R Kohno

A CT calibration method based on the polybinary tissue model for radiotherapy treatment planning

A method to establish the relationship between CT number and effective density for therapeutic radiations is proposed. We approximated body tissues to mixtures of muscle, air, fat and bone. Consequently, the relationship can be calibrated only with a CT scan of their substitutes, for which we chose water, air, ethanol and potassium phosphate solution, respectively. With simple and specific corrections for non-equivalencies of the substitutes, a calibration accuracy of 1% will be achieved. We tested the calibration method with some biological materials to verify that the proposed method would offer the accuracy, simplicity and specificity required for a standard in radiotherapy treatment planning, in particular with heavy charged particles.

Clinical implementation of a GPU-based simplified Monte Carlo method for a treatment planning system of proton beam therapy

We implemented the simplified Monte Carlo (SMC) method on graphics processing unit (GPU) architecture under the computer-unified device architecture platform developed by NVIDIA. The GPU-based SMC was clinically applied for four patients with head and neck, lung, or prostate cancer. The results were compared to those obtained by a traditional CPU-based SMC with respect to the computation time and discrepancy. In the CPU- and GPU-based SMC calculations, the estimated mean statistical errors of the calculated doses in the planning target volume region were within 0.5% rms. The dose distributions calculated by the GPU- and CPU-based SMCs were similar, within statistical errors. The GPU-based SMC showed 12.30-16.00 times faster performance than the CPU-based SMC. The computation time per beam arrangement using the GPU-based SMC for the clinical cases ranged 9-67 s. The results demonstrate the successful application of the GPU-based SMC to a clinical proton treatment planning.

Multi-layer energy filter for realizing conformal irradiation in charged particle therapy

A new type of filter for charged particle radiotherapy is developed to reduce unwanted dose transfer to the normal tissues around a tumor. The new filter can make a static irradiation field where the width of the spread-out Bragg peak (SOBP) is two-dimensionally adjusted. That makes the field conformal to the tumor three-dimensionally. The filter is made of many layers produced by using stereolithography. The layer has a miniaturized structure that has geometrical similarity to the conventional ridge filter. Shapes of cone and pyramid are also usable for the unit-cell constructing the layer. The spread of the field in the depth direction is decided by the thickness of the filter, or by the number of layers. The experimental result of the irradiation using the ridge-type construction shows a good agreement with an estimate by the Monte Carlo calculation. By combining this technique with intensity modulation that has lateral position dependence, the conformal irradiation can be achieved by a simple procedure.

Response characteristics of an imaging plate to clinical proton beams

Abstract For an application to a dose-distribution measurement, the response of an imaging plate (IP) has been studied with proton beams which are routinely utilized for the radiation therapy. The upper limit of measurable proton dose by an IP system is primarily controlled by the readout range of the scanner that is used. Within this limit, reasonable linear response of an IP to proton dose to water is maintained. Fading curves are neither sensitive to a small change of room temperature (22–26°C) nor to a variation of proton dose (0.0108–0.132xa0Gy). Reproducibility of the PSL intensity is fairly good if both the fading characteristics and the lot-dependence of the sensitivity of each IP are taken into account carefully. Stopping power dependence of the IP response has been examined at different positions in a Bragg curve.

Dosimetry of pulsed clinical proton beams by a small ionization chamber

Response of a micro volume (0.01 ml) ionization chamber has been studied with pulsed proton beams which are used for clinical purposes and has been compared with those of some JARP ionization chambers (0.6 ml). All chambers used had been calibrated by standard 60Co beams at the Electrotechnical Laboratory (ETL) and exposure calibration factors, Nx, were obtained on advance. Two methods are used to compensate the general recombination which occurs during pulsed beam irradiations: theoretical correction by a Boags formulation and a modified two-voltage technique. An evaluation of absolute absorbed dose-to-water is performed on the basis of the protocol provided by ICRU report 59. The results imply that, to a first approximation, both chambers indicate the almost same result within 2% when unknown chamber-dependent parameters of the micro chamber are tentatively assumed to be identical to those of the JARP chamber for the calibration with 60Co beams. The about 1.5% discrepancy observed in the response of both chambers is not discussible due to presumably 1-2% uncertainty of the protocol of ICRU report 59 which does not include any chamber-dependent corrections for the perturbation effects in proton beams.

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.

Respiratory-gated segment reconstruction for radiation treatment planning using 256-slice CT-scanner during free breathing

The conventional respiratory-gated CT scan technique includes anatomic motion induced artifacts due to the low temporal resolution. They are a significant source of error in radiotherapy treatment planning for the thorax and upper abdomen. Temporal resolution and image quality are important factors to minimize planning target volume margin due to the respiratory motion. To achieve high temporal resolution and high signal-to-noise ratio, we developed a respiratory gated segment reconstruction algorithm and adapted it to Feldkamp-Davis-Kress algorithm (FDK) with a 256-detector row CT. The 256-detector row CT could scan approximately 100 mm in the cranio-caudal direction with 0.5 mm slice thickness in one rotation. Data acquisition for the RS-FDK relies on the assistance of the respiratory sensing system by a cine scan mode (table remains stationary). We evaluated RS-FDK in phantom study with the 256-detector row CT and compared it with full scan (FS-FDK) and HS-FDK results with regard to volume accuracy and image noise, and finally adapted the RS-FDK to an animal study. The RS-FDK gave a more accurate volume than the others and it had the same signal-to-noise ratio as the FS-FDK. In the animal study, the RS-FDK visualized the clearest edges of the liver and pulmonary vessels of all the algorithms. In conclusion, the RS-FDK algorithm has a capability of high temporal resolution and high signal-to-noise ratio. Therefore it will be useful when combined with new radiotherapy techniques including image guided radiation therapy (IGRT) and 4D radiation therapy.

Deterioration of imaging plate by proton irradiation and an evidence of its recovery

Abstract An imaging plate (IP) was excessively irradiated by 250 MeV proton beam up to 4.3×10 13 protons/cm 2 in order to examine the resultant radiation damage. As a function of proton fluence, variations in photo-stimulated luminescence (PSL) signals were evaluated by comparing with the region without previous extensive proton irradiation on the same IP. Notable deterioration of PSL signal was found for the fluence more than 5×10 12 protons/cm 2 . Furthermore, partial recovery of the deterioration was evidently observed, which was promoted by keeping the IP at 50–70°C.

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.

Tumour shapes and fully automated range compensation for heavy charged particle radiotherapy

The idea of a computer-controlled range-compensating system for heavy charged particle radiotherapy, the multibar compensator, is proposed. By stacking multiple energy-absorbing layers along the beam, each of which has structure and behaviour similar to those of a multileaf collimator, variable range compensation will be achieved. The analysis of the conventional range compensators actually used for treatment concluded that the proposed system would not seriously degrade the treatment quality for the most cases, except for tumours in the head and neck region where 1 mm precision may be required. The system will even be able to coexist with the conventional range compensators to provide either method depending on clinical situations.