Biomedical subjects

A Brahme

Publications and source records attributed to A Brahme.

111 records · Page 7Linked to original sources

PubMedDate not supplied

Dosimetry and quality specification of high energy photon beams.

A number of quality descriptors are defined characterizing the photon attenuation and lepton contamination properties of high energy photon beams for radiation therapy. The dependence of the quality parameters on the design of the clinical beams such as the incident electron energy, target and filter thicknesses, field size and depth in the phantom are analyzed in some detail using analytical and Monte Carlo techniques. It is shown that the mean attenuation coefficient of the beam for a standard field size of 10 cm X 10 cm is related very accurately to the mean stopping power ratio for ionizing chamber dosimetry but also approximately to the equilibrium absorbed dose in the beam for a given photon energy fluence. This means that accurate photon dosimetry can be performed without knowing the acceleration potential, target design or filter thickness for the beam in use. Furthermore, the mechanism behind beam hardening and softening in the phantom are quantitized and suitable quality parameters for the lepton contamination are identified. The latter allow a determination of the lepton contamination for correction of the stopping power ratio near the surface if the contamination is large.

Monte Carlo Method↗

PubMedDate not supplied

Calculation and application of point spread functions for treatment planning with high energy photon beams.

A general dose calculation method for treatment planning with high energy photon beams, based on folding of the total energy released by primary photons per unit mass, the terma, with a fractional mean energy imparted point spread function is described. A set of point spread functions has been calculated with Monte Carlo technique for energies of primary photons between 100 keV and 20 MeV. Dose distributions have been calculated for a 6 MV bean using the method. The results clearly point out the considerably increased precision and flexibility achieved when calculating photon beam dose distributions from first principles using Monte Carlo generated point spread functions. The point spread functions calculated in this work are available on magnetic tape from the authors.

Humans↗

PubMedDate not supplied

Evaluation of fractionation regimens in stereotactic radiotherapy using a mathematical model of repopulation and reoxygenation.

PURPOSE: Various forms of stereotactic fractionation regimens have been used in the clinic. The purpose of this article is to evaluate the influence of the number of fractions on tumor control in stereotactic radiotherapy. METHODS: The 50% curative dose (D50) is calculated using a mathematical model of reoxygenation and repopulation. A review of the literature of stereotactic fractionation regimens is also presented. RESULTS: The number of fractions required to achieve the best therapeutic ratio is strongly influenced by the pattern of reoxygenation and the potential doubling time of the tumor. In the case of rapid repopulation, well-oxygenated clonogenic cells present a greater danger than hypoxic cells in a prolonged fractionated treatment schedule. In a slowly growing tumor, repopulation is relatively unimportant and the response is dominated by the pattern of reoxygenation. For a small number of large dose fractions, the potential doubling time and the time interval between fractions have a negligible effect on the D50 values. CONCLUSION: If a small number of large dose fractions is used in stereotactic radiotherapy and if reoxygenation occurs more efficiently with the passage of time, the therapeutic ratio can be enlarged by a sufficient time interval between fractions.

Brain Neoplasms↗