S Mori

Effects of a difference in respiratory cycle between treatment planning and irradiation for phase-controlled rescanning and carbon pencil beam scanning

OBJECTIVE To evaluate the impact of variation in respiratory cycle between treatment planning and irradiation for pencil beam scanning and phase-controlled rescanning (PCR) on the resulting dose distribution, we conducted a simulation study based on four-dimensional CT (4DCT) data for lung cancer patients. METHODS 4DCT data were acquired for seven patients with lung tumours. Treatment planning was designed to ensure the delivery of 95% of the prescribed dose to the clinical target volume in respective phases of the 4DCT by taking account of intrafractional beam range variations. Carbon ion pencil beam scanning dose distributions were calculated for various respiratory cycles that differed from the reference respiration (=4.4 s) but which stayed regular during irradiation. The number of rescannings was changed to 1, 4 or 8 times. PCR was correlated with the gating window in treatment planning to calculate the beam weighting map. RESULTS 8×PCR improved dose conformation to the target for all irradiation respiratory cycles. Minimum dose (Dmin) and lowest dose encompassing 95% of the target (D95) values with 4×PCR were decreased from 94.1% and 98.1% to 88.4% and 93.5% with an altered irradiation respiratory cycle of 2.4 s. However, these values were improved with 8×PCR to over 94.9% for Dmin and 98.6% for D95 for respective irradiation respiratory cycles. CONCLUSION Pencil beam scanning treatment with eight or more PCRs consistently improved dose conformation for moving lung targets even when different respiratory cycles were used for treatment planning and irradiation. ADVANCES IN KNOWLEDGE Scanning treatment with eight or more rescannings consistently improved dose homogeneity to a moving target even though respiratory cycles varied during treatment.

Impact of treatment planning with deformable image registration on dose distribution for carbon-ion beam lung treatment using a fixed irradiation port and rotating couch

OBJECTIVE When using a fixed irradiation port, treatment couch rotation is necessary to increase beam angle selection. We evaluated dose variations associated with positional morphological changes to organs. METHODS We retrospectively chose the data sets of ten patients with lung cancer who underwent respiratory-gated CT at three different couch rotation angles (0°, 20° and -20°). The respective CT data sets are referred to as CT0, CT20 and CT-20. Three treatment plans were generated as follows: in Plan 1, all compensating bolus designs and dose distributions were calculated using CT0. To evaluate the rotation effect without considering morphology changes, in Plan 2, the compensating boli designed using CT0 were applied to the CT±20 images. Plan 3 involved compensating boli designed using the CT±20 images. The accumulated dose distributions were calculated using deformable image registration (DIR). RESULTS A sufficient prescribed dose was calculated for the planning target volume (PTV) in Plan 1 [minimum dose received by a volume ≥95% (D95) > 95.8%]. By contrast, Plan 2 showed degraded dose conformation to the PTV (D95 > 90%) owing to mismatch of the bolus design to the morphological positional changes in the respective CT. The dose assessment results of Plan 3 were very close to those of Plan 1. CONCLUSION Dose distribution is significantly affected by whether or not positional organ morphology changes are factored into dose planning. ADVANCES IN KNOWLEDGE In treatment planning using multiple CT scans with different couch positions, it is mandatory to calculate the accumulated dose using DIR.

Effect of secondary particles on image quality of dynamic flat panels in carbon ion scanning beam treatment

OBJECTIVE Real-time markerless tumour tracking using radiographic fluoroscopic imaging is one of the better solutions to improving respiratory-gated radiotherapy. However, particle beams cause secondary particles from patients, which could affect radiographs. Here, we evaluated the quality of radiographs during carbon ion pencil beam scanning (CPBS) irradiation for respiratory gating. METHODS A water phantom and chest phantom were used. The phantoms were irradiated with CPBS at 290 MeV n(-1) from orthogonal directions. Dose rates were 3.4 × 10(8), 1.14 × 10(8) and 3.79 × 10(7) particles per second. A dynamic flat panel detector (DFPD) was installed on the upstream (DFPD1) or downstream (DFPD2) side of the vertical irradiation port. DFPD images were acquired during CPBS at 15.00, 7.50 and 3.75 frames per second (fps). Charge on the DFPD was cleaned using fast readout technique every 30 fps. DFPD images were acquired during CPBS with radiographic exposure, and results with and without fast readout technique were compared. RESULTS Secondary particles were visualized as spots or streak-like shapes. Capture of secondary particles from the horizontal beam direction was lower with fast readout technique than without it. With regard to beam irradiation direction dependency, CPBS from the horizontal direction resulted in a greater magnitude of secondary particles reaching DFPD2 than reaching DFPD1. When CPBS was delivered from the vertical direction, however, the magnitude of secondary particles on both DFPDs was very similar. CONCLUSION Fast readout technique minimized the effect of secondary particles on DFPD images during CPBS. ADVANCES IN KNOWLEDGE This technique may be useful for markerless tumour tracking for respiratory gating.

