Orthovoltage radiation therapy treatment planning using Monte Carlo simulation: treatment of neuroendocrine carcinoma of the maxillary sinus.

Dose distributions that result from treating a patient with orthovoltage beams are best determined with a treatment planning system that uses the Monte Carlo method, and such systems are not readily available. In the present work, the Monte Carlo method was used to develop a computer code for determining absorbed dose distributions in orthovoltage radiation therapy. The code was used in planning treatment of a patient with a neuroendocrine carcinoma of the maxillary sinus. Two lateral high-energy photon beams supplemented by an anterior orthovoltage photon beam were utilized in the treatment plan. For the clinical case and radiation beams considered, a reasonably uniform dose distribution (+/- 10%) is achieved within the target volume, while the dose to the lens of each eye is 4-8% of the prescribed dose. Therefore, an orthovoltage photon beam, when properly filtered and optimally combined with megavoltage beams, can be effective in the treatment of cancers below the skin, providing that accurate treatment planning is carried out to establish with accuracy and precision the doses to critical structures.

Designing, benchmarking, and applying a Monte Carlo electron transport code.

The design of a Monte Carlo electron transport code is described, with particular attention being given to the modeling of multiple-scattering, ionization, and bremsstrahlung production. Comparisons of code predictions with experimental data are presented, together with simple radiation therapy applications.

Optimal electron-beam treatment planning for retinoblastoma using a new three-dimensional Monte Carlo-based treatment planning system.

Electron-beam treatment planning for retinoblastoma was investigated and an optimal treatment plan was devised for a particular case using a new three-dimensional Monte Carlo-based treatment planning system known to be capable of correctly predicting dose perturbations caused by body surface obliquities and tissue heterogeneities. Computed tomography (CT) data files were used to construct a three-dimensional eye phantom representing the anatomy of a child's orbit. Dose distributions in sagittal, transverse, and coronal planes were predicted with 1-mm resolution. Study of these distributions led to an optimal treatment plan consisting of an anterior-lateral pair, with the anterior field being a 10-MeV, 30-mm-diam circular field, centrally blocked by a 10-mm-diam lucite lens shield and the lateral field being a 16-MeV, 30 x 25-mm D-shaped field. The anterior field delivers a therapeutic dose to the ora serrata, but it underdoses the posterior retinal surface behind the lens shield; the lateral field provides the necessary boost dose to the posterior retinal surface. An equally weighted combination of the two fields produces a dose distribution in which the entire retinal surface receives a therapeutic dose, with less than 10% of that dose being delivered to the lens, brain, and the contralateral orbit.

The accuracy of single-seed dose superposition for I-125 implants.

The Monte Carlo method was used to study perturbations of single I-125 seed dose distributions created by the presence of one or three neighboring seeds for the case of seeds immersed in a water phantom. Perturbation factors were determined within the geometric shadow of neighboring seeds for two-seed designs, four-seed spacings, and several choices of dose point. The results were compared to dose estimates obtained by the simple superposition of single-seed data for one- and two-plane implants. Some significant differences were found.

An improved electron multiple-scattering distribution for Monte Carlo transport simulation.

An improved electron multiple-scattering distribution is presented in the form of a composite function which combines three expressions valid over different scattering angle regions: a modified relativistic Mott single-scattering term for large angle scattering, a modified Moliere Gaussian term for small angle scattering, and an exponential term for the intermediate angle scattering region. The exponential term has two adjustable parameters which make possible the smooth transition from the large to the small scattering angle regions. The proposed distribution exhibits better agreement with experiment than other multiple-scattering distributions commonly used in Monte Carlo electron transport codes, is amenable to direct sampling over a continuous range of electron energies and step sizes, can be used for elements of any atomic number, and is particularly suitable for use on small memory computers.

Two-dimensional dose distribution around a commercial 125I seed.

