Objectives:We assessed the impact of three-dimensional (3D) conformal planning vs conventional planning of preoperative chemoradiotherapy (CRT) for locally advanced rectal cancer (LARC) on small bowel and bladder sparing and in optimising coverage of tumour target volume. Methods: Conformal and conventional plans were created for 50 consecutive patients. The conformal plan delineated a gross tumour volume (GTV), a clinical target volume (CTV) 1 to cover potential subclinical disease spread, a CTV2 to outline the mesorectum and lymph node areas at risk, and a planning target volume (PTV) to cover set-up error and organ movement. The conventional plan was created using digitally reconstructed radiographs (DRRs). Patients were treated with a dose of 45 Gy in 25 fractions with concurrent chemotherapy over 5 weeks. Dose-volume histograms (DVHs) were created and compared for GTV, PTV, small bowel and bladder. The GTV was covered by the conventional plan in all patients. Results: Significant differences were shown for median PTV coverage with conformal planning compared with conventional planning: 99.2% vs 94.2% (range 95.9-100% vs 75.5-100%); p,0.05. The median volume of irradiated small bowel was significantly lower for CT plans at all DVH levels. Median bladder doses did not differ significantly. Conclusion: 3D conformal CT planning is superior to conventional planning in terms of coverage of the tumour volume. It significantly reduces the volume of small bowel irradiated with no decrease in the rate of R0 resection compared with published data, and at the present time should be considered as the standard of care for rectal cancer planning. Chemoradiotherapy (CRT) followed by total mesorectal excision is the standard for care when MRI staging demonstrates threatened surgical margins in locally advanced rectal cancer (LARC) [1,2]. Radiotherapy planning for rectal cancer uses conventional orthogonal simulation with standardised radiation fields based on patterns of loco-regional relapse in relation to pelvic bony anatomy [3]. Three-field conventional orthogonal planning is considered an acceptable technique for planning preoperative CRT and major trials evaluating long-course chemoradiation for rectal cancer have permitted the use of conventional planning within their protocols [4,5]. In recent years, the treatment of rectal cancer has improved through advances in the planning and delivery of radiotherapy as well as improved preoperative imaging with MRI, the development of surgical techniques using total mesorectal excision (TME) and more accurate histopathological reporting [6]. Radiotherapy planning must ensure all clinically and radiologically identifiable disease is encompassed while still minimising the dose to the surrounding organs at risk, particularly the small bowel and bladder. Potential areas of microscopic spread and the appropriate pelvic lymph nodes should also be treated.Preoperative MRI has improved the knowledge of pelvic anatomy and identification of pelvic lymph nodes at risk according to tumour l...