Commissioning of a respiratory gating system involving a pressure sensor in carbon-ion scanning radiotherapy.

This study reports the commissioning methodology and results of a respiratory gating system [AZ - 733 V/733 VI (Anzai Medical Co., Japan)] using a pressure sensor in carbon-ion scanning radiotherapy. Commissioning includes choosing a location and method for pressure sensor installation, delay time measurement of the system, and the final flow test. Additionally, we proposed a methodology for the determination of a threshold level of generating an on/off gate for the beam to the respiratory waveform, which is important for clinical application. Regarding the location and method for installation of the pressure sensor, the actual person's abdomen, back of the body position, and supine/prone positioning were checked. By comparing the motion between the pressure sensor output and the reference LED sensor motion, the chest rear surface was shown to be unsuitable for the sensor installation, due to noise in the signal caused by the cardiac beat. Regarding delay time measurement of the system, measurements were performed for the following four steps: (a). Actual motion to wave signal generation; (b). Wave signal to gate signal generation; (c). Gate signal to beam on/off signal generation; (d). Beam on/off signal to the beam irradiation. The total delay time measured was 46 ms (beam on)/33 ms (beam off); these were within the prescribed tolerance time (<100 ms). Regarding the final flow test, an end-to-end test was performed with a patient verification system using an actual carbon-ion beam; the respiratory gating irradiation was successfully performed, in accordance with the intended timing. Finally, regarding the method for determining the threshold level of the gate generation of the respiration waveform, the target motion obtained from 4D-CT was assumed to be correlated with the waveform obtained from the pressure sensor; it was used to determine the threshold value in amplitude direction.

Scanned carbon-ion beam therapy throughput over the first 7 years at National Institute of Radiological Sciences.

INTRODUCTION: In the 7 years since our facility opened, we have treated >2000 patients with pencil-beam scanned carbon-ion beam therapy. METHODS: To summarize treatment workflow, we evaluated the following five metrics: i) total number of treated patients; ii) treatment planning time, not including contouring procedure; iii) quality assurance (QA) time (daily and patient-specific); iv) treatment room occupancy time, including patient setup, preparation time, and beam irradiation time; and v) daily treatment hours. These were derived from the oncology information system and patient handling system log files. RESULTS: The annual number of treated patients reached 594, 7 years from the facility startup, using two treatment rooms. Mean treatment planning time was 6.0 h (minimum: 3.4 h for prostate, maximum: 9.3 h for esophagus). Mean time devoted to daily QA and patient-specific QA were 22 min and 13.5 min per port, respectively, for the irradiation beam system. Room occupancy time was 14.5 min without gating for the first year, improving to 9.2 min (8.2 min without gating and 12.8 min with gating) in the second. At full capacity, the system ran for 7.5 h per day. CONCLUSIONS: We are now capable of treating approximately 600 patients per year in two treatment rooms. Accounting for the staff working time, this performance appears reasonable compared to the other facilities.

Carbon-Ion Pencil Beam Scanning Treatment With Gated Markerless Tumor Tracking: An Analysis of Positional Accuracy.

PURPOSE: Having implemented amplitude-based respiratory gating for scanned carbon-ion beam therapy, we sought to evaluate its effect on positional accuracy and throughput. METHODS AND MATERIALS: A total of 10 patients with tumors of the lung and liver participated in the first clinical trials at our center. Treatment planning was conducted with 4-dimensional computed tomography (4DCT) under free-breathing conditions. The planning target volume (PTV) was calculated by adding a 2- to 3-mm setup margin outside the clinical target volume (CTV) within the gating window. The treatment beam was on when the CTV was within the PTV. Tumor position was detected in real time with a markerless tumor tracking system using paired x-ray fluoroscopic imaging units. RESULTS: The patient setup error (mean ± SD) was 1.1 ± 1.2 mm/0.6 ± 0.4°. The mean internal gating accuracy (95% confidence interval [CI]) was 0.5 mm. If external gating had been applied to this treatment, the mean gating accuracy (95% CI) would have been 4.1 mm. The fluoroscopic radiation doses (mean ± SD) were 23.7 ± 21.8 mGy per beam and less than 487.5 mGy total throughout the treatment course. The setup, preparation, and irradiation times (mean ± SD) were 8.9 ± 8.2 min, 9.5 ± 4.6 min, and 4.0 ± 2.4 min, respectively. The treatment room occupation time was 36.7 ± 67.5 min. CONCLUSIONS: Internal gating had a much higher accuracy than external gating. By the addition of a setup margin of 2 to 3 mm, internal gating positional error was less than 2.2 mm at 95% CI.

Correction method for the physical dose calculated using Clarkson integration at the center of the spread-out Bragg peak for asymmetric field in carbon-ion radiotherapy.

