Volume 22, Issue 3, March 1995
Index of content:
22(1995); http://dx.doi.org/10.1118/1.597450View Description Hide Description
A model for collimator photon scatter calculations is presented. The scatter from a collimating block is separated into two categories, one being the scatter released from photons entering the block through the surface facing the source, and one from photons striking the block tangentially through the side tangential to the beam. Both sources of scatter are analyzed by means of scatter kernels, defined as the scatter fluence distribution from a narrow line beam striking a collimator block. Kernels for both cases are calculated analytically using first scatter models based on Klein–Nishina cross sections including corrections for binding effects and coherent scattering. For 1‐MeV primary photons, Monte Carlo calculations (EGS4) are used to show that the collimator scatter is dominated by first scatter. The kernels for the two types of scattering geometries are shown to be related and thus the total collimator scatter kernel can be derived from the kernel for the photons which have entered the block through the source facing side. Using a set of photon beam spectra, the kernels are parametrized as functions of beam energy in a form suitable for implementation in treatment planning systems. The parametrization is used to derive collimator scatter distributions for broad beam geometries of clinical interest. It is shown that scatter from the primary collimator, due to its location close to the beam source, is a major source of scatter that amounts to several percent of the primary fluence. The scatter from beam shaping collimators, however, generally accounts for less than 1% of the primary fluence. Although the beam‐shaping collimators generate little scatter, the model used to calculate the resulting dose component is simple to implement and separates the different sources of scattered photons from the accelerator head. The latter provides generality and accuracy in dose per monitor unit calculations in treatment planning. Due to the low magnitude of collimator scatter it is recommended to use the phrase ‘‘head scatter’’ instead of ‘‘collimator scatter’’ when addressing the overall subject of extra‐focal radiation from medical accelerators.
22(1995); http://dx.doi.org/10.1118/1.597451View Description Hide Description
Dose‐surface histograms are studied and compared with dose‐volume histograms, as an evaluation tool for prostate treatment planning. For thin walled hollow organs, such as the rectum and bladder, the surface area irradiated is a more appropriate measure of the biological effect than the full volume. It is also more accurate and efficient to define the surface for a hollow structure and compute the surface area histograms. Application of the dose‐surface histograms provide new insights into prostate treatment planning. A simple idealized geometry model demonstrates that the percentage surface area intersected by the geometric beam edge differs from the percentage volume intersected. For a group of prostate patients, it is shown that the dose‐surface histograms yield substantially different results from the dose‐volume histograms in ranking four‐, six‐, and, eight‐field treatment plans and in calculating the fraction of the rectum irradiated to high dose. The difference in terms of surface area between these plans in the high‐dose region is usually less than that in terms of the volume, and a reverse of plan ranking order can consequently occur. The percentage of organsurface irradiated to high dose is typically greater than the percentage volume by 5% to 10%. The use of the dose‐surface histograms in analysis of organ motion and/or patient setup uncertainty, and analysis of rectal complications, is also discussed.
22(1995); http://dx.doi.org/10.1118/1.597452View Description Hide Description
22(1995); http://dx.doi.org/10.1118/1.597453View Description Hide Description
This investigation evaluated samples of three phantom materials designed as substitutes for water for electron beamcalibration and depth ionization measurements. Two of the materials are commercially available (photon–electron Solid Water and Plastic Water), while the third (Homat) is not. Applying the values for water for all factors used in the calibration protocol of the American Association of Physicists in Medicine [Task Group 21, Med. Phys. 10, 741–771 (1983)] results in a discrepancy in calculated peak dose rates. Eliminating this discrepancy requires the additional inclusion of a multiplicative correction factor of approximately 1.015 for beam energies below 10 MeV, 1.01 for beam energies between 10 and 12 MeV, and 1.005 for beam energies above 12 MeV. Measurements for R 50 and extrapolated range may be made in these materials with no corrections. Some improvement can be made in the performance of the phantom material by optimizing the match to water specifically for electron beams without regard for photon beam response. As with all radiationoncology apparatus, calibration phantoms need acceptance testing before routine use.
Control of the necrosed tissue volume during noninvasive ultrasound surgery using a 16‐element phased array22(1995); http://dx.doi.org/10.1118/1.597603View Description Hide Description
Focused high‐power ultrasound beams are well suited for noninvasive local destruction of deep target volumes. In order to avoid cavitation and to utilize only thermal tissue damage, high frequencies (1–5 MHz) are used in ultrasonic surgery. However, the focal spots generated by sharply focused transducers become so small that only small tumors can be treated in a reasonable time. Phased arrayultrasound transducers can be employed to electronically scan a focal spot or to produce multiple foci in the desired region to increase the treated volume. In this article, theoretical and experimental studies of spherically curved square‐element phased arrays for use in ultrasonic surgery were performed. The simulation results were compared with experimental results from a 16‐element array. It was shown that the phased array could control the necrosed tissue volume by using closely spaced multiple foci. The phased array can also be used to enlarge a necrosed tissue volume in only one direction at a time, i.e., lateral or longitudinal. The spherically curved 16 square‐element phased array can produce useful results by varying the phase and amplitude setting. Four focal points can be easily generated with a distance of two or four wavelengths between the two closest peaks. The maximum necrosed tissue volume generated by the array can be up to sixteen times the volume induced by a similar spherical transducer. Therefore the treatment time could be reduced compared with single transducer treatment.
