Clinical implementation of intensity modulated arc therapy (IMAT) for rectal cancer

Abstract

3685 Background: In rectal cancer, radiotherapy decreases local recurrence rates and improves survival. Small bowel (SB) radiation toxicity is strongly influenced by the volume of irradiated SB. Intensity modulation (IM) could avoid the irradiation of large volumes of SB. A planning strategy for intensity modulated arc therapy (IMAT) was developed and clinically implemented. We report on the planning results of the first seven patients treated with IMAT. Methods: Seven patients consented to participate to the study. Delineation of target volumes was done on a fused CT and MRI, with the help of a radiologist. For the IMAT plan, machine states (MS) were generated every 8 degrees using an anatomy-based segmentation tool. Adjacent MSs of the same class were stratified in arcs. The optimization of the arcs was done by weight optimization and leaf position optimization. Arcs with a constant angular delivery rate were extracted semi-automatically. The IMRT plans, which were made for comparison, used seven incidences. Segmentation and optimization tools were identical as for IMAT. The 3D plans used three conventional - partially wedged - beams (one posterior and two lateral fields), of which the outlines were shaped to the planning target volume (PTV). All plans were normalized to a median dose of 45 Gy in the PTV. Results: The IMAT plan resulted in 3–6 arcs and a mean delivery time of 6.3 minutes. The minimal dose in the PTV was not significantly different in any comparison between 3D, IMRT and IMAT plans (42, 41 and 41 Gy, respectively). Dose inhomogeneity in the PTV was highest for the IMAT plans (14.1%) and lowest for the 3D plans (9.9%). The volume of SB receiving more than 90% of the prescribed dose was 19% for 3D, 10% for IMRT and 7% for IMAT. Mean dose to the SB was significantly lower for the IMRT (12.1 Gy) and IMAT plans (12.4 Gy) than for the 3D plans (17.0 Gy). The volume of SB receiving a specified dose was lower for the IM plans when compared to the 3D plans. IMAT resulted in the lowest SB volume irradiated to doses between 20 and 45 Gy. Conclusions: IMAT is deliverable within a standard clinical time slot, and results in a significantly lower dose to the SB, without compromising the dose to the PTV. No significant financial relationships to disclose.