Katsuyoshi Tabushi

An experimental attenuation plate to improve the dose distribution in intraoperative electron beam radiotherapy for breast cancer

Intraoperative electron beam radiotherapy (IOERT) is a technique in which a single-fraction high dose is intraoperatively delivered to subclinical tumour cells using an electron beam after breast-conserving surgery. In IOERT, an attenuation plate consisting of a pair of metal disks is commonly used to protect the normal tissues posterior to the breast. However, the dose in front of the plate is affected by backscatter, resulting in an unpredictable delivered dose to the tumour cells. In this study, an experimental attenuation plate, termed a shielding plate, was designed using Monte Carlo simulation, which significantly diminished the electron beam without introducing any backscatter radiation. The plates performance was verified by measurements. It was made of two layers, a first layer (source side) of polymethyl methacrylate (PMMA) and a second layer of copper, which was selected from among other metals (aluminium, copper and lead) after testing for shielding capability and the range and magnitude of backscatter. The optimal thicknesses of the PMMA (0.71 cm) and copper (0.3 cm) layers were determined by changing their thicknesses during simulations. On the basis of these results, a shielding plate was prototyped and depth doses with and without the plate were measured by radiophotoluminescence glass dosimeters using a conventional stationary linear accelerator and a mobile linear accelerator dedicated for IOERT. The trial shielding plate functioned as intended, indicating its applicability in clinical practice.

Dose distribution near thin titanium plate for skull fixation irradiated by a 4-MV photon beam

To investigate the effects of scattered radiation when a thin titanium plate (thickness, 0.05 cm) used for skull fixation in cerebral nerve surgery is irradiated by a 4-MV photon beam. We investigated the dose distribution of radiation inside a phantom that simulates a human head fitted with a thin titanium plate used for post-surgery skull fixation and compared the distribution data measured using detectors, obtained by Monte Carlo (MC) simulations, and calculated using a radiation treatment planning system (TPS). Simulations were shown to accurately represent measured values. The effects of scattered radiation produced by high-Z materials such as titanium are not sufficiently considered currently in TPS dose calculations. Our comparisons show that the dose distribution is affected by scattered radiation around a thin high-Z material. The depth dose is measured and calculated along the central beam axis inside a water phantom with thin titanium plates at various depths. The maximum relative differences between simulation and TPS results on the entrance and exit sides of the plate were 23.1% and – 12.7%, respectively. However, the depth doses do not change in regions deeper than the plate in water. Although titanium is a high-Z material, if the titanium plate used for skull fixation in cerebral nerve surgery is thin, there is a slight change in the dose distribution in regions away from the plate. In addition, we investigated the effects of variation of photon energies, sizes of radiation field and thickness of the plate. When the target to be irradiated is far from the thin titanium plate, the dose differs little from what it would be in the absence of a plate, though the dose escalation existed in front of the metal plate.

Two‐radiograph reconstruction using six geometrical solution sets and least‐squares method

When two radiographic projections are available for reconstruction, it was found that six different combinations of equations could be used to obtain the geometrical solutions for the position of any point. No errors in the image coordinates read from the radiographs resulted in identical solutions for the six equations. Inaccuracies or errors present in the image coordinates generated differences among the six solutions. In this case, a least-squares method could be used to determine the optimum position. The utility of such a least-squares optimizing approach is presented in the context of a clinical example.

A method for calculating the optimum irradiation condition for intracavitary radiotherapy using quadratic programming

A method of calculating optimum irradiation conditions for intracavitary radiotherapy using quadratic programming has been formulated and then modified for practical application. The allowable range of obtained dose, which is usually fixed in advance, is automatically computed to be as small as possible. The variance of the product of the activity and the irradiation time of the tandem source is also minimised to avoid the occurrence of cold and/or hot spots. Optimum irradiation conditions for conventional intracavitary radiotherapy of carcinoma of the uterine cervix were obtained on the basis of isodose curves passed through the points A of the Manchester system. Those for carcinoma of the other organs and special cases of carcinoma of the uterine cervix can be determined after consideration of the tumour state.