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Cho, Kim, Kang, Kim, Kim, Cheon, and Suh: Study of Motion-induced Dose Error Caused by Irregular Tumor Motion in Helical Tomotherapy
Original Article

Progress in Medical Physics 2015;26(3):119-1126.

Published online: 20 September 2015

DOI: https://doi.org/10.14316/pmp.2015.26.3.119

Study of Motion-induced Dose Error Caused by Irregular Tumor Motion in Helical Tomotherapy

Min-Seok Cho∗, †, Tae-Ho Kim∗, †, Seong-Hee Kang∗, †, Dong-Su Kim∗, †, Kyeong-Hyeon Kim∗, †, Geum Seong Cheon∗, †, Tae Suk Suh∗, †

∗Department of Biomedical Engineering, Seoul, Korea

†Research Institute of Biomedical Engineering, College of Medicine, The Catholic University of Korea, Seoul, Korea

Correspondence:Tae Suk Suh(suhsanta@catholic.ac.kr) Tel:82-2-2258-7232, Fax:82-2-2258-7506

Received 30 July 2015       Revised 28 August 2015       Accepted 7 September 2015

Copyright © 2015 Korean Society of Medical Physics

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The purpose of this study is to analyze motion-induced dose error generated by each tumor motion parameters of irregular tumor motion in helical tomotherapy. To understand the effect of the irregular tumor motion, a simple analytical model was simulated. Moving cases that has tumor motion were divided into a slightly irregular tumor motion case, a large irregular tumor motion case and a patient case. The slightly irregular tumor motion case was simulated with a variability of 10% in the tumor motion parameters of amplitude (amplitude case), period (period case), and baseline (baseline case), while the large irregular tumor motion case was simulated with a variability of 40%. In the phase case, the initial phase of the tumor motion was divided into end inhale, mid exhale, end exhale, and mid inhale; the simulated dose profiles for each case were compared. The patient case was also investigated to verify the motion-induced dose error in ‘clinical-like' conditions. According to the simulation process, the dose profile was calculated. The moving case was compared with the static case that has no tumor motion. In the amplitude, period, baseline cases, the results show that the motion-induced dose error in the large irregular tumor motion case was larger than that in the slightly irregular tumor motion case or regular tumor motion case. Because the offset effect was inversely proportion to irregularity of tumor motion, offset effect was smaller in the large irregular tumor motion case than the slightly irregular tumor motion case or regular tumor motion case. In the phase case, the larger dose discrepancy was observed in the irregular tumor motion case than regular tumor motion case. A larger motion-induced dose error was also observed in the patient case than in the regular tumor motion case. This study analyzed motion-induced dose error as a function of each tumor motion parameters of irregular tumor motion during helical tomotherapy. The analysis showed that variability control of irregular tumor motion is important. We believe that the variability of irregular tumor motion can be reduced by using abdominal compression and respiratory training.

Keywords: Motion-induced dose error, Tumor motion, Helical tomotherapy

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Fig. 1.
Schematic of acquiring the dose profile.
pmp-26-119f1.tif
Fig. 2.
Position versus time curve of the treatment field in the (a) room coordinate system and (b) tumor coordinate system.
pmp-26-119f2.tif
Fig. 3.
(a) Dose profiles in the amplitude case. (b) Position versus time curve in the amplitude case.
pmp-26-119f3.tif
Fig. 4.
(a) Dose profiles in the period case. (b) Position versus time curve in the period case.
pmp-26-119f4.tif
Fig. 5.
(a) Dose profiles in the baseline case. (b) Position versus time curve in the baseline case.
pmp-26-119f5.tif
Fig. 6.
Dose profiles in the phase case (end inspiration, mid expiration, end expiration, and mid inspiration). The tumor motion was divided in (a) regular tumor motion case and (b) irregular tumor motion case.
pmp-26-119f6.tif
Fig. 7.
(a) Dose profiles in the patient case: no tumor motion case, regular tumor motion case, and real tumor motion case were compared. (b) Position versus time curve in the patient case.
pmp-26-119f7.tif
Table 1.
Experimental conditions used to acquire the static does.
Part Parameters Experimental condition
Gantry Source to axis distance (SAD) 85 cm
  Beam delivery mode Static mode
  Beam energy 6 MV
  Treatment field width 2.5 cm
  Angle of exposed beam 90o
Phantom Type of phantom Cheese phantom
  Centered point Isocenter
Film Type of film Gafchromic EBT2
  Expiration date June 2015
  Lot number 06241301
Table 2.
Helical tomotherapy param information used in the simulation.
Parameters Experimental condition
Total number of rotation Static mode
Gantry rotation period (Tg) 16 s
Projection number per rotation Treatment field width 51 times 2.5 cm
Pitch factor 0.287
PTV dimension (Cylindrical shape) Isocenter
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