Biomedical subjects
Publications and source records attributed to I J Das.
A model based on an approximation called the partial fluence approximation is presented for the calculation of dose distributions in the vicinity of medium interfaces in photon beams. The predictions of the model are compared with dose distributions measured in layered phantoms consisting of aluminum and polystyrene, for photon beams ranging in energy from 60Co to 24 MV.
The description of the quality of a photon beam has usually been characterized by a single value such as the half-value layer, the effective attenuation coefficient, the percent depth dose, and most recently by the ionization ratio (IR). Although the IR is simple and easy to measure, it lacks sensitivity at photon energies above 10 MV. This paper describes a method based on dose perturbation at an interface and defines the forward dose perturbation factor (FDPF) as a measure of beam quality. Comparisons between the two methods are given for photon energies ranging from 60Co to 24 MV. The results show that the FDPF method is more sensitive to spectral changes at photon energies above 10 MV than the IR.
The energy averaged Boltzmann equation model is applied to the determination of dose distributions in infinite, homogeneous media with uniform, monoenergetic, isotropic source distributions in spherical regions of radius larger than the electron range. The generalization to the case of spherically symmetric source distributions is made. Comparisons with dose distributions calculated by the integration of dose point kernels derived from Monte Carlo calculations are presented.
Most computer algorithms used clinically for photon beam treatment planning are unable to predict the effect of electron backscattering on dose distribution from high atomic number materials. It has been observed that there is a significant dose enhancement at such an interface. We define the dose enhancement in terms of backscatter dose factor (BSDF), which depends on the energy of the photon beam, thickness and width of the inhomogeneity, distance from the interface, and the atomic number of the inhomogeneity. For all energies studied, the dose fall-off is initially very rapid and disappears beyond a few millimeters upstream from the interface. Empirically derived equations are presented for dose calculation at the interfaces of various media, including bone and soft tissue, for photon energies in the range of Co-60 gamma rays to 24 MV x rays.
An improved, quantitative version of the partial fluence model [Med. Phys. 14, 585 (1987)] for the calculation of dose perturbations at media interfaces in photon beams is presented and compared with measurements made at interfaces between polystyrene and materials ranging in atomic number from aluminum to lead, for photon beams ranging in energy from 60Co to 24 MV.
Measurement of dose or dose perturbation factors at high atomic number interfaces are usually performed with a thin-window parallel-plate ion chamber. In a transition region, under nonequilibrium conditions, accuracy of ion chamber readings for the dose measurements has often been questioned. This paper critically analyzes the factors (stopping power ratio and charge collection) for the dose measurements at interfaces. Monte Carlo simulations were performed to investigate the secondary electron spectrum produced by photon beams and to calculate the stopping power ratios at the point of measurement. The validity of dose measurements was studied for the photon beams in the range of Co-60 gamma rays to 24-MV x rays at bone and lead interfaces with polystyrene, using thermoluminescent dosimeters, extrapolation chamber and several types of commercially available parallel-plate ion chambers. It is observed that for energies greater than 10 MV most parallel-plate chambers can be used to measure dose accurately. At lower energies, however significant differences between measured doses with different detectors were noticed. It is suggested that at high-Z interfaces and lower energies, the dose measurements should be performed with ultrathin-window parallel-plate ion chambers or extrapolation chambers.
The physical characteristics of the interactions of megavoltage photons and electrons with matter provide distinct advantages, relative to low-energy (orthovoltage) x rays, that lead to better radiation dose distributions in patients. Use of these high-energy radiations has resulted in better patient care, which has been reflected in improved radiation treatment outcome in recent years. But, as the desire for higher energy radiation beams increases, it becomes important to determine whether the physical characteristics that make megavoltage beams beneficial continue to provide a net advantage. It is demonstrated that, in fact, there is an energy range from 4 to 15 MV for photons and 4 to 20 MeV for electrons that is optimally suited for the treatment of cancer in humans. Radiation beams that exceed these maximum energies were found to add no advantage. This is because the costs (price of unit, installation, maintenance, shielding for neutron and photons) are not justified by either improved physical characteristics of the radiation (penetration, skin sparing, dose distribution) or treatment outcome. In fact, for photon beams some physical characteristics result in less desirable dose distributions, less accurate dosimetry, and increased safety problems as the energy increases for example, increasingly diffuse beam edges, loss of electron equilibrium, uncertainty in dose perturbations at interfaces, increased neutron contamination, and potential for higher personnel dose. The special features that make electron beams useful at lower energies, for example, skin sparing and small penetration, are lost at high energies. These physical factors are analyzed together with the economic factors related to radiation therapy patient care using megavoltage beams.