A computer algorithm has been developed for electron beam treatment planning. The method uses a limited amount of stored experimental dose distribution data, performs interpolation, and stores generated beam information on an optimized fan line-depth line grid. Experimental verification of the computer program showed agreement within +/-5% for beam generation and air gap correction.

Computers↗

PubMedDate not supplied

Revision of tissue-maximum ratio and scatter-maximum ratio concepts for cobalt 60 and higher energy x-ray beams.

The concept of tissue-maximum ratios (TMR) as a basis of dose computations has posed problems when applied to a whole range of clinically used megavoltage beams. Another problem is the definition of scatter-maximum ratio (SMR) which assumes a value of zero at the depth of maximum dose. This paper describes new methods of measuring collimator and phantom scatter correction factors. The definitions of TMR and SMR are modified in order to compute phantom scatter at any depth, including depth of maximum dose. The revised concept is basic and general enough that it can be applied to x-ray beams of any energy fields of any shape and isocentric as well as non-isocentric modes of treatment.

Cobalt Radioisotopes↗

PubMedDate not supplied

Lead shielding for electrons.

Using a 13 MeV electron beam as an example, transmission curves for various thicknesses of lead were measured. The data indicated that the choice of shielding thickness depends greatly on the depth at which the measurements are made. The importance of this reference depth and criteria for shield design when shields of minimum thickness are required is discussed.

Electrons↗

PubMedDate not supplied

Film dosimetry of megavoltage photon beams: a practical method of isodensity-to-isodose curve conversion.

The central problems of photon beam film dosimetry are the dependence of film response upon photon energy, processing conditions, and film plane orientation. We have overcome these problems by accurately fitting the depth-dependent sensitometric curve of Kodak XV-2 film (exposed parallel to beam axis) to the equation OD(D,d) = ODs(1 - exp [- alpha 0[1 + beta(d-dm)]D]) where OD(D,d) is the optical density for dose D at depth d. ODs, alpha 0, and beta are constants characteristic of the film and beam energy but are independent of field size. Only central axis depth dose data for a single field are required to determine their values. A computer program based upon this equation has been written which successfully generates single field isodose curves from film data for a variety of field sizes (including wedged fields) with an accuracy of +/- 3%. Data are presented for 60Co, and 4 and 10 MV x rays.

Film Dosimetry↗

PubMedDate not supplied

A model for calculating electron beam scattering in treatment planning.

The Fermi-Eyges theory of electron scattering overestimates the scattering of electron beams used in radiation therapy. The reason for this overestimate is the neglect of the loss of electrons which are scattered into highly oblique paths and removed from the beam at relatively shallow depths. A modification of Eyges' solution to Fermi's equation is presented to take this loss of electrons into account. Equations for the calculation of isodose distributions for any medium using pencil beams are developed. Experimental confirmation is presented for electron beams of 13 and 18 MeV in homogeneous water, polystyrene, Lucite, and aluminum phantoms.

Electrons↗

PubMedDate not supplied

Dose calibrator response to brachytherapy sources: A Monte Carlo and analytic evaluation.

It is well known that the dose calibrator response/unit exposure rate depends significantly upon source energy. However, investigation of 137Cs, 192Ir, and 226Ra brachytherapy sources by empirical, analytical, and Monte Carlo techniques shows that source filtration significantly affects the calibrator reading to exposure rate conversion factor. The results demonstrate that for each clinically used filtration thickness an exposure calibrated standard source is required to establish the response of the well chamber. An interesting consequence of this analysis is that the Sievert point dose algorithm for clinical sources overestimates the dose on the order of 3% at distances of approximately 3.5 cm from the source.

Brachytherapy↗

PubMedDate not supplied

Model for calculating depth dose distributions for broad electron beams.

Central axis electron beam depth dose distributions can be transformed by replacing dose by fluence and depth by a measure of angular dispersion. This transformation was applied to a set of broad beam central axis depth dose distributions calculated by Monte Carlo code for beams with initial energies ranging from 1 to 60 MeV in homogeneous media of water, aluminum, and copper. The resulting fluence distributions belong to a family of curves that can be parametrized by a single-valued function of initial beam energy and medium and can be used to calculate fluence distributions for accelerators. Fluence curves can be easily transformed to depth dose curves.

Models, Theoretical↗

PubMedDate not supplied

Obliquity correction for electron beams.

Uneven air gaps, beam obliquity, or sloping contours create complex dosimetry problems in electron beam treatment planning. We find that the depth dose distribution in an obliquely incident beam can be calculated from the data for a normally incident beam by using inverse square law and obliquity factors. The obliquity factors have been determined for various energy beams in the range of 6-22 MeV and can be used for manual treatment planning.

Electrons↗

PubMedDate not supplied

Dosimetry of asymmetric x-ray collimators.

We have studied the dosimetry of an independent jaw system (provided with the Varian Clinac 2,500) using ionometric measurements performed both in air and in a water phantom. Our study shows that the effect of the independent jaw on the dose distribution is similar to that of secondary blocking except for changes produced in the collimator scatter. A system of dose calculation was developed which takes into account the changes in the collimator scatter as well as in the isodose distribution. A method is described to correctly generate isodose curves for fields shaped by an independent jaw using a modified AECL TP11 treatment planning system. The primary modification in the program consists of correcting the zero-area tissue-maximum ratios for the off-axis variation in beam quality.

Humans↗

PubMedDate not supplied

Validity of lung correction algorithms.

Our studies have compared the "effective tissue-air ratio (TAR) method" (ICRU Report No. 24), "equivalent TAR method," and the "generalized Batho method" (currently used by the TP-11 computer treatment planning system) with measured results for different energy photon beams using two lung inhomogeneities to simulate a lateral chest field. Significant differences on the order of 3%-15% were found when comparing these various methods with measured values.

Algorithms↗

PubMedDate not supplied

The production of secondary electrons in an electron beam.

Convenient methods for calculating the ratio of restricted to unrestricted collision stopping power in water over a wide range of initial and cutoff energies, the rate of production of secondary electrons, and the primary electron dose distribution in an electron beam, are presented.

Electrons↗

PubMedDate not supplied

Dose perturbations at interfaces in photon beams.

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.

Humans↗