Effect of jaw size in megavoltage CT on image quality and dose.
OBJECTIVE
Recently, the jaw size for the TomoTherapy Hi-Art II(®) (TomoTherapy Inc., Madison, WI) was reduced from 4 mm (J4) to 1 mm (J1) to improve the longitudinal (IEC-Y) resolution in megavoltage computed tomography (MVCT) images. This study evaluated the effect of jaw size on the image quality and dose, as well as the dose delivered to the lens of the eye, which is a highly radiosensitive tissue.


METHODS
MVCT image quality (image noise, uniformity, contrast linearity, high-contrast resolution, and full width at half-maximum) and multiple scan average dose (MSAD) were measured at different jaw sizes. A head phantom and photoluminescence glass dosimeters (PLDs) were used to measure the exposed lens dose (cGy). Different MVCT scan modes (pitch = 1, 2, and 3) and scan lengths (108 mm, 156 mm, and 204 mm) were applied in the MSAD and PLDs measurements.


RESULTS
The change in jaw size from J4 to J1 produced no change or only a slight improvement in image noise, uniformity, contrast linearity, and high-contrast resolution. However, the full-width at half-maximum reduced from approximately 7.2 at J4 to 4.5 mm at J1, which represents an enhancement in the longitudinal resolution. The MSAD at the center point changed from approximately 0.69-2.32 cGy (peripheral: 0.83-2.49 cGy) at J4 to 0.85-2.81 cGy (peripheral: 1.05-2.86 cGy) at J1. The measured lens dose increased from 0.92-3.36 cGy at J4 to 1.06-3.91 cGy at J1.


CONCLUSIONS
The change in jaw size improved longitudinal resolution. The MVCT imaging dose of approximately 3.86 cGy, 1.92 cGy, and 1.22 cGy was delivered at a pitch of 1, 2, and 3, respectively, per fraction in the head and neck treatment plans. Therefore, allowance for an approximately 15% increase in lens dose over that with J4 should be provided with J1.
Department of Radiation Oncology, College of Medicine, Soonchunhyang University Bucheon Hospital, Bucheon 1174, Korea.
Jae Hong
JH
Jung
Kwang Hwan
KH
Cho
Yong Ho
YH
Kim
Seong Kwon
SK
Moon
Chul Kee
CK
Min
Woo Chul
WC
Kim
Eun Seog
ES
Kim
Ah Ram
AR
Chang
Tae Ho
TH
Kim
Jai-Woong
JW
Yoon
Tae-Suk
TS
Suh
Hyun Do
HD
Huh
D000465:Algorithms; D005898:Glass; D006257:Head / Q000473:pathology / Q000528:radiation effects; D006801:Humans; D007091:Image Processing, Computer-Assisted; D007568:Jaw / Q000473:pathology / Q000528:radiation effects*; D008027:Light; D049449:Luminescence; D019047:Phantoms, Imaging; D011874:Radiometry / Q000379:methods*; D015203:Reproducibility of Results; D014057:Tomography, X-Ray Computed / Q000379:methods*; D014867:Water / Q000737:chemistry
D014867:Water