By I. Basir. Teachers College. 2018.
The two elastic scattering processes accounts for less than 10 % of the interactions in the diagnostic energy range purchase clomiphene 25mg on line. The purpose for discussing these details about absorption and scat- tering is to give some background knowledge of the physics of the x-ray picture generic clomiphene 25mg line. It is differential attenuation of photons in the body that produces the contrast which is responsible for the information. The attenuation of the radiation in the body depends upon; the density, the atomic num- ber and the radiation quality. In mammography one are interested in visualizing small differences in soft tissue – and we use low energy x-rays (26 – 28 kV) to enhance the tissue details. In the case of chest pictures the peak energy must be larger because the absorbing body is very much larger – and some radiation must penetrate the body and reach the detector. It is the transmitted photons that reach the detector that are responsible for the picture. The detector system A number of different detectors (flm, ionization chambers, luminescence and semiconductors) have been used since the beginning of x-ray diagnostic. The x-ray picture was created when the radiation was absorbed in the flm emul- sion consisting of silver halides (AgBr as well as AgCl and AgI). In the usual morning meeting the doctors were often gath- ered in front of the “light box” to discuss the patients (see illustration). Consequently, in order to increase the sensitiv- ity, intensifying screens were introduced. The screen is usually a phosphor scintillator that converts the x-ray photons to visible light that in turn expose the flm. The introduction of intensifying screens was made already in 1896 by Thomas Alva Edison. He introduced the calcium tungstate screens which were dominating up to the 1970-ties. We do not intend to go through the technical details with regard to intensifying screens – nor to the many technological details within x-ray diagnostic. In order to ensure that the photoelec- tric effect is dominant lower energies are used. Energies lower than 30 kV are used for mammog- raphy – which is very effective for seeing details in soft tissue. However, this energy range is only useful for tissue thicknesses of a few centimeter. Mammography X-ray tube In mammography the goal is to see the contrast between different den- sity of soft tissue, fat and blood ves- sels without use of contrast media. The x-ray energy is between 25 and 30 kV in order to ensure that the photoelectric effect is dominant. This also result in absorption of ra- diation and an increase of the patient dose. Detector 181 Examples Tumor It is sometimes very convincing to see a mammogram like that shown to the right. It is also amazing that we can see details like this in soft tissue without using contrast media to enhance the difference in electron density. To the left is a modern digital picture whereas the other is a flm-based mammography. Implants Muscle It is obvious, even for the layman, that the presence of breast implants does interfere and makes it more diffcult to obtain good information with mammography. The presence of implants affects the way mammograms are done, since additional views are needed during routine screening mammography to visualize all of the breast tissue. The lesson to learn from this is that implants could be an impediment to cancer detection. Implant We can conclude that you have to be well trained to give a good de- scription.
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The red stars represent plans for which one cannot improve any of the two criteria without diminishing the other cheap clomiphene 25 mg without a prescription. It is observed that a radiation treatment prescription commonly contains multiple buy clomiphene 50 mg with mastercard, mutually conﬂicting objectives. In general, the goal of full target coverage is set against the need to spare healthy tissues and organs at risk. The relative weighting of these different treatment objectives represents a trade-off that is seldom expressed specifically in the prescription. Instead, this trade-off is usually explored by investigating multiple treatment plans, either from a pre-calculated database or, more often, in an iterative process. The Pareto evaluation concept is based on a set of Pareto optimal solutions/treatment plans. The definition of a Pareto optimal solution, in this context, is the fact that one objective cannot be improved without worsening another objective (see Fig. The plans can be Pareto optimal from a mathematical or from a clinical point of view. The mathematical Pareto front is often used in the optimization to find the best solution. The clinical method is used to compare different techniques for the same patient or to visualize the trade-off between contradicting organs and tumours. Further information regarding plan evaluation and the uncertainties in this sub-process are described in Refs [3, 4]. Tools Delineation conformity can be achieved by consensus discussion with groups of radiation oncologists locally within a department. However, it can be more advantageous if, for example, several hospitals in a region perform such tasks together. An interesting paper was recently published from Canada regarding plan quality and the relationship with the experience of the radiation oncologist . One should remember that rounds offer a great opportunity for education of all participants. Especially the latter may be of importance for the individual patient concerning positioning accuracy, intra- and inter movements, etc. Transfer of data In this case, only the transfer from an approved treatment plan to the control or record and verify system is discussed. In the first case, the information is kept within the same vendor’s environment and for the user it appears as though all the information is available from the same source. The opposite solution is having data within different systems which requires that information has to be exported from one system and then imported to the next system through a process which requires certain quality controls to ensure correct data transfer. The first solution should, in principle, be the safest method from a patient’s view; however, accidents have occurred where information was lost between treatment planning and delivery systems in such an environment (cf. Other problems that have also been reported are when an old method for data transfer still exists after the introduction of new systems (see Glasgow accident [7, 8]). The hard or technical solutions can be watch-dogs or independent dose calculation (included in the linac/control system asking the operator whether they really want to deliver this dose to the patient), and, in many cases, an integrated environment will improve safety. The soft solutions include awareness, training, knowledge and understanding, and not forgetting communication among all staff involved in radiation oncology. Commissioning The commissioning part of a medical device, such as a linear accelerator with the capabilities of delivering high doses within a very short time period, is one of the most critical steps in radiation oncology. Errors made at this stage will give rise to systematic deviations for the lifetime of the equipment. Such errors have occurred repeatedly; a couple of examples are given: 60 (a) Exeter (1988): error during calibration of a replaced Co source, measurements performed at 0. The physicist managed it as a linear accelerator but for calculation of output factors for field limiting cones 2 other than 10 × 10 cm the backscatter factor was missing, leading to dose differences of up to 10% in specific cases; most patients were undertreated . It should be noted that in these accidents, as well as in others, only a single physicist performed the duties, and neither double-checking appears to have occurred nor any internal or external audit. In the Exeter case, it was the national audit in the United Kingdom that discovered the problem.