International Journal of Radiation Oncology*Biology*Physics
Physics ContributionIntrafractional Baseline Shift or Drift of Lung Tumor Motion During Gated Radiation Therapy With a Real-Time Tumor-Tracking System
Introduction
Radiation therapy for lung tumors requires careful attention to intrafractional respiratory motion 1, 2, 3, 4, 5, 6. Tumors with large respiratory motion can be covered with large margins for the intrafractional tumor motion, but this may increase the volume of normal lung tissue that is irradiated and increase the risk of radiation pneumonitis 7, 8, 9. Various procedures have been developed for motion management in radiation therapy, and clinical guidelines are available for how to use them (10). Respiratory gating using surface markers, tracking irradiation using a prediction model based on the predetermined relationship between the internal motion and the surface marker, or respiratory holding using the surface motion are used to solve this problem 5, 11, 12, 13, 14, 15, 16. However, these techniques are often vulnerable to the intrafractional changes in the relationship between internal tumor motion and the external surface 17, 18, 19.
The average tumor position in both the expiratory phase and inspiratory phase changes during beam delivery, and the variation in average tumor position differs in the various directions. Baselines of respiratory motion of internal tumor locations have been known to change in the cranio-caudal (CC) and antero-posterior (AP) directions during the delivery of radiation (20). Seppenwoolde et al have introduced the term baseline shift, but the term used in recent studies has been baseline drift 6, 21, 22, 23, 24. Drift is a term expressing continuous gradual changes, and shift expresses noncontinuous changes. In our experience, the baseline of the respiratory motion may change either continuously or noncontinuously, and here we use the term baseline shift/drift to include both continuous or noncontinuous changes. Without paying attention to the baseline shift/drift of the tumor motion during the delivery of irradiation, there may be incorrect targeting in stereotactic body radiation therapy for the lungs and liver (5). Inhomogeneous dose distributions due to the baseline shift/drift may happen in intensity modulated radiation therapy, spot-scanning proton beam therapy, and in intensity modulated proton beam therapy 4, 25, 26.
We have developed and used a real-time tumor-tracking radiation therapy (RTRT) system since 1999 and used it in stereotactic body radiation therapy (SBRT) for peripheral lung tumors (27). In our treatment protocol, the position of the patient was adjusted remotely by pausing the irradiation and making adjustments to the patient table. Retrospectively, it is possible to determine the extent and frequency of the intrafactional baseline shift/drift during the delivery of the radiation. In this study we have investigated the frequency of baseline shift/drift during the delivery of SBRT using the RTRT system.
Section snippets
Methods and Materials
The SBRT technique using the RTRT system has been described elsewhere 27, 28, 29, 30. Patients with peripheral stage I non-small cell lung carcinomas who were mainly treated with SBRT using the RTRT system from 2008 March to October 2010 were the candidates for this study. During this period, 83 patients were registered, and of these, 15 were excluded from the study. The exclusion criteria were (1) patients who could not lie steadily on a couch for a long time and were clinically evaluated not
Results
Treatment time, defined as the accumulation of beam delivery time for 1 fraction, ranged from 2.6 to 82.6 minutes, with the median duration of 23.9 minutes. The wide range in the treatment time arises as follows: (1) 1 patient was treated with 40 Gy in 20 fractions; and (2) the treatment time is not affected when the motion of the fiducial marker is within the GW of ±2.0 mm, but it is lengthened when the marker motion is irregular and/or there are a large number of intrafractional baseline
Discussion
The intrafractional baseline shift/drift of lung tumor motion was first reported by Seppenwoolde et al in 2002 (20). They found that there were interfractional shifts that may be due to residual setup errors, and intrafractional gradual shift/drift, which may be due to patient motion or internal tumor motion (change in breathing intensity). Guckenberger et al (23) have reported that, by repeated helical 4D-CT 4 times at 10-minute intervals, there was a drift of the mean tumor position greater
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This research was supported by Grant-in-Aid for Scientific Research no. 24791261 and the Global Institution for Collaborative Research and Education, Hokkaido University, founded by the Ministry of Education, Culture, Sports, Science and Technology, Japan.
Conflict of interest: H.S. received grants from Hitachi, Ltd, Mitsubishi Heavy Industries, Ltd, and Shimadzu Corporation during the conduct of the study.