The Monte Carlo method was used to investigate the dose distribution around a 3M Company model 6711 125I seed immersed in a water phantom. Dose rate per unit activity data are presented as a matrix of 63 points surrounding the seed. Relative dose data are presented graphically for two mutually perpendicular directions and compared with the corresponding data for the only other 125I seed currently available, the 3M Company model 6702 125I seed. The 6711 relative dose distribution decreases more rapidly with distance from the seed than does the 6702 relative dose distribution. Uncertainties in the 6711 seed dose distribution produced by end-weld thickness variations were investigated and found to be substantial at certain points.

Monte Carlo simulation of the dose distribution around 125I seeds.

The Monte Carlo method was used to investigate dose distributions around the 3M Company model 6701 and model 6702 125I brachytherapy seeds. The transverse axis dose distributions of the two seed models were found to be nearly identical, but the longitudinal axis dose distributions differed significantly. Seed design influences upon dose distributions also were investigated.

Removal of streaking artifact in computed tomography.

In transmission computed tomography, relative X-ray attenuation measurements are made at various angles around a patient's body. These input projection data are reconstructed to yield a cross-sectional view of internal structure. If the body section contains material that severely attenuates the X-ray beam (e.g., surgical clips, lead fragments), high-density streaks that obliterate internal structure will be produced in the process of image reconstruction. This loss of diagnostic information renders the scan useless. A technique has been developed that removes this imaging artifact. The approach views the affected projection data as misinformation. These data are assigned new values, and image reconstruction is performed without changing existing computer hardware or software. Projection data for a head section containing a lead fragment were obtained by Monte Carlo simulation. Three methods of obtaining replacement data were examined. A nearest-neighbor pattern recognition technique yielded excellent results.

An interactive program to calculate the treatment time for 60Co single beam, opposing beam, and rotational beam radiation therapy.

An interactive computer program for calculating treatment times for 60Co single beam, opposing beam, or rotational beam radiation therapy is described. The program allows the user to enter parameters specific to his machine. Analytic expressions are used to represent the depth dose, backscatter factor, tissue-air ratio, collimator factor, and beam attenuation of a block-carrying tray.

Confirmation of a speculation concerning average tissue-air ratio determinations for rotational Co-60 radiation therapy treatment planning.

The question of the number of radii which are necessary to accurately determine the average tissue-air ratio (TAR) to be used in 60Co rotational radiation therapy treatment planning was studied using actual patient contours. It was found that 12 radii adequately determine the average TAR for clinical purposes.

A pattern recognition method for the removal of streaking artifact in computed tomography.

A new method is presented for the removal of streaking artifact in computed tomography. The method uses a nearest-neighbor pattern recognition technique to redetermine ray sums that intersect the foreign object responsible for the streaking. When the method is applied to the removal of streaking artifact caused by lead in a skull phantom, excellent results are obtained.

A system for checking radiation therapy calculations for treatment time or monitor setting.

Errors in the calculation of radiation therapy treatment time or monitor setting are likely to occur with regularity unless adequate quality control procedures are utilized. These errors can be greatly reduced if appropriate computer systems are employed with programs which utilize analytical representations of dosimetric quantities. A system wherein dosimetry technologists routinely check treatment calculations using a computer terminal has been in operation in our department for several years and has proved helpful in uncovering calculation errors.

Optimal radiotherapy treatment planning for carcinoma of the cervix uteri.

A study is described in which an optimal time-dose-fractionation (TDF) value for radiotherapy of carcinoma of the cervix uteri is found using clinical data for points 2 cm from the cervical os proximally along the direction of the cervical canal and 2 cm laterally to the canal. The optimal TDF value was found to be 127 +/- 10 for regimens utilizing external beam cobalt 60 irradiation (or external irradiation of equivalent quality) in combination with intracavitary radium irradiation.

A formalism for the assignment of uncertainties to clinical probabilities of hypothyroidism, euthyroidism, and hyperthyroidism.

A formalism for the optimal estimation of the probabilities of hypothyroidism, euthyroidism, and hyperthyroidism is outlined. The scheme provides a means for assigning uncertainties to the estimates. These uncertainties can be used to determine whether or not the difference between two probabilities is statistically significant. Clinical data are utilized to illustrate the method.