[Measurement and analysis of the imaging dose with megavoltage computed tomography for helical tomotherapy].
OBJECTIVE
During the helical tomotherapy process, megavoltage computed tomography (MVCT) images are usually used for guiding the precise setup of patients before/after treatment delivery, which would certainly increase the total dose for patients. This study was to investigate the imaging dose of MVCT using the body and head phantom on a tomotherapy machine.


METHODS
A set of cylindrical body and head phantoms was adopted for scanning with different pitch values (1.0/2.0/3.0), lengths (4.8/7.2/9.6/12/14.4 cm) and patient setups on the couch of tomotherapy system. The average MVCT imaging doses were measured using A1SL chambers inserted in the phantoms with preset geometry. The dose uniformity was qualitatively analyzed.


RESULTS
The MVCT scanning dose for the body phantom was between 0.599 and 2.876 cGy during each treatment delivery, while the dose for the head phantom was between 0.913 and 3.231 cGy. Two major parameters, the assigned pitch numbers and scanning lengths, were the most important impacts to the dose variation. The MVCT dose was inversely proportional to the CT pitch value. With respect to the scanning length, the doses responded differently along the radial direction of the phantoms with different setup criteria.


CONCLUSIONS
The results may provide a reliable guidance for proper planning design of the scanning region, which is valuable to help minimize the extra doses to patient.
Xu
Shou-Ping
SP
Department of Radiation Oncology, PLA General Hospital, Beijing, 100853, P.R. China. xshp228@163.com
Xie
Chuan-Bin
CB
Ju
Zhong-Jian
ZJ
Dai
Xiang-Kun
XK
Gong
Han-Shun
HS
Guo
Yan-Yan
YY
Wang
Lian-Yuan
LY
D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D036542:Tomography, Spiral Computed / Q000379:methods*