Changes in chest wall thickness during four-dimensional CT in particle lung treatment planning

Four-dimensional (4D) CT images for charged particle lung therapy were acquired using a 256 multislice CT without couch movement. The thickness of the posterior right chest wall changed with respiration with a water equivalent path length (WEL) of more than 5 mm over the mid-exhalation phase but the thickness of the left chest wall did not vary.

SU-FF-T-50: Reduction of Computed Tomography Metal Artifacts Due to I-125 Seeds for Post Implant Analysis in Prostate Permanent Brachytherapy

Purpose: Postimplant analysis in I‐125 permanent brachytherapy for prostate cancer plays important role in improving the techniques of implant. Although American Brachytherapy Society recommends CT based postimplant analysis, the identification of prostate bundle is quite difficult. In addition, metal artifacts from the I‐125 seeds implanted make it more difficult to identify the prostate. Thus accuracy of the dosimetric parameters associated with volume of prostate such as D90 and V100 may be unclear. The purpose of this study is to mitigate CT metal artifacts due to I‐125 seeds to provide more accurate postimplant analysis. Method and Materials: The prostate phantom that was implanted 3 to 10 seeds per a plane was scanned using 16‐detector raw CT. The sinogram was modified by our algorithm that is similar to projection‐interpolation method to CTimages containing artifacts from I‐125 seeds. The regions of projection data existing I‐125 seeds were identified by observing differences of X‐ray intensity between the phantom and the seeds. Then the regions were interpolated to remove the metal artifacts. The new images were reconstructed with the corrected sinogram. We compared these with the CTimages that are corrected by commercially available metal artifact reduction method. Results: The metal artifacts caused by a small number of I‐125 seeds were completely eliminated by our correcting method. On the other hand, the magnitude of the artifact with the commercially available method was insufficient. With regard to many seeds in the same plane, the metal artifact was mitigated by our method although the contrast of images was degraded. Conclusion: Our method would mitigate metal artifacts caused by I‐125 seeds and be helpful to identify prostate. Although some problems have still remained to improve, our approaches would be adapted to clinical field.

SU‐FF‐T‐576: Effects of Respiration On Proton Dose Distributions and DVHs in Pancreatic Cancer as Assessed by 4D Treatment Planning

Purpose: To study the effects of respiratory motion on dose distributions and DVHs in patients with pancreatic cancer treated by proton beam therapy. Methods and Materials: A 4D CT scan was acquired on 10 patients studied; target and 11 organs‐at‐risk structures were contoured on the T30 phase scan. Contours were propagated to other respiratory phases using B‐spline deformable registration. A compensating bolus was designed to cover the CTV for both gated and ungated treatment. Field arrangements included laterals and a posterior field. Dose distributions were calculated for each respiratory phase and then mapped to a single anatomical reference phase. A time weighted DVH was calculated for the VOIs. Results: Impact of motion on dosimetry was evaluated by two approaches. Data browsing of the 4D dose distribution movies showed robustness to motion in target coverage. During breathing, ripples in the high dose gradient are small (∼few mm). In the organs eye view reference frame, isodose lines move and the principal observation is that lower isodose lines expand into the liver during inhalation by as much as 1–2cm, resulting in higher DVHs at the 25% dose level. The dynamic range of volume irradiated during respiration can be 15% for >10mm COM motion of the liver. The time weighted DVH of the liver is ∼6% greater than the T50 liver DVH. Conclusion: 4D dose distributions show that proton beam coverage of the target can be robust during light breathing. DVHs of the liver are increased typically by ∼5% at lower isodose values when ungated treatment is delivered, and can be improved through gating. 4D treatment planning can be completed in ∼2 hours, making it feasible to analyze the effects of motion routinely.

SU‐FF‐J‐127: Four‐Dimensional Carbon Ion Dose Assessment in Respiratory‐Gated Lung Therapy: Simulation Study in Respiratory Pattern Variation Cases

Purpose: To estimate the effects of the irregular respiratory pattern during the carbon‐beam treatment of lungcancer in the four‐dimensional treatment planning, we simulated the irregular respiratory motion by the NCAT phantom and calculated the accumulated dose for respiratory gated and ungated treatment. Method and Materials: 4D NCAT phantom was used to simulate the irregular respiratory tumor motion during several cycles of respiration. The treatment planning was performed based on the first cycle of the respiration. The cycle was subdivided into 10 phases and the prescribed dose was distributed so that all points in the planning target volume (PTV) receives more than 90 percent of the total prescribed dose. The total of 52.8 GyE was delivered from four beam ports with angles selected to be 340, 70, 20, and 110 degrees. The plan was scheduled both for ungated and gated treatment. For the respiratory‐gated treatment, the gating window was set in T40%–T60% phases. The dose calculation was based on the pencil beam algorithm. The accumulated dose was evaluated and compared between gated‐ and ungated‐ results in terms of V20, D95 and dose‐volume histogram (DVH). Results: For the ungated case the dose conformation was deteriorated compared to the ideal case where the respiratory pattern was completely the same as in the treatment planning, while, for the gated treatment, the improved dose conformation was observed in terms of both D95 and V20. Conclusion: Though the 4D planning is effective to estimate internal margin, the irregular respiratory pattern may deteriorate the conformation of the accumulated dose during the whole treatment. The gating treatment, if appropriately synchronized, is effective both for the dose conformity and the reduction of the extra dose to the normal tissues or organs at risks.

