An empirical relationship for determining photon beam quality in TG-21 from a ratio of percent depth doses.

A key component of the Radiological Physics Center's (RPC) on-site dosimetry review visits are photon beam calibrations for which determination of the energy of the x ray is a key element. The ratio of ionizations, TPR20/TPR10, for a 10 cm x 10 cm field at depths of 20 and 10 cm for a constant SCD is used as a quantitative measure of beam quality in the Task Group 21 protocol. The RPC has measured both TPR20/TPR10 and the corresponding ratio of percent depth dose (D20/D10) at a constant SSD for 685 photon beams (4-25 MV) for most makes and models if accelerators. A strong correlation between TPR20/TPR10 and D20/D10 is presented which allows the determination of the TPR ratio from the measurement of the ratio of percent depth doses. An analysis of the uncertainty introduced in the TG-21 factors (L/rho, Pwall, Prepl) caused by the spread in the measured data and translated into the determination of the TPR ratio results in an insignificant error (< 0.3%). This empirical relationship provides an alternate technique for quantifying the beam quality defined in the TG-21 protocol without surrendering any loss of precision in output calibration. This technique may be found by those who calibrate at a fixed SSD to be an easier and quicker method.

A generic off-axis energy correction for linac photon beam dosimetry.

Cooperative clinical trial group protocols frequently require off-axis point dose calculations. The Radiological Physics Center uses the calculative technique developed by Hanson et al. [Med. Phys. 7, 145-146 (1980); 7, 147-150 (1980)] to verify these calculations. In order to correct for off-axis energy changes, this technique requires off-axis half-value layer data, HVL, as a function of off-axis ray angle for the specific beam. This paper presents a formulism based on HVL mesurements on a limited number of therapy beams, which allows the calculation of an off-axis energy-correction factor for any clinical photon beam created by a linear accelerator using conventional flattening filters.

Depth of ionization chamber in water.

The device developed by the authors and described here enables the user to measure the depth from the water surface to the point of measurement for a cylindrical ion chamber with a waterproof plastic cap in a water phantom, free of surface-tension error with a high precision. The device seeks vertical orientation and provides the convenience of hands-free operation. The measurement process is simple and quick with a precision of 0.1 mm. (The device is currently available as a 'water phantom depth gauge' from Nuclear Associates, Division of Victoreen Inc., Clare Place, NY, USA.)

How water equivalent are water-equivalent solid materials for output calibration of photon and electron beams?

The water equivalency of five "water-equivalent" solid phantom materials was evaluated in terms of output calibration and energy characterization over a range of energies for both photon (Co-60 to 24 MV) and electron (6-20 MeV) beams. Evaluations compared absorbed doses calculated from ionization measurements using the same dosimeter in the solid phantom materials and in natural water (H2O). Ionization measurements were taken at various calibration depths. The Radiological Physics Center's standard dosimetry system, a Farmer-type ion chamber in a water phantom, was used. Complying with the TG-21 calibration protocol, absorbed doses were calculated using eight measurement and calculational techniques for photons and five for electrons. Results of repeat measurements taken over a period of 2 1/2 years were reproducible to within a +/- 0.3% spread. Results showed that various combinations of measurement techniques and solid phantom materials caused a spread of 3%-4% in the calculation of dose relative to the dose determined from measurements in water for all beam energies on both modalities. An energy dependence of the dose ratios was observed for both photons and electrons.

[Registry of activities of the Hemodynamics and Interventional Cardiology Section in 1993]. / Registro de actividad de la Sección de Hemodinámica y Cardiología Intervencionista del año 1993.

Results of the Spanish Registry for Interventional Cardiology 1993 are presented, as previous years, by the Section of Haemodinamics and Interventional Cardiology of Sociedad Española de Cardiología. There are 72 participating cardiac catheterization laboratories. That represents 100% of those laboratories who had activity in 1993, Public medicine (51 centers) and Private practice (21 centers). From those, 8 laboratories exclusively performed pediatric cases. There have been performed 47353 diagnostic procedures and 2647 endomiocardial biopsies. In diagnostic cases greater proportion corresponds to coronariography (37591, 75.2%). Therapeutic interventionalism is also accomplished in 61 laboratories, on which 7807 balloon PTCA, 535 coronary atherectomies, 825 mitral valvotomies were performed, and 503 endocoronary prostheses were implanted. Mean rate of coronary interventionalism was 222 procedures by a million of inhabitants. This activity represents approximately a 17% increase from 1992, in diagnostic as well interventional procedures. Results of therapeutic cases did not show any significant changes, balloon PTCA primary success rate is 91.5%, with an incidence of 4.2% of complications that includes 0.7% mortality rate.

Dosimetric accuracy at low monitor unit settings.

Dosimetric accuracies at low monitor units are evaluated for linear accelerators from various manufacturers. A large error is observed in the majority of the accelerators. The error can be positive or negative. Although the error can exceed 20% for the first few monitor units, it is usually less than 5% when more than 10 monitor units are delivered. When low doses are required proper precautions should be taken for dosimetric accuracy including the beam energy, beam flatness and dose per monitor unit.