SU‐E‐T‐502: Investigation of Interfraction Setup Error from Using Non‐Extended Standard Thermoplastic Mask for Head and Neck IMRT Patients

Abstract

Purpose: Setup accuracy of our head and neck IMRT patients based on the non‐extended standard thermoplastic masks was examined using Varian onboard imaging system. Methods: Retrospective analysis of thirty head and neck IMRT patients treated during April 2009–July 2010 was performed. All 2DkV and 3D CBCT images were acquired weekly during the same session and reviewed by oncologists using an offline review 8.6 program on Eclipse TPS. Couch shifts predicted by software between 2DkV and CBCT images were recorded. PTV margin using Van Herk's margin formula was calculated. Results: Four‐hundred‐forty‐four images of 2DkV and CBCT alignment were analyzed. Positioning errors within 3 mm. were shown in 84.07% of 2DkV radiographs and 85.84% of CBCT images. Average displacement found in AP, SI and LR axes, were 0.3±2.0 mm, 0±1.7 mm., 0.5 ±1.5 mm. for 2DkV, and 0.3±2.3, 0.7±2.1, 0.4 ±2.1 mm. for CBCT data set, respectively. Systematic and random variations from both methods were seen in the range of 0.5–1.8 mm. PTV margins determined from 2DkV pair images, in AP, CC and LR direction were presented at 4.60, 3.80 and 2.41 mm. when compared to 5.4, 4.32 and 4.35 mm from 3D CBCT. Adaptive treatment planning on six patients were as well undertaken owing to the great benefit of CBCT to detect the patient's contour changes,which can be seen in the range of 1.20–3.12 cm Conclusions: Based on our immobilization masks and laser‐based positioning, majority of the treatment setups were accurate within our acceptably criteria. Both 2DkV and CBCT were insisted to be an effective method to reduce the residual setup error. Results from this study are used as a baseline for further improving the setup accuracy for head and neck IMRT patients at our institution. © 2011, American Association of Physicists in Medicine. All rights reserved.