SU‐EE‐A1‐03: Range Fluctuation Analysis Due To Respiratory Motion in Charged Particle Lung Therapy

Purpose: Water equivalent pathlength (WEL) variations due to respiration can change the penetration of a charged particlebeam, and result in beam overshoot to critical organs or undershoot to the tumor. We have analyzed range fluctuations by analyzing four‐dimensional CT (4DCT) data and quantitatively assessing potential beam overshoot. Methods and Material: 4DCT images were acquired with a multi‐slice CT scanner. The maximum intensity volume (MIV) was calculated by temporal maximum intensity projection (MIP) processing. Two targets were designed for charged particlebeam therapy. The first target volume calculates the MIV over the entire respiratory cycle (ITV‐Rx), while the second target volume is the MIV corresponding to gated radiotherapy (over a 30% phase window around exhale). These targets were used to calculate boli that were then applied to the 4DCT data to estimate beam penetration. Analysis metrics include range fluctuation, overshoot volume, both as a function of gantry angle. We compared WEL fluctuations observed in treating the ITV Vs gated treatment in 11 lung patients. WEL fluctuation and beam overshoot into normal lung are displayed over a beams‐eye view display. Results: WEL fluctuations were less than 29.8 mm‐WEL and 12.0 mm‐WEL for ITV‐Rx and gated‐Rx, respectively for all patients. Gated‐Rx reduced beam overshoot volume by approximately a factor of four compared to ITV treatment. Such range fluctuations can affect the efficacy of treatment, and result in excessive dose to a distal critical organ.Conclusions: Time varying WEL range fluctuationanalysis provides information useful to determine appropriate patient specific treatment parameters in the charged particleradiotherapy. This analysis can also be useful for optimizing planning and delivery.

SU‐FF‐J‐47: Determination of Internal Target Volume Reconstruction Algorithm Beyond the Time Dimension Using Second Model of a 256‐Slice CT

To observing trajectory of moving tumor under free breathing, we compared two image‐processing methods and two reconstruction algorithms based on the FDK and adapted to the 256‐slice CT. These algorithms were namely 4D image average (4DIA) and 4D image maximum intensity projection (4DIM), 4D projection data average (4DPA) and 4D projection data maximum intensity projection (4DPM). The concept of 4DIA and 4DIM was generated on CTimage after backprojection process. 4DIA was averaged each pixel value on the volumetric CT data along the time axis, and 4DIM was selected maximum each pixel value along the time axis. With regard to the 4DPA and 4DPM, these essential concepts are to process projection data along the time axis, rather than reconstructedCTimages as 4DIA and 4DIM. Evaluations of these algorithms were done in the imagenoise, CT‐number accuracy, and target moving distance with various reconstruction time conditions using lungcancer patients and compared these results with those with volumetric cine images. As the results, it is difficult to observe the edge of the tumor in 4DIA and 4DPA images due to decreasing CT number from the original tumorCT number. While 4DIM images emphasized pulmonary vessels as increasing the processing time ranges and it makes difficult to observe the accurate tumor edge. From these results, 4DPM provides the accurate tumor movement and accurate CT‐number independent of the reconstruction conditions.

SU‐FF‐J‐23: Improved Temporal Resolution by Respiratory Gated Segment Reconstruction: Towards Four‐Dimensional (4D) Radiation Therapy for Heavy Ion Beams Using the 256‐Detector‐Row CT‐Scanner

Purpose: To perform more precise treatment planning for respiratory-moving tumors, we developed a respiratory gated segment reconstruction method (RS) based on the Feldkamp-Davis-Kress algorithm (FDK) which can achieve high temporal resolution and high signal-to-noise ratio. We compared full scan (FS-FDK) and RS-FDK with regard to the image quality and the obtained dose distributions for heavy ion treatment planning. Method and Materials: Data acquisition for RS-FDK relies on the assistance of the respiratory sensing system in a cine scan mode with a 256-detector row CT. We compared the image quality for RS-FDK to that for FS-FDK in phantom and animal studies. To evaluate the accuracy of the actual irradiation for the moving tumors, we compared the dose distributions of both algorithms in heavy ion treatment planning with the beam parameters of FS-FDK. Results: RS-FDK provided images without motion artifacts and visualized the edges of the liver and pulmonary vessels more clearly than FS-FDK. With regard to the iso-dose distributions, FS-FDK covered the target volume. RS-FDK, however, had an insufficient dose to the target and a considerable dose was deposited to the normal tissue around the target. Conclusion: RS-FDK has good capabilities for providing useful information to give accurately prescribed dose-to-target volume. It is possible to achieve more precise radiotherapy including 4D radiation therapy using the RS-FDK. Now we investigated 4D radiation therapy using the moving phantom, however we can summarized RS-FDK using lung cancer patients and 4D radiation therapy planning using carbon ion beam and its movie files by AAPM annual meeting.