PURPOSE: We previously proposed a calculation method using Clarkson integration to obtain the physical dose at the center of the spread-out Bragg peak (SOBP) for a treatment beam, the measurement point of which agrees with the isocenter [Tajiri et al. Med. Phys. 2013; 40: 071733-1-5]. However, at the measurement point which does not agree with the isocenter, the physical dose calculated by this method might have a large error. For this error, we propose a correction method. MATERIALS AND METHODS: To confirm whether the error can be corrected using in-air off axis ratio (OAR), we measured the physical dose at the center of an asymmetric square field and a symmetric square field and in-air OAR. For beams of which the measurement point does not agree with the isocenter, as applied to prostate cancer patients, the physical dose calculated using Clarkson integration was corrected with in-air OAR. RESULTS: The maximum difference between the physical dose measured at the center of an asymmetric square field and the product of in-air OAR and the physical dose at the center of a symmetric square field was - 0.12%. For beams as applied to prostate cancer patients, the differences between the measured physical doses and the physical doses corrected using in-air OAR were -0.17 ± 0.23%. CONCLUSIONS: The physical dose at the measurement point which does not agree with the isocenter, can be obtained from in-air OAR at the isocenter plane and the physical dose at the center of the SOBP on the beam axis.

Calculation method using Clarkson integration for the physical dose at the center of the spread-out Bragg peak in carbon-ion radiotherapy.

PURPOSE: In broad-beam carbon-ion radiotherapy performed using the heavy-ion medical accelerator in Chiba, the number of monitor units is determined by measuring the physical dose at the center of the spread-out Bragg peak (SOBP) for the treatment beam. The total measurement time increases as the number of treatment beams increases, which hinders the treatment of an increased number of patients. Hence, Kusano et al. [Jpn. J. Med. Phys. 23(Suppl. 2), 65-68 (2003)] proposed a method to calculate the physical dose at the center of the SOBP for a treatment beam. Based on a recent study, the authors here propose a more accurate calculation method. METHODS: The authors measured the physical dose at the center of the SOBP while varying the circular field size and range-shifter thickness. The authors obtained the physical dose at the center of the SOBP for an irregularly shaped beam using Clarkson integration based on these measurements. RESULTS: The difference between the calculated and measured physical doses at the center of the SOBP varied with a change in the central angle of the sector segment. The differences between the calculated and measured physical doses at the center of the SOBP were within ± 1% for all irregularly shaped beams that were used to validate the calculation method. CONCLUSIONS: The accuracy of the proposed method depends on both the number of angular intervals used for Clarkson integration and the fineness of the basic data used for calculations: sampling numbers for the field size and thickness of the range shifter. If those parameters are properly chosen, the authors can obtain a calculated monitor unit number with high accuracy sufficient for clinical applications.

Correction method for in-air output ratio for output variations occurring with changes in backscattered radiation.

PURPOSE: The in-air output ratio (S(c)) for a rectangular field is usually obtained using an equivalent square field formula. However, it is well-known that S(c) obtained using an equivalent square field formula differs slightly from the measured S(c). Though several correction methods have been suggested for the monitor-backscatter effect, the authors propose a more simple correction method for a rectangular field. METHODS: For rectangular fields and equivalent square fields, the authors assumed that the output variation was the product of six output variations for each backscattering area at the top of the collimator jaws, and the correction factor was the ratio of the output variation for a rectangular field to the output variation for an equivalent square field. The output variation was measured by using a telescope measurement. RESULTS: The differences between the measured and corrected S(c) ranged from -0.20% to 0.28% for symmetric rectangular fields by applying the correction factor to S(c) obtained using an equivalent square field formula. This correction method is also available for asymmetric rectangular fields. CONCLUSIONS: The authors propose a method to correct S(c) obtained using an equivalent square field formula, and a method to obtain the output variation for a field defined by collimator jaws.

New shielding materials for clinical electron beams.

Since lead has recently been recognized as a source of environmental pollution, we have investigated new electron shielding materials that do not contain lead. We compared the shielding thicknesses of a hard plate and a sheet composed of the new materials with that of lead for electron beams. The shielding thickness was evaluated as the thickness required for shielding primary electrons. The comparison revealed the shielding ability of the hard plate and sheet is approximately equivalent to 1.0 and 0.9 times that of lead, respectively. The thickness (in millimeters) required for shielding by the hard-plate, as well as the thickness of lead, is related to approximately half of the electron-beam energy (in MeV). The shielding ability of the sheet is also equivalent to that of Lipowitz alloy. Moreover these materials are environmentally friendly, and can be easily customized into arbitrary shapes. Therefore they can be used as lead substitutes for shielding against electron beams.

A new radiation shielding block material for radiation therapy.

In recent years, lead has been recognized as a source of environmental pollution; this includes lead use for radiation shielding in radiotherapy. We looked for a new material that could be a lead substitute. We chose a material composed of tungsten and resin. We compared the attenuation coefficient of the material with those of lead and Lipowitz's metal, and found the material has a higher attenuation coefficient than the other two. The material may be used as a substitute for lead because it is easy to fabricate and friendly to the environment.