Intensity-Modulated Radiotherapy With a Simultaneous Integrated Boost in Rectal Cancer

Dose-response curves suggest that higher doses of radiotherapy improve the complete response rate in rectal cancer. The UK adopted the EXPERT trial dose and fractionation, 45 Gy in 25 fractions to the pelvis with a sequential 9 Gy in five fractions to the gross tumour, in patients where the aim was to maximise the complete response. In the Oxford University Hospital NHS Foundation Trust (Oxford, UK) we deliver a biological equivalent dose (BED5) in selected patients using intensity-modulated radiotherapy (IMRT) with a simultaneous integrated boost (SIB) in 25 fractions. We carried out a retrospective analysis of our series to: (i) document the toxicity of this protocol; (ii) ascertain whether dose constraints from RTOG 0822 were appropriate; (iii) assess the response.

The demographics and treatment details for all consecutive patients treated with this protocol were collected using electronic systems. Patients received 45 Gy to the elective nodes and 52 Gy using a SIB to the gross tumour with capecitabine chemotherapy using IMRT or RapidArc plans. Acute toxicity was collected prospectively during weekly reviews. For the purpose of this study, a dedicated gastrointestinal radiologist reviewed all baseline and post-treatment magnetic resonance images and assigned a magnetic resonance tumour regression grade (mrTRG).

Seventy-one patients were identified. Seventy completed radiotherapy with a median overall treatment time of 34 days (range 32-36 days); 67.6% received full-dose chemotherapy, with 21.2% receiving a reduced dose. There was a 4.2% incidence of grade 3+ non-haematological toxicity and 1.5% grade 3 + haematological toxicity. 4.2% were admitted during their radiotherapy, with one death due to a pelvic abscess. The RTOG 0822 constraints were achieved in ≥75% of cases, other than the high-dose bladder constraint. mrTRG 1-2 was seen in 47.8%, with mrTRG 1 seen in 23.9%.

We suggest that our protocol shows acceptable acute toxicity, with promising mrTRG results, and could be adopted by centres as an IMRT equivalent dose for EXPERT dose and fractionation.