Measuring linac photon beam energy through EPID image analysis of physically wedged fields.

PURPOSE: Electronic portal imaging devices (EPIDs) have proven to be useful tools for measuring several parameters of interest in linac quality assurance (QA). However, a method for measuring linac photon beam energy using EPIDs has not previously been reported. In this report, such a method is devised and tested, based on fitting a second order polynomial to the profiles of physically wedged beams, where the metric of interest is the second order coefficient α. The relationship between α and the beam quality index [percentage depth dose at 10 cm depth (PDD10)] is examined to produce a suitable calibration curve between these two parameters. METHODS: Measurements were taken in a water-tank for beams with a range of energies representative of the local QA tolerances about the nominal value 6 MV. In each case, the beam quality was found in terms of PDD10 for 100 × 100 mm(2) square fields. EPID images of 200 × 200 mm(2) wedged fields were then taken for each beam and the wedge profile was fitted in MATLAB 2010b (The MathWorks, Inc., Natick, MA). α was then plotted against PDD10 and fitted with a linear relation to produce the calibration curve. The uncertainty in α was evaluated by taking five repeat EPID images of the wedged field for a beam of 6 MV nominal energy. The consistency of measuring α was found by taking repeat measurements on a single linac over a three month period. The method was also tested at 10 MV by repeating the water-tank crosscalibration for a range of energies centered approximately about a 10 MV nominal value. Finally, the calibration curve from the test linac and that from a separate clinical machine were compared to test consistency of the method across machines in a matched fleet. RESULTS: The relationship between α and PDD10 was found to be strongly linear (R(2) = 0.979) while the uncertainty in α was found to be negligible compared to that associated with measuring PDD10 in the water-tank (± 0.3%). The repeat measurements over a three month period showed the method to be reasonably consistent (i.e., well within the limits defined by local QA tolerances). The measurements were repeated on a matched machine and the same linear relationship between α and PDD10 was observed. The results for both machines were found to be indistinguishable across the energy range of interest (i.e., across and close to the thresholds defined by local QA tolerances), hence a single relation could be established across a matched fleet. Finally, the experiment was repeated on both linacs at 10 MV, where the linear relationship between α and PDD10 was again observed. CONCLUSIONS: The authors conclude that EPID image analysis of physically wedged beam profiles can be used to measure linac photon beam energy. The uncertainty in such a measurement is dominated by that associated with measuring PDD10 in the water-tank; hence, the accuracies of these two methods are directly comparable. This method provides a useful technique for quickly performing energy constancy measurements while saving significant clinical downtime for QA.

The relationship between the mitochondrial gene T-urf13 and fungal pathotoxin sensitivity in maize.

Mitochondria isolated from maize containing cms-T cytoplasm are specifically sensitive to pathotoxins (T-toxins) produced by the fungi Bipolaris maydis race T and Phyllosticta maydis. T-toxins interact with a 13 kDa membrane-bound toxin receptor protein, URF13, to produce hydrophillic pores in the membrane. Expression of URF13 in Escherichia coli produces bacterial cells that form hydrophillic pores in the plasma membrane when exposed to T-toxin or methomyl. Topological studies have established that URF13 contains three membrane-spanning alpha-helices, two of which are amphipathic and may contribute to pore formation. URF13 specifically binds T-toxin in a cooperative manner. Oligonucleotide-directed mutagenesis of URF13 led to the isolation of methomyl/T-toxin-resistant mutations at 39 separate positions throughout the URF13 primary sequence. Chemical cross-linking of URF13 demonstrated the presence of URF13 oligomers and established that the pore-forming species is oligomeric. The ability of the carboxylate-specific reagent, dicyclohexycarbodiimide to cross-link URF13 has been used in conjunction with site-directed mutagenesis to establish that the URF13 tetramer has a central core consisting of a four-alpha-helical bundle that may undergo a conformational change after T-toxin or methomyl binding. Experimental evidence indicates that URF13 acts as a ligand-gated, pore-forming T-toxin receptor.

