Randomized controlled trial in which the effect is investigated of a radiation boost in addition to standard chemoradiation in patients with locally advanced rectal cancer on complete response rate defined as pathological complete response, in those who undergo surgery, or 2-years local recurrence-free survival (2y-LRFS), in those who opted for a wait and see approach. Secondary objectives are adverse events due to chemoradiation (acute, perioperative and late toxicity), tumor response assessed with MRI, the impact of the boost on local and distant recurrence and survival as well as patient-reported quality of life and workability. The need for this comprehensive study is emphasized by the sub-optimal (radiation-) methods, heterogeneity between and poor reporting in the few previous trials in this field.

Rationale: The current treatment for locally advanced rectal cancer consists of pre-operative chemoradiation treatment (CRT) (50 Gray (Gy) in 25 fractions) followed by surgical resection, according to T-/N-stage, circumferential resection margin (CRM) and tumor localization (see table 1). After this neo-adjuvant treatment approximately 15% of patients show pathological complete response (pCR), i.e.no residual tumor in the resected specimen on pathologic examination. Patients with pCR have a lower risk of local and distant recurrences and significantly longer disease-free and overall survival. Furthermore, in these patients surgery could possibly have been omitted. Selected patients with a clinical complete response (cCR), defined prior to surgery by rectoscopy, rectal examination and magnetic resonance imaging (MRI), may opt for an organ-preserving therapy, a so called wait and see approach.

Response to chemoradiation occurs in a dose dependent fashion. Therefore, recent trials aimed to improve prognosis by radiation dose-escalation that resulted in improved pCR rates. Toxicity rates associated with radiation doses above 60 Gy are manageable and differ between studies; from increased to comparable or even lower toxicity. Moreover, dose escalation may increase the proportion of patients eligible for organ-preserving therapy.

Objective: We study whether addition of a radiation boost to standard chemoradiation in patients with locally advanced rectal cancer increases the complete response rate defined as pathological complete response, in those who undergo surgery, or 2-years local recurrence-free survival (2y-LRFS), in those who opted for a wait and see approach. Secondary objectives are adverse events due to chemoradiation (acute, perioperative and late toxicity), tumor response assessed with MRI, the impact of the boost on local and distant recurrence and survival as well as patient-reported quality of life and workability. The need for this comprehensive study is emphasized by the sub-optimal (radiation-) methods, heterogeneity between and poor reporting in the few previous trials in this field.

Study design: Multicentre Randomized Controlled Trial, nested within a prospective cohort according to the 'cohort multiple randomized controlled trial' (cmRCT) design.

Study population: Rectal cancer patients participating in a prospective cohort (the PLCRC project) and diagnosed with adenocarcinoma of the rectum whom will undergo chemoradiation based on clinical criteria (see table 1 section 1.2.1).

Intervention: An irradiation boost of 15 Gy delivered to the gross tumor volume (GTV) in 5 fractions in addition to the standard chemoradiation treatment of 50 Gy. Thereby increasing the total GTV dose to 65 Gy.

Main study parameters/endpoints: The primary endpoint is complete response either defined as pathological complete response (pCR) in patients who undergo surgery, assessed by standardized pathologic examination of the surgical specimen, or 2-years local recurrence-free survival (LRFS) after chemoradiation in patients who opted for a wait and see approach. Secondary outcomes are treatment acute, perioperative and late toxicity, tumor response assessed with MRI, patient-reported quality of life and workability, local recurrence and (disease-specific) survival.

Inclusion criteria

• Participant in the PLCRC project (ClinicalTrials.gov: NCT02070146)
• Informed consent obtained for being offered experimental interventions within the PLCRC project
• Informed consent obtained for questionnaires on patient reported outcomes within the PLCRC project
• WHO: 0-2
• Indication for chemoradiation based on primary tumor, regional nodes, metastasis (TNM) stage
• Referred for chemoradiation
• No contra-indication for MRI
• Tumor distance from ano-rectal transition ≤10 cm

Exclusion criteria

• <18 years
• No indication for chemoradiation according to Dutch guidelines based on TNM staging.
• Inflammatory bowel disease
• Prior pelvic radiotherapy
• At least one contra-indication for Capecitabine administration (based on Dihydropyrimidine dehydrogenase (DPD)-deficiency, bloodcount, liver malfunction, renal failure (Creatinine clearance <30 ml/min), medical history such as recent cardiac events
• Recent pregnancy ≤ 1 year ago
• Inadequate understanding of the Dutch language in speech and/or writing