Presentation Dosimetric impact of 4DCT artifact in carbon-ion scanning beam treatment : worst case analysis in lung and liver treatments

森, 慎一郎

1. IntroductionMost treatment centers used still less than 64MDCT, typically 40 mm or less scan region was acquired. Multiple CT scans in cine mode at respective couch position were necessary to obtain scan region enough to treatment planning. Therefore, the resorting process in 4DCT is based on the respiratory phase, however, it is well-known that respiratory pattern is not strictly regular but varies in amplitude and period in respective respiratory cycles. Tumor position could not be always same even though it is same respiratory phase. This inconsistency between respiratory phase and tumor position could cause geometrical error (4DCT artifact). These artifacts hamper quantitative analysis and it is questionable whether 4DCT images with 4DCT artifact provide accurate dose distribution in actual situation. Here, we evaluated the impact of the 4DCT artifacts on carbon-ion pencil beam scanning (C-PBS) dose distributions for lung and liver treatment.2. Materials and Methods A total of 20 lung and liver cancer patients were randomly selected from patients at our institution and gave informed consent before participation. 4DCT image sets were acquired by a 320-slice CT. A single respiratory cycle in the 4DCT data set was subdivided into 10 phases (T00: peak inhalation, T50: around peak exhalation).To simulate 4DCT imagery acquired by cine MDCT, we sorted 4DCT images at respective couch positions by selecting 4DCT images at other respiratory phases. The couch moving step was 20 mm. The external respiratory monitor does not obtain actual tumor position, but rather abdominal surface motion. Moreover, the 4DCT sets were subdivided based on respiratory phase, not amplitude. However, 10-phase 4DCT data sets were available, and respiratory amplitude variation could not be made to strictly corresponded to respiratory phase information. We approximated these assignments for the present study. The 4DCT respiratory phase at two sigma statistical variation position from peak exhalation was close to T30 and T70. Adopting a worst-case scenario, we randomly selected 4DCT images at plus/minus 20% respiratory phases from the reference phase. To satisfy the simulated 4DCT pattern statistically, we calculated 50 simulated 4DCT patterns for each patient.Reference dose distribution was calculated using the original CT data, which is a 3DCT acquired for the treatment planning. CTV was defined by adding a 10-mm margin to the GTV. PTV was kept equal to the CTV with a 0-mm setup margin and 0-mm internal margin. Beam weight maps for the PTV were optimized using the relative biological effectiveness (RBE)-weighted absorbed dose. The total prescribed dose of 60 Gy(RBE) was delivered to the PTV via four different beam fields from the ipsilateral rather than contralateral side of the tumor for lung cases and two different beam fields for liver cases. Hybrid C-PBS was applied and the iso-energy layer was changed using a range shifter and 11 synchrotron energies. Spot spacing was 2.0 mm laterally and 3.0 mm in the beam direction, and lateral scatter (80-20%) was approximately 5 mm.Treatment planning parameters were optimized using the simulated CT data. C-PBS dose distribution (simulated dose distribution) was calculated using these treatment planning parameters, but with the original CT replaced by the simulated CT data. We assumed that the original CT images represented the actual patient geometrical shape because they were 4DCT artifact-free. By doing this, actual C-PBS dose distribution was evaluated when we used 4DCT acquired by MDCT.3. Results and DiscussionLiver casesDose assessment metrics (D95, Dmax, Dmin, HI and V10) averaged over all liver cases are summarized in Table 1. For the reference dose, D95, Dmax, Dmin and HI mean values for CTV (mean ± SD) were 97.6 ± 0.6%, 101.4 ± 0.6%, 90.0 ± 1.4% and 0.9 ± 0.2% of the prescribed dose, respectively, and liver-V10 mean value was 15.5 ± 8.6%. With regard to the dose assessment metrics for the simulated dose, CTV-D95 mean value was 89.0 ± 14.0%, but showed large variation between 14.8% and 99.0%. CTV-Dmax value was almost the same as that for the reference dose, whereas CTV-Dmin, CTV-HI and liver-V10 values were significantly degraded (48.2 ± 0.6%, 5.0 ± 4.7%, 15.9 ± 8.8%, respectively).Lung caseDose assessment metrics (D95, Dmax, Dmin, HI and V20) averaged over all lung cases are summarized in Table 1. CTV-Dmax value for the simulated dose (= 101.8 ± 0.6%) was almost the same as that for the reference dose (= 102.0 ± 0.5%). CTV-D95 maximum value for the simulated dose (= 97.4%) was also similar to that for the reference dose (= 97.5%). However, CTV-D95 mean value for the simulated dose (95.1 ± 1.5%) was significantly degraded compared with that for the reference dose (= 84.8 ± 12.8%).4. Conclusion4DCT artifact was shown to degrade dose conformation to the target if medical staff did not remake the 4DCT to improve image quality. Medical staff should pay particular attention to checking the quality of 4DCT images as a function of respiratory phase, because it is difficult to recognize 4DCT artifact on a single phase in some cases.

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