Magnetic displacement force and torque on dental keepers in the static magnetic field of an MR scanner.

PURPOSE: To evaluate the effect of the static magnetic field of magnetic resonance (MR) scanners on keepers (ie, ferromagnetic stainless steel plate adhered to the abutment tooth of dental magnetic attachments). MATERIALS AND METHODS: Magnetically induced displacement force and torque on keepers were measured using 1.5 Tesla (T) and 3.0 T MR scanners and a method outlined by American Society for Testing and Materials (ASTM). Changes in magnetic flux density before and after exposure to scanner static magnetic field were examined. RESULTS: The maximum magnetically induced displacement forces were calculated to be 10.3 × 10(-2) N at 1.5 T and 13.9 × 10(-2) N at 3.0 T on the cover surface. The maximum torques exerted on the keeper (4 mm in diameter) were 0.83 N × 4 mm at 1.5 T and 0.85 N × 4 mm at 3.0 T. These forces were considerably higher than the gravitational force (7.7 × 10(-4) N) of the keeper but considerably lower than the keeper-root cap proper adhesive force. The keepers' magnetic flux density remained less than that of the Earth. CONCLUSION: Magnetically induced displacement force and torque on the keeper in the MR scanner do not influence the keeper-root cap proper adhesive force.

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.

[Improvement of dose distributions at NIRS's 70 MeV proton eye treatment beam course]

In order to improve dose distributions at NIRS's 70 MeV proton eye treatment beam course, we introduced finer bar ridge filters, and examined the effects of range compensators. The pitch of new bar ridge filters was 5mm in contrast to 15mm pitch of old ones. A NC-machine recently available enabled this refinement. The spread out Bragg peak (SOBP) widths were 10, 15, 20 and 30 mm. The new ridge filters improved the field uniformity considerably. In the ridge filter design we assumed parallel beam condition in which the mono-energetic proton should proceed in parallel with the central axis, and bar ridges only changes the proton ranges. We searched empirically for the optimum wobbler radius in view of field flatness and depth dose distribution. Range shifter and compensator did not affect the field flatness and depth dose distribution at the optimum condition thus searched. We measured dose distribution in a phantom using a compensator of stairs-shape, which fairly modulated the beam. A 50% isodose line almost coincided with the compensator shape, and these results suggested that improvements of dose distributions should be possible using compensators. However width between 50% and 80% isodose lines depended on the thickness of phantom. This might be due to scattering in the compensator and suggests that it is necessary to calculate dose distribution taking account of such effects.