Ken Yusa

Treatment planning for the layer-stacking irradiation system for three-dimensional conformal heavy-ion radiotherapy

We have upgraded a heavy-ion radiotherapy treatment-planning system to adapt for the layer-stacking irradiation method, which is to conform a variable spread-out Bragg peak to a target volume by means of dynamic control of the conventional beam-modifying devices. The biophysical model, the beam-setup logic, and the dose-calculation algorithm implemented for the layer-stacking method are described and the expected clinical usability is discussed. The layer-stacking method was integrated in perfect accordance with the ongoing conventional treatments so that the established protocols, which are the clinically optimized dose fractionation schemes, will still be valid. On the other hand, a simulation study indicated a substantial improvement of dose distribution with the layer-stacking method though the significance may depend on the size, shape, and location of the tumor. The completed treatment system will provide an option for improved conformal radiotherapy without interfering with the conventional method and we expect a gradual expansion of the clinical cases applicable to the layer-stacking method.

Carbon Ion Radiotherapy at the Gunma University Heavy Ion Medical Center: New Facility Set-up

Carbon ion radiotherapy (C-ion RT) offers superior dose conformity in the treatment of deep-seated tumors compared with conventional X-ray therapy. In addition, carbon ion beams have a higher relative biological effectiveness compared with protons or X-ray beams. C-ion RT for the first patient at Gunma University Heavy Ion Medical Center (GHMC) was initiated in March of 2010. The major specifications of the facility were determined based on the experience of clinical treatments at the National Institute of Radiological Sciences (NIRS), with the size and cost being reduced to one-third of those at NIRS. The currently indicated sites of cancer treatment at GHMC are lung, prostate, head and neck, liver, rectum, bone and soft tissue. Between March 2010 and July 2011, a total of 177 patients were treated at GHMC although a total of 100 patients was the design specification during the period in considering the optimal machine performance. In the present article, we introduce the facility set-up of GHMC, including the facility design, treatment planning systems, and clinical preparations.

Design of ridge filters for spread-out Bragg peaks with Monte Carlo simulation in carbon ion therapy

Spread-out Bragg peaks made by ridge filters or wheel range modulators are used in charged particle therapy with passive methods to achieve uniform biological responses in irradiated tumors. Following the biological responses needed to design the ridge filters, which were developed at the National Institute of Radiological Sciences in Japan, new ridge filters were designed using recent developments in heavy-ion reactions and dosimetry. The Monte Carlo code of Geant4 was used to calculate the qualities of carbon ion beams in a water phantom. The results obtained from the simulation were corrected so that they agreed with the measurements of depth dose distributions. The calculations of biological responses to fragments other than carbon ions were assumed to be for helium ions. The measured dose distributions with the designed ridge filters were compared to the calculated distributions. A beam modifying system using this adaptable method was successively applied to carbon ion therapy at Gunma University.

The compact electron cyclotron resonance ion source KeiGM for the carbon ion therapy facility at Gunma University

A high-energy carbon-ion radiotherapy facility is under construction at Gunma University Heavy Ion Medical Centre (GHMC). Its design was based on a study of the heavy ion radiotherapy at the National Institute of Radiological Sciences (NIRS) in order to reduce the size and construction cost of the facility. A compact electron cyclotron resonance ion source (ECRIS) for Gunma University, called KeiGM, was installed in 2008. It is almost a copy of the prototype ECRIS Kei2 which was developed by NIRS; meanwhile this prototype produced over 1 e mA of C(4+) using C(2)H(2) gas (660 W and 40 kV). The beam intensity of C(4+) was 600 e microA with CH(4) gas (250 W and 30 kV). The beam intensity satisfies the required value of 300 e microA.

Three-dimensional velocity mapping of lung motion using vessel bifurcation pattern matching.

We present a new quantification technique for three-dimensional (3D) lung motion by means of tracking the anatomical features inside the lung using a set of sequential 3D-CT images (a 4D-CT image). The method is based on the conservation of topology, such as connections and junctions of vessels, during the motion. Lung CT images are used to do lung volume modeling, lung vessel extracting and thinning, and coordinates of vessel bifurcations are derived as feature points. Such feature points are tracked in a series of 3D-CT images, i.e., the points are individually tracked between two successive 3D-CT images, in which the lung is deformed. Consequently, 3D displacement vectors are obtained. The feature point tracking is carried out using point pattern matching with a probabilistic relaxation method. We examined this technique using a lung 3D-CT image and artificially deformed one, and separately scanned CT images for a rigid bifurcation phantom. The studies estimated that the error of the vectors is within approximately 1 voxel, i.e., 1 mm or less. Therefore, the accuracy is expected to be high enough for radiation therapy. This technique enables us to quantify realistic 3D organ motion without any fiducial markers. It can be applied to the quantification of tumor (target volume) deformation by gridding interpolation into all voxels. We expect it to be useful for dose estimation in mobile organs and for 4D treatment planning in radiation therapy.

Evaluation of a pencil beam algorithm for therapeutic carbon ion beam in presence of bolus.

