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Background The acquisition of detailed computerized tomography (CT) imaging at the time of simulation, along with three-dimensional (3D) treatment planning software has been integrated with radiation delivery hardware to create the modality known as 3D conformal radiotherapy (3DXRT). This approach provides, in theory, a means to selectively subtract the anal sphincter from the high-dose field of irradiation in patients with stage II and III adenocarcinomas of the mid-rectum scheduled for low anterior resection (LAR). Hypothesis Implementation of 3DXRT with sphincter blocking may be a feasible strategy to reduce the dose of radiation distributed to the anal canal without reduction in the dose distribution to the gross tumour volume (GTV) plus adequate margins. Methods Pretreatment simulation CT scans of 10 patients with rectal cancers located between 5 and 10 cm from the anal verge were retrieved from a computerized database. Radiation oncologists and colorectal surgeons defined the contours of the GTV and the anal sphincter, respectively, on successive CT scan slices. These contours provided the volumetric data required to quantify dose distribution and compute dose-volume histograms. The standard mode of pelvic irradiation planned with CT simulation was compared with a ‘virtual CT simulation’ approach, in which a sphincter block was added to the protocol. Results The mean distance of tumours from the anal verge was 6.3 cm. In the virtual simulation treatment plan, a 2-cm margin separated the sphincter block from the lower limit of the GTV. The mean volume of the anal sphincter was 16.1 ± 3.5 cm3. The dose distributed to the GTV in the real plan and in the virtual simulated block plan were 51.7 ± 1.4 and 51.6 ± 1.4 Gy respectively ( P = 0.85). By comparison the mean dose distributed to the anal sphincter was dramatically reduced by using a sphincter block (33.2 ± 12 Gy vs 6.4 ± 4.1 Gy, P < 0.001). Conclusion During a course of radiotherapy for most low- or mid-rectal cancers, the anal canal is included within the field of irradiation with a mean dose distribution to the sphincter of 33 Gy. Evaluation of 3DXRT with full sphincter block (mid-rectum) and partial sphincter block (distal rectum) is a feasible strategy to decrease the volume of anal sphincter carried to full dose without reduction in dose to the GTV. This approach, by minimizing treatment-induced damage to the anal sphincter, might improve functional outcome of LAR. [ABSTRACT FROM AUTHOR]