Scattered radiation from dental metallic crowns in head and neck radiotherapy.

We aimed to estimate the scattered radiation from dental metallic crowns during head and neck radiotherapy by irradiating a jaw phantom with external photon beams. The phantom was composed of a dental metallic plate and hydroxyapatite embedded in polymethyl methacrylate. We used radiochromic film measurement and Monte Carlo simulation to calculate the radiation dose and dose distribution inside the phantom. To estimate dose variations in scattered radiation under different clinical situations, we altered the incident energy, field size, plate thickness, plate depth and plate material. The simulation results indicated that the dose at the incident side of the metallic dental plate was approximately 140% of that without the plate. The differences between dose distributions calculated with the radiation treatment-planning system (TPS) algorithms and the data simulation, except around the dental metallic plate, were 3% for a 4 MV photon beam. Therefore, we should carefully consider the dose distribution around dental metallic crowns determined by a TPS.

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 plate's 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.

Calculation of 10 MV x-ray spectra emitted by a medical linear accelerator using the BFGS quasi-Newton method.

To calculate photon spectra for a 10 MV x-ray beam emitted by a medical linear accelerator, we performed numerical analysis using the aluminium transmission data obtained along the central axis of the beam under the narrow beam condition corresponding to a 3x3 cm2 field at a 100 cm distance from the source. We used the BFGS quasi-Newton method based on a general nonlinear optimization technique for the numerical analysis. The attenuation coefficients, aluminium thicknesses and measured transmission data are necessary inputs for the numerical analysis. The calculated x-ray spectrum shape was smooth in the lower to higher energy regions without any angular components. The x-ray spectrum acquired by the employed method was evaluated by comparing the measurements along the central axis percentage depth dose in a water phantom and by a Monte Carlo simulation code, the electron gamma shower code. The values of the calculated percentage depth doses for a 10x10 cm2 field at a 100 cm source-to-surface distance in a water phantom were obtained using the same geometry settings as those of the water phantom measurement. The differences in the measured and calculated values were less than +/-1.0% for a broad region from the shallow part near the surface to deep parts of up to 25 cm in the water phantom.

[Method for determining optimal intracavitary radiotherapy conditions for carcinoma of the uterine cervix with tumor infiltration of the vaginal wall].

Stage III carcinoma of the uterine cervix is occasionally accompanied by tumor infiltration of the vaginal wall. Currently, the vaginal wall has to be irradiated in the same manner as the uterine cervix. The authors have developed a system for determining the optimal irradiation conditions for treating the two regions, uterine cervix and vaginal wall, at the same time. A comparison of two methods is shown in simulation, and then a clinical case is reported. The first method consists of two treatment plans, one for the uterine cervix without tumor infiltration of the vaginal wall, and the other for the vaginal wall without carcinoma of the uterine cervix. The second, newly developed method considers the two regions together. Irradiation times of ovoid sources obtained with the second method are 15-25% less than those of the first method. Isodose curves obtained with the two methods are very different, and thus the uterine cervix and vaginal wall must be considered together in order to determine irradiation conditions.

[Method of calculating TDF biological equivalent for optimal treatment dose in fractionated intracavitary irradiation of carcinoma of the uterine cervix].

Intracavitary irradiation therapy for carcinoma of the uterine cervix used with high or low dose rate irradiation is fractionated in Japan. The optimal treatment dose is determined according to the biological effect on both diseased and healthy tissues. The equations of modified NSD and TDF biological equivalents were recalculated from Arai's clinical data, which were used to examine the optimal time-dose-fractionation relationship for high and low dose rate intracavitary irradiation on squamous cell carcinoma of the cervix uteri. The optimal time-dose-fractionation relationship at point A is expressed as follows: D = NSD N0.26 T0.06 where NSD is 17.75 get for high dose rate and 31.78 get for low dose rate. TDF = K n d1.47 x-0.09 where K is 1.46 for high dose rate and 0.62 for low dose rate. The range of the optimal total dose to point A given by one fraction per week was 30.7 Gy for 4 fractions and 38.3 Gy for 8 fractions in high dose rate irradiation. In the case of low dose rate irradiation, the optimal total dose given by one fraction per week and the dose rate of 75.0 cGy/h was 55.0 Gy for 4 fractions. The maximum dose difference between our result and Arai's was about +/- 10%. The dose modification ratio for high dose rate and low dose rate is 1.79.

[Techniques for determining position from more than two radiographs intersecting at arbitrary angles in brachytherapy].

