Pulmonary embolism in pregnant patients: a survey of practices and policies for CT pulmonary angiography.

OBJECTIVE: We surveyed the practices and policies of the radiology departments of the Society of Thoracic Radiology members regarding the use of CT pulmonary angiography in pregnant patients suspected of having pulmonary embolism. MATERIALS AND METHODS: Surveys were mailed electronically to the 403 members of the Society of Thoracic Radiology (403 addresses). Respondents were asked to send one response from each institution or department. Information gathered included use of CT angiography in relation to ventilation-perfusion imaging in pregnant patients, written policies, informed consent procedures, and modifications of standard protocols for dose reduction. RESULTS: Fifty-seven members responded; 43 (75%) reported that they perform CT angiography in pregnant patients suspected of having pulmonary embolism. Of the 43 respondents who perform CT angiography in pregnant patients, 23 (53%) generally perform CT angiography as the initial study rather than ventilation-perfusion scanning, 26 (60%) require informed consent from the patient, seven (16%) have a written policy concerning CT angiography in pregnant patients, and 17 (40%) modify standard imaging protocols for pregnant patients. The most common modification for dose reduction is decreasing the scanning area along the z-axis. CONCLUSION: Most respondents perform CT angiography in pregnant patients suspected of having pulmonary embolism, but their policies and practices vary considerably.

Proton dosimetry in bone using electron spin resonance.

We have studied the ESR response of proton-irradiated (in vitro) bone. The ESR response as a function of proton (E = 105 MeV) dose to bone was linear from 0 to 50 Gy and similar to the photon (E = 6 MV) dose response. The ESR depth response (Bragg) curve was depressed as compared to a depth-response curve determined with a parallel plate ionization chamber (PPIC). There was a short-term ESR signal fade in the Bragg peak region, likely attributable to the organic component in bone. We are continuing to investigate these latter two effects.

Spatial distribution of bremsstrahlung in a dual electron beam used in total skin electron treatments: errors due to ionization chamber cable irradiation.

The spatial distribution of the bremsstrahlung dose in total skin electron therapy and large electron fields has been assumed to be symmetric along the match line; however, the measured data in the entire plane have not been available. Das et al. ["Spatial distribution of bremsstrahlung in total skin electron irradiation," Med. Phys. 19, 810 (1992)] reported the ion-chamber measured bremsstrahlung data in the Stanford six-field technique and found that the bremsstrahlung dose profile was 2.5 times greater in the caudal plane (beam facing toward floor) as compared to the cephalic plane. This observation was recently quoted by Klevenhagen [Physics and Dosimetry of Therapy Electron Beams (Medical Physics, Madison, WI, 1993)]. Further study using thermoluminescent dosimeters and careful measurements using ion chambers showed that the bremsstrahlung radiation was indeed symmetric at the match line as theoretically predicted and was not a function of the beam angle or the type of floor. The earlier observations of Das et al. were found to be due to cable effects. Ionization chamber cable response in a large electron field and the associated errors in the bremsstrahlung distribution are presented. It is suggested that a cable-free dosimeter should be used for measuring low levels of bremsstrahlung radiation in large electron fields and at extended distances.

Spectral energy effects in ESR bone dosimetry: photons and electrons.

The spectral energy-dependence of the radiation-induced ESR signal has been studied in ovine cortical bone. Crushed bone samples were irradiated using photon beams with effective energies in the range from 0.06 to 6 MeV, and electron beams with mean energies in the range from 2 to 10 MeV. The photon and electron data were normalized to a dose to bone of 50 Gy and the results are reported as response relative to the ESR signal for photon irradiation at 1.25 MeV (60Co). The photon irradiation results show that the ESR response is greatest at low energies with a relative value of 1.2 at 0.06 MeV. The relative response decreases, as the energy increases, to approximately 0.85 in the region of 2 to 3 MeV. These variations in the relative ESR responses are significantly less than the ESR energy-dependent responses reported in the literature for human tooth enamel and synthetic hydroxyapatite. An explanation for this difference is offered. For electron beam irradiations, the ESR signal is fairly constant with energy, and approximately equal to that at a photon energy of 1.25 MeV. Implications of these results are discussed.

Electron beam modifications for the treatment of superficial malignancies.

For the treatment of superficial tumors, the surface dose should be high; unfortunately, because of pronounced dose buildup in low energy electron beams, their efficacy for such treatment is reduced. Electron beams can be modified by placing a low atomic number material called a beam spoiler in the beam. In general, the surface dose is a function of electron energy, source to surface distance, field size, thickness of beam spoiler, distance of beam spoiler from surface, atomic number of beam spoiler, and angle of the beam. The effects of these parameters are evaluated with respect to surface dose, bremsstrahlung dose, and field size changes for small fields at standard SSD and electron energies from 6 to 17 MeV. It was found that the use of a beam spoiler can generally increase the surface dose to values exceeding 90% of the maximum buildup value while maintaining a bremsstrahlung dose less than 3%. Changes in field size related to the placement of the beam spoiler were considerable in some cases.