Dosimetric characterization of a multileaf collimator for a new four-dimensional image-guided radiotherapy system with a gimbaled x-ray head, MHI-TM2000.
OBJECTIVE
To present the dosimetric characterization of a multileaf collimator (MLC) for a new four-dimensional image-guided radiotherapy system with a gimbaled x-ray head, MHI-TM2000.


METHODS
MHI-TM2000 has an x-ray head composed of an ultrasmall linear accelerator guide and a system-specific MLC. The x-ray head can rotate along the two orthogonal gimbals (pan and tilt rotations) up to +/- 2.5 degrees, which swings the beam up to +/- 41.9 mm in each direction from the isocenter on the isocenter plane perpendicular to the beam. The MLC design is a single-focus type, has 30 pairs of 5 mm thick leaves at the isocenter, and produces a maximum field size of 150 x 150 mm2. Leaf height and length are 110 and 260 mm, respectively. Each leaf end is circular, with a radius of curvature of 370 mm. The distance that each leaf passes over the isocenter is 77.5 mm. Radiation leakage between adjacent leaves is minimized by an interlocking tongue-and-groove (T&G) arrangement with the height of the groove part 55 mm. The dosimetric characterizations including field characteristics, leaf position accuracy, leakage, and T&G effect were evaluated using a well-commissioned 6 MV photon beam, EDR2 films (Kodak, Rochester, NY), and water-equivalent phantoms. Furthermore, the field characteristics and leaf position accuracy were evaluated under conditions of pan or tilt rotation.


RESULTS
The differences between nominal and measured field sizes were within +/- 0.5 mm. Although the penumbra widths were greater with wider field size, the maximum width was < 5.5 mm even for the fully opened field. Compared to the results of field characteristics without pan or tilt rotation, the variation in field size, penumbra width, flatness, and symmetry was within +/- 1 mm/l% at the maximum pan or tilt rotational angle. The leaf position accuracy was 0.0 +/- 0.1 mm, ranging from -0.3 to 0.2 mm at four gantry angles of 0 degrees, 90 degrees, 180 degrees, and 270 degrees with and without pan or tilt rotation. The interleaf leakage was up to 0.21%, whereas the intraleaf leakage was < 0.12%. T&G decreased the doses by 10.7%, on average.


CONCLUSIONS
This study demonstrated that MHI-TM2000 has the capability for high leaf position accuracy and low leakage, leading to highly accurate intensity-modulated radiotherapy delivery. Furthermore, substantial changes in the dosimetric data on field characteristics and leaf position accuracy were not observed even at the maximum pan or tilt rotation.
Department of Radiation Oncology and Image-Applied Therapy, Graduate School of Medicine, Kyoto University, Kyoto 606-8507, Japan.
Mitsuhiro
M
Nakamura
Akira
A
Sawada
Yoshitomo
Y
Ishihara
Kenji
K
Takayama
Takashi
T
Mizowaki
Shuji
S
Kaneko
Mikiko
M
Yamashita
Hiroaki
H
Tanabe
Masaki
M
Kokubo
Masahiro
M
Hiraoka
D011874:Radiometry; D011878:Radiotherapy / Q000295:instrumentation* / Q000379:methods; D011879:Radiotherapy Dosage; D012399:Rotation