THeragnostic Utilities for Neoplastic DisEases of the Rectum by MRI Guided Radiotherapy (THUNDER2)

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.