An analysis of some dosimetric uncertainties in radiation therapy.

A phenomenological analysis of the effect of several parameters on the precision of delivered dose is presented; experimental data are used to demonstrate the effect of these parameters on the delivered dose. It is shown that some setups possess inherently better precision than others; the need for strict quality control during patient treatment is supported by the data.

Acceptance testing of medical imaging equipment: CT and MRI.

Acquisition of medical imaging equipment should be followed by verification of its ability to satisfy clinical objectives. This verification normally is fulfilled by initial acceptance testing before clinical use and by an ongoing quality-assurance program thereafter. This article describes our experience with the acceptance testing of computed tomography scanners and magnetic resonance imaging systems.

A quality-assurance program for mobile CT scanners.

This article describes our six-year experience with several mobile computed tomography scanners. Our quality-assurance program includes acceptance testing, a digital image-performance program, evaluation of scanner reliability, and consideration of human/environment factors.

A technique for field matching in primary breast irradiation.

The intrinsic divergence of photon beams presents serious matching problems in three-field treatment of the breast and the adjoining supraclavicular area. A method is presented in which appropriate beam blocking combined with suitable isocentric rotation of the treatment couch neutralize the affects of divergence so that proper matching is achieved at all depths. The geometric principles and the set-up procedures are discussed and illustrated.

Dose to lung in primary breast irradiation.

Using anatomic data derived from computerized tomography (CT) scans of the torso, the volume of lung irradiated during primary breast treatment has been measured for a variety of irradiation techniques. Two-field tangential plans which are angled into lung to treat also the internal mammary nodes have been compared to three-field plans which include a separate internal mammary field (IMF). The volume of lung achieving high dose (greater than 3000 rad) is similar in both techniques when photons only are used. Electron beam treatment of the IMF is successful in lowering the lung dose. Additional treatment plans that angle the IMF parallel to the tangential fields may offer some theoretical advantage.

A system for electron therapy dosimetry surveys with thermoluminescence dosimeters.

Radiation-therapy dosimetry surveys employing thermoluminescence dosimeters (TLDs) are now being considered for high-energy electron beams. Using a system of individually calibrated pressed LiF TLDs in a water and a polystyrene phantom, we established that the distortions of depth-dose distributions in non-conducting materials previously observed at high absorbed doses and high dose rates were not detectable in the present geometry at doses and dose rates as much as 40 times higher than those employed in radiation therapy. The system was then used to measure TLD response in water and in polystyrene in the nominal electron-energy range from 7 to 18 MeV. In the water phantom, the well-known trend for TLD response to decrease with increasing electron energy was observed. In the polystyrene phantom, TLD response was found to be independent of electron energy.

The time course of radioprotection by WR 2721 in mouse skin.

The radioprotective effect of 400 mg/kg of WR 2721 on mouse skin has been investigated over a series of times from 5 to 60 minutes after intravenous or intraperitoneal injection of the drug. The acute desquamation reaction on the hind leg of white mice was studied. Dose response curves were obtained for the average reaction over 15 to 25 days after single-dose irradiation. A high dose-rate electron beam (13--17 Gy/min) was used to minimize the irradiation time. Single doses of 20 to 60 grays were given. Dose modifying factors (DMFs) of 1.7--2.1 were obtained at 30 to 60 minutes, but only 1.1 to 1.3 at 5 minutes and intermediate values at 10 and 15 minutes. DMFs rose slightly faster and to slightly higher values after i.v. than after i.p. injection of WR 2721. We conclude that 30 minutes is the shortest interval which should be used between injection of Wr 2721 and irradiation with this normal tissue.

The stability of teletherapy-beam symmetry with gantry angle.

The authors describe a rapid means of evaluating the stability of beam symmetry as the gantry angle is varied. Two radiation detectors, a ratio circuit, and an X-Y recorder are used. The teletherapy machine is operated in the rotational mode, and the ionization-current ratio is recorded as a function of the gantry angle. This test can provide an early warning of beam-symmetry instabilities.

Localization in interstitial dosimetry utilizing the CT scanner.

A simple method utilizing CT scanners which were readily available is described in which alignment of the isodose curves can be made directly on the printout of the CT scan. This has obvious practical and theoretical advantages. In particular, the dose to vital structures, such as spinal cord and other critical viscera, can be well described.

Electron contamination of a high-energy X-ray beam.

The Lorentz force on an electron is utilised to separate out the electron component from the open beam of a Clinac-18, operating in the X-ray mode. The experimental arrangement is such that the mixed beam (comprising the primary photons, the scattered photons and the contamination electrons) enters an external magnetic field, which is perpendicular to the central axis of the beam. The electron contribution is analysed as a function of collimator setting, depth and distance. Based on the data, one is led to believe that the shift in dmax with field size is due to a relative enhancement in the soft X-ray component as the collimator jaws are opened.

Experience with a pair of matched diodes for constancy checks.

A unit is discussed which employs a pair of matched silicon diode detectors to check the constancy of the output, beam quality, field flatness, and symmetry of teletherapy equipment. The unit has proven useful, but the long-term stability of the diode-amplifier system must be considered, because two years of experience has demonstrated significant sensitivity changes in the diodes.

Field flood uniformity correction: benefits or pitfalls?

Strict quality control of scintillation camera images is increasingly recognized as important in a nuclear medicine laboratory. The field flood uniformity of the camera should be tested daily. A variety of commercially available data systems facilitate the above task. Concomitantly, scintigrams are increasingly being corrected for uniformity, namely, unbalanced photo-multiplier tubes, uniformity correction can occasionally, depending on the scatter fraction, produce clinically significant artifacts. This effect is due to the application of a linear correction factor to a nonlinear phenomenon.

Size distribution of human albumin microspheres by forward scattering of monochromatic light.

Optical diffraction is used to study the structure of the Fourier transform of a thin suspension of human albumin microspheres. The intensity profile in the transform plane is then used to derive information about the particle size distribution.

Quantitative assessment of field uniformity for gamma cameras.

Objective criteria for judging the uniformity of field floods are developed and the effects of the energy window and scatter material on uniformity are examined. There is a highly significant correlation (p = 0.99) between a 14% difference in average count among adjacent photomultiplier tube regions and observer detection of detuned areas. Similarly, a 10% average count difference is not significantly recognized as a nonuniform area. Results indicate that there is a complex interaction between uniformity, energy window (15-30% symmetrical) and 0-7.6 cm of tissue-equivalent scatter medium interposed between the flood source and the collimator. Quantitation of the flood should be performed to indicate which regions need retuning.

Liver size determination in pediatrics using sonographic and scintigraphic techniques.

Knowledge of the normal liver size is essential for making the scintigraphic estimate of hepatomegaly. A nomogram for sonographic liver size versus height of the patient was developed for the normal pediatric patient. Liver size was measured as the longitudinal liver length in the plane midway between the xiphoid and the right lateral liver margin. Scintigraphic and sonographic measurements showed a good correlation. The scintigraphic nomogram was developed using the experimentally determined relationship between the two modalities.