International Journal of Radiation Oncology*Biology*Physics
Physics ContributionImpact of Real-Time Image Gating on Spot Scanning Proton Therapy for Lung Tumors: A Simulation Study
Introduction
Spot scanning proton therapy is increasingly used for treating tumors. It offers more flexible and conformal dose distributions than passive scattering methods, and it minimizes the extra dose to normal tissue. However, if a tumor is moving, the interplay effect between the scanning beam and the motion of the target can result in hot and cold spots in the target 1, 2, 3. Possible solutions to suppress the interplay effect proposed to date include rescanning (1), beam gating 4, 5, beam tracking (6), phase-controlled fast scanning (7), and breath-holding (8).
Previous studies have investigated gated scanning beams with regular motion 3, 9 or external marker motion (10). Mori et al (11) evaluated gating based on fluoroscopic images under irregular motion, but their study did not assess dose distributions. In a previous study (12), we used 3-dimensional internal fiducial marker trajectories, including irregular motion, to investigate our gating system known as real-time-image gated proton beam therapy (RGPT). In RGPT, 2 fluoroscopic images of an internal fiducial marker near the tumor are monitored in real time, and the proton beam is gated when the marker enters a preassigned gating window (GW). We demonstrated that RGPT yielded good dose distributions with a ±2-mm GW without greatly extending the treatment time 4, 12. However, those studies were limited to relatively homogeneous regions, such as water phantoms and the liver, and assumed rigid motion. For the lung, where tissue heterogeneity and anatomic deformation during respiration have to be taken into account, the effectiveness of RGPT has yet to be studied. Although Grassberger et al (9) investigated the dosimetric impact of gating to mitigate interplay effects during lung treatments, they assumed regular motion extracted from 4-dimensional computed tomography (4DCT) images, and the gating was based on the duty cycle, not the tumor position.
In the present study, therefore, we used a simulation process to investigate the 4D dose distributions and treatment times of RGPT lung treatments, taking into account the anatomic deformation and irregular motion of the lung. To assess the effectiveness of RGPT, the results were compared with those from free-breathing proton therapy (FBPT). In addition, we evaluated the most suitable size for the GW, which previously has only been investigated for 1-dimensional motion (3), taking account of the trade-off between dose distribution and treatment time.
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Patient data
The study included 35 patients with peripheral stage I non-small cell lung cancer (NSCLC) treated with x-ray real-time tumor-tracking radiation therapy (RTRT) 13, 14 and acquired 4DCT from 2012 to 2014. The accuracy of 4DCT is limited by artifacts related to respiratory motion and reconstruction processing 15, 16, 17; Yamamoto et al (15) showed that 1 or more artifacts were observed in approximately 90% of cases. For good 4DCT image quality, the marker should move with the phase order; we
Dose distribution
Figure 3 shows an example of the dose distributions of the 4D simulations for the 7 treatment plans. For reference, the dose distributions of the static plans are shown together. With a small GW (±1 or ±2 mm), the isodose line of the prescribed dose sufficiently covered the CTV, whereas with larger GWs (±5 or ±8 mm), hot and cold spots were created. The isodose lines broadened widely with increasing GW, as they did in the original plan. Greater dose heterogeneity was observed with FBPT.
The same
Discussion
It is important to reduce the dose to the normal lung as far as possible because the risk of symptomatic radiation-induced pneumonitis is related to the irradiation and gradually increases with the irradiated volume (24). Compared with photon therapy, proton beam therapy can spare more normal lung tissue from irradiation for stage I and III NSCLCs and so may reduce the risk of lung toxicity for these diseases (25).
Real-time-image gated proton beam therapy was designed to minimize the
Conclusions
For the selected patients in this simulation study, RGPT using a ±2-mm GW size provided good dose preservation within an acceptable treatment time while reducing the dose to normal lung tissue.
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2020, Technical Innovations and Patient Support in Radiation OncologyCitation Excerpt :Additionally, since the maximum MU delivered to each spot is limited, more rescanning was used [37]. Kanehira et al. investigated various windows of gating and found that decreasing the gating window improves the D99%, HI, and lung V20Gy [15]. A 2 mm gating window was chosen as it had adequate coverage while having an acceptable treatment time of approximately 3.5 minutes.
This research was supported by the Translational Research Network Program, JSPS KAKENHI grant no. 15K09984, 24861049, and the Global Institution for Collaborative Research and Education (GI-CoRE), Hokkaido University, founded by the Ministry of Education, Culture, Sports, Science and Technology, Japan.
Conflict of interest: H.S. has received grants from Hitachi, Ltd, Mitsubishi Heavy Industries, Ltd, and Shimadzu Corporation.