The least squares method and geometrical solution for calculating position were used in the two-projection method. Five coordinate systems were defined as a normal system, image system on film intersecting the beam central axes at an arbitrary angle and the projection, virtual coordinate and virtual image coordinate systems with beam central axis as one of the three coordinate axes to determine the geometrical relationship between a point and image on the film. Normal coordinates of the point were calculated by six geometrical solution sets and two forms of the least squares method using the rotation matrixes of the coordinate systems. One least squares method solves simultaneous nonlinear equations, and the other derives a strict solution from simultaneous linear equations. The latter least squares method has little physical meaning and is not as useful as the former. Although the former has physical meaning, the iterative approximation method should be used to determine position since a strict solution cannot be obtained directly. By these least squares methods, position is determined with less error using the projections at more than two focal spots.

[Determination of source position for four radiography systems with orthogonal projections in brachytherapy].

Four configurations of two X-ray tube positions are available for determining the position of a point using two orthogonal films. For each configuration, there are many formulas for calculating the coordinate of a point: the least squares methods with and without physical meaning, such as six sets of geometrical solutions, an approximation method with constant magnifications and so on. It is troublesome for a person in charge of treatment planning to directly derive a formula or select an appropriate formula from numerous ones for the four configurations. Thus, a method to easily apply the published formula for a configuration to the other three configurations is described in simulations and a clinical case using rotation matrixes of the right-handed coordinate system. Each diagonal element of the rotation matrixes is 1 or -1, and the other elements are 0.

[Determination of the point position for brachytherapy from the shift-projection method using the least squares method and geometrical solutions].

We have derived formulas by the least squares method and six sets of geometrical solutions for calculating the position for brachytherapy from shift-projection images on one or two films. There are four types of expression for each coordinate in both methods, the shift-film technique with double exposures on a film and the stereo-shift technique with two films. Some geometrical solutions for the shift-film technique are equal to well-known conventional formulas. In the case of the stereo-shift technique, each conventional formula for the two coordinates is equal to the mean of the two geometrical solutions of each coordinate, and the one for the other coordinate is equal to one of the geometrical solutions. Formulas for the shift-film technique can be easily reduced to those for the stereo-shift technique. The error of the position calculated by the least squares method was the smallest among all the formulas for the shift-film technique in simulation.

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.

[Experiences in clinical application of an automated calculation system that determines the optimum intracavitary irradiation condition in the treatment of carcinomas of the uterine cervix].

An automated calculation system that determines the optimum intracavitary irradiation condition for treating carcinomas of the uterine cervix has been developed on the basis of the Manchester system. Using Vax-11 computer, the system has been successfully applied in treating 105 patients since December, 1984. Trouble with the system almost occurred during its use in the first year of its clinical application. One problem related to the method of calculation, which was quickly settled, and the other was mainly a human error. The calculation system has proven especially useful for radiotherapists.

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.

[An analysis of tumor status of stage 3 carcinoma of the uterine cervix according to Cox's theory].

Four hundred and eighty three cases of primary stage 3 carcinoma of the uterine cervix which were treated at the National Institute of Radiological Sciences were analyzed with reference to the tumor status at the first examination, using Cox's proportional hazard model. Five prognostic factors, i.e. growth type of tumor, size of tumor at portio, size of uterus (including cervix and body), degree of tumor infiltration of parametrium and degree of tumor infiltration of vaginal wall, were considered. A coefficient of correlation, parameters of a hazard model, relative risk and survival functions were calculated with a program developed by us. The coefficient of correlation between the size of the uterus and the size of the tumor at the portio or the parametrial infiltration was about -0.5. Other combinations of prognostic factors indicated only a small correlation. When five prognostic factors were analyzed independently, the size of the uterus was found to have the greatest influence on the patient's prognosis and infiltration of the vaginal wall had the least. A combination of prognostic factors was selected according to the criterion of small correlation with each other. And then, the combination was evaluated for prognosis and to find the level of parameters under the asymtotic normal distribution. The best combination consisted of a growth type of tumor, a size of tumor at portio and tumor infiltration into the parametrium. Parameters of each prognostic factor which were calculated simultaneously from marginal likelihood were 0.236, 0.534 and 0.151, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

[RCT image study by angular interval of sampling].

A multi-purpose gamma camera system which had 2 detectors of opposed position and was capable of radionuclide computed tomography (RCT) imaging was installed in Division of Nuclear Medicine, Chiba Cancer Center Hospital. In this study, the variance of RCT image due to the angular interval of sampling was analyzed. The full width at half maximum (FWHM) and full width at tenth maximum (FWTM) of point source were almost same for all sampling intervals investigated. The distortion of RCT images with uniform activity source could not be detected even for larger intervals. When the RCT images were obtained using a phantom contained 3 line sources and a bar phantom for RCT which were located at right angle with the plane of reconstruction, the images did not change between 1 degrees and 6 degrees of sampling interval, but for 9 degrees or more the distortion of images occurred explicity. For our system, sampling interval of 2 degrees was predicted from the theorectical calculation considering both resolution and statistical noise. But, results from the phantom studies of present work showed that sampling interval of 6 degrees was sufficient to reconstruct RCT images without distortion, and could be applicable to dynamic RCT images.