22(1995); http://dx.doi.org/10.1118/1.597454View Description Hide Description
The relatively precise placement of brachytherapy sources afforded by stereotactic frames for brain implants is not generally achievable for other sites, which lack the fixed geometry of the cranium and its contents. An exception is a source‐containing rigid mold that delivers brachytherapy when inserted securely in a surgical defect. A technique has been developed in which an acrylic mold of the region to be treated is suspended in a demountable aluminum box, which is then filled to a suitable level with dental stone to form a casting that supports the mold and that can be removed intact. First, the box is aligned on a mill table and a ball mill is used to drill three parallel holes in the acrylic mold, with precisely known locations and depths and as widely separated as possible. The spherical air cavities that result from plugging these holes with ball‐milled acrylic rods become reference markers in subsequent computed tomography(CT) scans. After optimum CT‐coordinate locations have been planned for 125I seeds in catheters, they are transformed to mill coordinates using a matrix developed from the known marker coordinates in the two systems. Catheter holes are then drilled with the mold in the reassembled casting and box. The method has been used to treat both recurrent maxillary cancer and recurrent orbital rhabdomyosarcoma.
22(1995); http://dx.doi.org/10.1118/1.597455View Description Hide Description
The dose distribution surrounding uniform cylinders of 32P has been calculated in order to obtain information relevant to the utilization of radioactive stents. The radiological impact on two major components of vascular tissue, smooth muscle, and endothelial cells is also considered.
Determination of dose components in phantoms irradiated with an epithermal neutron beam for boron neutron capture therapy22(1995); http://dx.doi.org/10.1118/1.597447View Description Hide Description
The application of activation foils, thermoluminescent detectors, and ionization chambers has been investigated for the determination of the different dose components in phantoms irradiated with a mixed gamma‐ray and epithermal neutron beam for boronneutron capture therapy. The thermal neutron fluence has been determined using a set of AuAl and MnNi activation foils. TLD‐700 and a Mg(Ar) ionization chamber have been used for the determination of the gamma‐ray dose. The dose from epithermal neutrons has been determined using a TE(TE) ionization chamber. The detector characteristics and the relative sensitivities of the various detectors to the different dose components in the phantom have been determined. The following accuracies (1 standard deviation) in the determination of the different components have been obtained: thermal neutron fluence rate: 5%; gamma‐ray dose rate: 7%; epithermal neutron dose rate: 15%. These values make these detectors suitable for obtaining the complete set of clinical dosimetry data required for patient dose assessment.
Specific absorbed fractions of energy from internal photon sources in brain tumor and cerebrospinal fluid22(1995); http://dx.doi.org/10.1118/1.597448View Description Hide Description
Transferrin, when injected intracranially into glioblastoma multiforme lesions, acts as a cytotoxic substance. Transferrin, radiolabeled with In‐111, can be coinjected and subsequent scintigraphic imaging can demonstrate the biokinetics of the cytotoxic transferrin. The administration of 111In transferrin into a braintumor results in distribution of radioactivity in the brain,braintumor, and the cerebrospinal fluid (CSF). Information about absorbed radiation doses to these regions, as well as other nearby tissues and organs, is important for evaluating radiation‐related risks from this procedure. The radiation dose is usually estimated for a mathematical representation of the human body. We have included source/target regions for the eye, lens of the eye, spinal column, spinal CSF, cranial CSF, and a 100‐g tumor within the brain of an adult male phantom developed by Cristy and Eckerman. The mathematical models of the spinal column, spinal CSF, and the eyes were developed previously, however, these source/targets have not been routinely included in photon transport simulations. Specific absorbed fractions (SAFs) as a function of photonenergy were calculated using the ALGAMP computer code, which utilizes Monte Carlo techniques for simulating photon transport. The ALGAMP code was run three times, with the source activity distributed uniformly within the tumor, cranial CSF, and the spinal CSF volumes. These SAFs, which were generated for 12 discrete photonenergies ranging from 0.01 to 4.0 MeV, were used with decay scheme data to calculate S‐values needed for estimating absorbed doses. S‐values for 111In are given for three source regions (braintumor, cranial CSF, and spinal CSF) and all standard target regions/organs, the eye and lens, as well as to tissues within these source regions. S‐values for the skeletal regions containing active marrow are estimated. These results are useful in evaluating the radiation doses from intracranial administration of 111In transferrin. The SAFs are also generally useful for calculation of absorbed dose from any radionuclide in these source regions.
Erratum: ‘‘Two‐dimensional registration of magnetic resonance brain images’’ [Med. Phys. 21, 1333–1337 (1994)]22(1995); http://dx.doi.org/10.1118/1.597449View Description Hide Description