No abstract available

[en] In the present study, the impact of respiratory gating on the beam characteristics of a linear accelerator is investigated. The main focus is put on the influence of the duty cycle. Measurements were performed on a linear accelerator type Oncor (Siemens) with photon energies 6 MV and 15 MV, equipped with the Anzai gating system AZ-733V. Depth dose curves and beam profiles were found not to be significantly altered by gating even for duly cycles down to 5% for realistic respiration frequencies (dose variations all <2.5%). However, for very small duty cycles, the absolute dosimetry changes significantly (dose deviations > 10%), leading to clinically relevant underdoses. The crucial parameter for the dosimetry is the number of monitor units per gate. Our results imply that treatment planning for respiratory gating can be performed on the basis of data obtained under ungated operation if and only if the absolute gates sizes during treatment are sufficiently large. The limiting values for the gate sizes have to be determined individually for each accelerator. (orig.)

No abstract available

[en] Obtaining accurate %DD values for routine treatment calculations is essential in radiation therapy. Many papers have presented expressions to calculate this or related parameters but these expressions often required that the parameters needed by the equation be determined for each individual treatment unit. This paper presents an expression that calculates %DD values with a mean-square accuracy of ∼1.0% versus measured values. The expression is applicable to beam energies ranging from Co-60 to 24 MV, field sizes from 4x4 to 40x40 cm², and depths from 1 cm deeper than d_max to 30 cm. The only information required by this expression that is machine specific is the ionization ratio

[en] A modified version of the Shabason and Hendee form of the Fermi-Dirac distribution function is proposed. The simple analytic expression characterizes the central axis depth-dose for electrons from the surface down to the 10% level with reasonable accuracy. (U.K.)

No abstract available

[en] Values of measured maximum depth dose for doubly-curved radon applicators confirms that, as the degree of curvature is increased, the value of maximum depth dose also increases. These figures indicate that radon applicators prescribed for doubly-curved sites on the human body can deliver a maximum dose up to 61% in excess of the calculated minimum depth dose. The clinically accepted excess is 33%

[en] Measurements described have been carried out to solve the problem of the high measured values of maximum dose delivered by doubly-curved Radon-222 applicators. In no case was the minimum measured depth dose more than 2% lower than the calculated value. The maximum depth dose did not exceed the minimum by more than the clinically acceptable 33%. The problem has been solved by increasing the diameter of the area containing the source and by placing 5% of the total activity in the centre of the applicator

[en] Published in summary form only

[en] We have examined a number of analytic representations of electron central-axis depth dose data current in the literature, testing them against sets of standard depth dose data. One of them, a two-parameter model of Shabason and Hendee, is recommended in situations in which good accuracy (approximately 2%) is desired, with the values of the parameters determined by an approximation formula which we have developed elsewhere. For higher accuracy, we have developed a polynomial model which gives, typically, a standard deviation of the fitting polynomial from the data points of 1%, and a maximum deviation of 2%. Fitting polynomials obtained with this method possess the property of having zero slope at the position of actual maximum dose, and generally a fifth-order polynomial (requiring four nonzero coefficients) provided the most acceptable fit. The four parameters involved are determined through inversion of a 4 x 4 matrix, and we have tabulated these four coefficients for the standard data sets. The polynomial model is designed for interpolation in the range between the 100% dose depth and the 10% dose depth, and another fitting curve of the same type can be adjoined to cover depths less than the 100% dose depth