The people and the tiger.

PIP: Wildlife protection activities in Ranthambhore in Rajasthan state in 1993, and conditions of overpopulation and expanded grazing were described. Project Tiger is an ambitious program begun in the 1970s to protect the habitat of the tiger in Ranthambhore National Park. Forest park protection is limited by corrupt, poorly trained staff, who are unmotivated to protect the established sanctuaries. Tiger counts declined from 45 in the 1980s to 25 in 1993. Between 1988 and 1993, the Ranthambhore Foundation has worked to change village attitudes and practices that are based on forest exploitation. The aim was to show how the park and livelihoods can coexist without destruction of the park through excess grazing, and to initiate the planting of trees in a largely arid, treeless region. Demonstrating new ways of living had to be achieved first by building trust. Thapar, an author and environmentalist, has established his own land as an example of how irrigation and fallow land can return the land to productive potential. A mix of Jersey and Holstein cattle are bred to show how nonrange-fed cattle can be produced and deliver higher milk yields than the straggly range-fed ones. A dairy cooperative has been organized in the district, and the milk productivity has increased 1000%. Several bio-gas cookers are used to demonstrate the effectiveness of dung for fuel, instead of wood from the forest. Women become aware of the advantages of shorter walks and the end of potential arrests from taking forest protected fuelwood. There is still work to be done in learning how to work with local people. One foundation program works with instilling ideas about conservation among 3500 village children. Forest Protection Societies have been organized to grow and defend their own trees. Village men are organized to help police unauthorized grazing in protected areas. Not all programs have been successful, and lax administration has been a problem. Tourism has become a problem of too many parked along protected routes. In the past year, two tiger mothers have produced 5 cubs, which increased the population 25%; this is a success against poachers, lax park management, woodcutters, and graziers.^ieng

The protein-encoding gene T-urf13 is not edited in maize mitochondria.

RNA editing of T-urf13, a gene specific to the mitochondria of cytoplasmic male-sterile, type-T (cms-T) maize, and an adjacent, cotranscribed gene orf221, have been studied by cDNA sequencing. No editing was detected in 22 cDNA clones. This is the only report of a polypeptide-encoding gene in higher-plant mitochondria that is not edited. T-urf13 may not be edited because it is derived largely from the coding and flanking regions, which are rarely edited, of a ribosomal RNA gene. orf221 is edited; however, the similarity between the predicted amino acid sequences of orf221 in cms-T and normal cytoplasms is not increased.

Legal standards of causation in chemical exposure litigation.

A discussion of the legal process that resolves disputes involving the questions of causation of exposure to chemical substances is presented. Toxicologists and pharmacologists called upon to testify in private litigation of whether or not exposure to a particular chemical caused an illness will have a clearer view of the law's notion of causation by the examples cited. The trends in the law on this subject are reviewed and ways are offered for scientists to influence future chemical exposure litigation.

Drug abuse in anesthesia training programs. A survey: 1970 through 1980.

Experience at our institution with drug dependence among anesthesia residents, coupled with a lack of published data, prompted us to survey US anesthesia training programs. Two hundred eighty-nine programs were surveyed, 247 (85.5%) responded, and 184 (74%) of these programs had at least one suspected incident of drug dependence to report. Three hundred thirty-four confirmed persons were reported, including a substantial number of instructors. Meperidine and fentanyl were the most frequently mentioned drugs. Behavior changes were frequently noted by staff personnel, and in general such changes led directly to detection. After confirmation of abuse, the majority of impaired anesthetists were referred for psychiatric care, with few in need of actual detoxification. Detailed follow-up was available for about 40% of the total; 71 persons were offered a return to their original place of employment, while 30 persons died of drug overdose. Chemical impairment may be more common than usually thought in anesthesia, perhaps in part because of drug availability.