatment and a lower probability of developing distant relapse.

The primary aim of this clinical trial is to evaluate the impact of adaptive radiotherapy dose escalation in rectal cancer by identifying poor responders during treatment using ERI and increasing the prescribed radiation dose in these patients. Using this adaptive boosting strategy, an increase of approximately 10% in the complete response rate is expected. All enrolled patients receive chemoradiotherapy delivered using a 0.35 Tesla magnetic resonance-guided radiotherapy (MRgRT) system.

The radiotherapy schedule consists of 55 Gy in 25 fractions to the gross tumor volume and corresponding mesorectum, together with 45 Gy in 25 fractions to the whole pelvis. Concomitant chemotherapy consists of continuous oral capecitabine or 5-fluorouracil. Oxaliplatin may be added in patients considered at high risk of recurrence. Daily magnetic resonance imaging is performed throughout treatment. ERI is calculated at fraction 10. Patients with an ERI value below 13.1 continue the standard treatment schedule, whereas patients with an ERI value above 13.1 undergo adaptive treatment replanning with dose escalation up to 60.1 Gy to the residual tumor volume. For selected high-risk patients, consolidation chemotherapy with three cycles of the FOLFOX regimen may be administered during the interval before surgery. Patients achieving a complete clinical response may be managed using a Watch-and-Wait strategy according to multidisciplinary team evaluation.

Following protocol amendments, the study has been expanded to include additional patients for longitudinal multi-omics analyses. Stool samples are collected for gut microbiome characterization, including metagenomic, metabolomic, and metatranscriptomic analyses. Blood samples are collected for circulating tumor DNA (ctDNA) analyses and other blood-based biomarkers. These data are integrated with magnetic resonance imaging-derived delta-radiomics features to develop composite biomarker models aimed at improving prediction of treatment response and supporting personalized treatment strategies in locally advanced rectal cancer.

Inclusion Criteria:

• Histological proven adenocarcinoma of the rectum;
• cT2-4, or cN0-2 (also extramesorectal nodes), or mesorectal fascia involvement for tumor (MRF+) or extramural venous invasion (EMVI+);
• cM0;
• No prior radiotherapy in pelvic region;
• Tumour located between 0 and 15 cm above the anal verge;
• Not mesorectal fascia involvement for tumor;
• No extramesorectal nodes involvement;
• No extramural venous invasion (EMVI);
• No rectal mucinous adenocarcinoma histology;
• No contra-indications for MRI;
• ECOG 0-1;
• Age over 18 years;
• Adequate hematological function: granulocyte count > 1500/microl, Hemoglobin level > 10 g/dl, Platelet count > 100000/microl, ALT/AST: 7-45 UI/L;
• No other malignancies in the previous history (except skin and initial cervical cancer);
• Absence of important comorbidities: severe cardiac or coagulative disease, moderate or severe restrictive/obstructive lung deficit, severe cognitive impairment, moderate and severe renal and hepatic impairment;
• Absence of any psychological, familial, sociological or geographical condition potentially hampering compliance with the study protocol and follow-up schedule; those conditions should be discussed with the patient before registration in the trial;
• Absence of pregnancy or lactating female patients;
• Written informed consent.

Exclusion Criteria:

• Distant metastases (cM1);
• Prior radiotherapy in the pelvic region;
• Tumour located lower than 0 cm or higher than 15 cm above the anal verge;
• Rectal mucinous adenocarcinoma histology;
• Contra-indications for Magnetic Resonance Imaging (MRI);
• Eastern Cooperative Oncology Group (ECOG) performance status score >= 2;
• Age under 18 years;
• Inadequate hematological function (Granulocyte count <= 1500/microl, Hemoglobin level <= 10 g/dl, Platelet count <= 100000/microl, ALT/AST outside 7-45 UI/L);
• History of other malignancies (except skin cancer and initial cervical cancer);
• Presence of severe comorbidities (severe cardiac or coagulative disease, moderate or severe restrictive/obstructive lung deficit, severe cognitive impairment, moderate/severe renal or hepatic impairment);
• Any psychological, familial, sociological, or geographical condition potentially hampering compliance with the study protocol and follow-up schedule;
• Pregnant or lactating female patients.