Hot- and cold-dose spots at a shallow depth in a target are formed by carbon ions passing through the bolus with sharp gradients. These spots are caused by sidescatter disequilibrium due to various multiple scattering effects in the different bolus thicknesses. When the dose calculation method by the broad beam algorithm (BBA) is used for treatment planning, these spots cannot be predicted, because the BBA neglects the multiple scattering effects in materials (rms error of 3.9%). On the other hand, since the dose calculation method by the pencil beam algorithm (PBA) takes into account the scattering effects, the results calculated by the PBA agreed better than the BBA with the measured hot- and cold-dose spots, having a rms error of 1.9%. Thus, dose calculation by the PBA improves the accuracy of dose prediction at the shallow depth. However, since dose distributions at deeper positions are affected by many light fragment particles generated by fragment reactions, the results calculated by the PBA disagree with the experimental ones. It is necessary that even the PBA accurately models behavior of fragment particles.

Reconstruction of biologically equivalent dose distribution on CT-image from measured physical dose distribution of therapeutic beam in water phantom.

From the standpoint of quality assurance in radiotherapy, it is very important to compare the dose distributions realized by an irradiation system with the distribution planned by a treatment planning system. To compare the two dose distributions, it is necessary to convert the dose distributions on CT images to distributions in a water phantom or convert the measured dose distributions to distributions on CT images. Especially in heavy-ion radiotherapy, it is reasonable to show the biologically equivalent dose distribution on the CT images. We developed tools for the visualization and comparison of these distributions in order to check the therapeutic beam for each patient at the National Institute of Radiological Sciences (NIRS). To estimate the distribution in a patient, the dose is derived from the measurement by mapping it on a CT-image. Fitting the depth-dose curve to the calculated SOBP curve also gives biologically equivalent dose distributions in the case of a carbon beam. Once calculated, dose distribution information can be easily handled to make a comparison with the planned distribution and display it on a grey-scale CT-image. Quantitative comparisons of dose distributions can be made with anatomical information, which also gives a verification of the irradiation system in a very straightforward way.

Experimental evaluation of analytical penumbra calculation model for wobbled beams.

The goal of radiotherapy is not only to apply a high radiation dose to a tumor, but also to avoid side effects in the surrounding healthy tissue. Therefore, it is important for carbon-ion treatment planning to calculate accurately the effects of the lateral penumbra. In this article, for wobbled beams under various irradiation conditions, we focus on the lateral penumbras at several aperture positions of one side leaf of the multileaf collimator. The penumbras predicted by an analytical penumbra calculation model were compared with the measured results. The results calculated by the model for various conditions agreed well with the experimental ones. In conclusion, we found that the analytical penumbra calculation model could predict accurately the measured results for wobbled beams and it was useful for carbon-ion treatment planning to apply the model.

Changes in Rectal Dose Due to Alterations in Beam Angles for Setup Uncertainty and Range Uncertainty in Carbon-Ion Radiotherapy for Prostate Cancer

Background and Purpose Carbon-ion radiotherapy of prostate cancer is challenging in patients with metal implants in one or both hips. Problems can be circumvented by using fields at oblique angles. To evaluate the influence of setup and range uncertainties accompanying oblique field angles, we calculated rectal dose changes with oblique orthogonal field angles, using a device with fixed fields at 0° and 90° and a rotating patient couch. Material and Methods Dose distributions were calculated at the standard angles of 0° and 90°, and then at 30° and 60°. Setup uncertainty was simulated with changes from −2 mm to +2 mm for fields in the anterior-posterior, left-right, and cranial-caudal directions, and dose changes from range uncertainty were calculated with a 1 mm water-equivalent path length added to the target isocenter in each angle. The dose distributions regarding the passive irradiation method were calculated using the K2 dose algorithm. Results The rectal volumes with 0°, 30°, 60°, and 90° field angles at 95% of the prescription dose were 3.4±0.9 cm3, 2.8±1.1 cm3, 2.2±0.8 cm3, and 3.8±1.1 cm3, respectively. As compared with 90° fields, 30° and 60° fields had significant advantages regarding setup uncertainty and significant disadvantages regarding range uncertainty, but were not significantly different from the 90° field setup and range uncertainties. Conclusions The setup and range uncertainties calculated at 30° and 60° field angles were not associated with a significant change in rectal dose relative to those at 90°.

Evaluation of an empirical monitor output estimation in carbon ion radiotherapy.

PURPOSE A conventional broad beam method is applied to carbon ion radiotherapy at Gunma University Heavy Ion Medical Center. According to this method, accelerated carbon ions are scattered by various beam line devices to form 3D dose distribution. The physical dose per monitor unit (d/MU) at the isocenter, therefore, depends on beam line parameters and should be calibrated by a measurement in clinical practice. This study aims to develop a calculation algorithm for d/MU using beam line parameters. METHODS Two major factors, the range shifter dependence and the field aperture effect, are measured via PinPoint chamber in a water phantom, which is an identical setup as that used for monitor calibration in clinical practice. An empirical monitor calibration method based on measurement results is developed using a simple algorithm utilizing a linear function and a double Gaussian pencil beam distribution to express the range shifter dependence and the field aperture effect. RESULTS The range shifter dependence and the field aperture effect are evaluated to have errors of 0.2% and 0.5%, respectively. The proposed method has successfully estimated d/MU with a difference of less than 1% with respect to the measurement results. Taking the measurement deviation of about 0.3% into account, this result is sufficiently accurate for clinical applications. CONCLUSIONS An empirical procedure to estimate d/MU with a simple algorithm is established in this research. This procedure allows them to use the beam time for more treatments, quality assurances, and other research endeavors.