Assessment of Interfraction Setup Error of Non-Extended Standard Thermoplastic Mask for Head- and- Neck Intensity-Modulated Radiation Therapy Patients

Abstract

Objective: Setup accuracy of our head and neck intensity-modulated radiation therapy (IMRT) patients based on the nonextended
standard thermoplastic masks was examined using a Varian onboard imaging system.
Methods: Retrospective analysis of thirty head and neck IMRT patients treated during April 2009-July 2010 was performed.
All two-dimensional kilovoltage (2DkV) and cone-beam computed tomography (CBCT) images were acquired weekly during
the same session and reviewed by oncologists using an offline review 8.6 program on an Eclipse treatment planning system.
Couch shifts predicted by software between 2DkV and CBCT images were recorded. The planning target volume (PTV)
margin using VanHerk’s margin formula was calculated.
Results: Four-hundred and 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 anteroposterior
(AP), cranio-cuadal (CC) and left-right (LR) axes , were 0.3±2.0 mm , 0±1.7 mm, 0.5±1.5 mm for the 2DkV, and
0.3±2.3, 0.7±2.1, 0.4±2.1 mm for the CBCT data set, respectively. Systematic and random variations from both methods
which were seen in the range of 0.5-1.8 mm. PTV margins determined from 2DkV pair images, in AP ,CC and LR directions
were presented at 4.60, 3.80 and 2.41 mm. when compared to 5.4, 4.32 and 4.35 mm from CBCT, respectively. Adaptive
treatment planning on six patients were undertaken as well 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.
Conclusion: Based on our immobilization masks and laser-based positioning, the majority of treatment setups were accurate
within our acceptable criteria. Both 2DkV and CBCT were shown to be effective methods to reduce the residual setup
error. The results from this study will be used as a baseline for further improving the setup accuracy for head and neck IMRT
patients at our institution.