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  • Some final comments
    it requires perhaps infinite deliberation before every act Rule utilitarian approaches skirt this difficulty but have their own problems We might do this exercise with the other two ethics tests and come to similar conclusions Thus if the only goal of a course is to teach students the complexities of these ethical approaches we would not recommend using these tests though our perverse imaginations suggest ways even this might be done These tests are designed for applied courses whose goals are to help students learn to think ethically within their profession see our comments on goals in ethics courses There is good reason to believe that it is a bad idea to treat these ethical approaches as foundational rules from which correct answers can be derived see for example Stephen Tolumin The Tyranny of Principles The Hastings Center Report 1981 Stephen Tolumin How Medicine Saved the Life of Ethics Perspectives in Biology and Medicine 25 4 Summer 1982 As you can see from this essay this is not the way we think these tests should be used And therefore we think that their direct correspondence to the ethical approaches becomes less of an issue These are guidelines not first principles in a deductive solution Finally we do not think these test should be used alone Much research on moral reasoning suggests that experts in professional ethics spend much of their time thinking at an intermediate level about the ethical issues at stake in a case see for example Keefer M Ashley K D 2001 Case based Approaches to Professional Ethics A Systematic Comparison of Students and ethicists Moral Reasoning Journal of Moral Education 30 377 398 Bebeau M J Thoma S J 1999 Intermediate Concepts and the Connection to Moral Education Educational Psychology Review 11 4 343 360 So we

    Original URL path: http://computingcases.org/general_tools/teaching_with_cases/ethics_tests/final_comments.html (2016-04-30)
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  • Therac Abstract
    setup easier the computer also monitored the machine for safety With the advent of computer control hardware based safety mechanisms were transferred to the software Hospitals were told that the Therac 25 medical linear accelerator had so many safety mechanisms that it was virtually impossible to overdose a patient Normally when a patient is scheduled to have radiation therapy for cancer he or she is scheduled for several sessions over

    Original URL path: http://computingcases.org/case_materials/therac/supporting_docs/therac_case_narr/therac_abstract.html (2016-04-30)
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  • Therac Case Intro
    doctor decides that a patient needs radiation therapy that patient is given a prescription that indicates to the medical linear accelerator operator how many rads radiation absorbed dose the patient should receive over how many total treatments In addition the prescription indicates the location where the radiation should be applied The patient schedules a time or times to receive treatment Standard procedures then determine whether on any particular appointment the operator is to set up the equipment for electron or X ray beam treatment The patient is asked to lie in the appropriate position on the treatment table and the table is rotated to place the diseased part of the patients body in the path of and at the appropriate distance from the radiation beam The operator then does whatever mechanical setup is required and leaves the room to program the treatment data into the machine After doing this the operator presses the button that actuates the treatment routine The patient is then helped off the treatment table and ushered out After the appropriate forms have been filled out the next patient is admitted Therac 25 s computerization made this laborious process much easier for operators and allowed them to

    Original URL path: http://computingcases.org/case_materials/therac/supporting_docs/therac_case_narr/therac_case_intro.html (2016-04-30)
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  • How Radiation Therapy Works
    the most noticeably hurt tissues after treatment and treatment may produce skin lesions and hair loss These tissues have cells that rapidly divide and the radiation halts their development But they are usually able to recover from this assault and return to normalcy Nevertheless skin lesions and hair loss are not an unusual side effect of radiation therapy What a Treatment Session is Like Radiation therapy is usually done in a series of sessions occurring over several weeks allowing the effect of the radiation to build up over time The treating doctor will determine the specific number of treatments the dosage at each treatment and the schedule During treatment the doctor will usually see the patient once a week to check on general health side effects and the progress of the treatment Before the series of treatments occurs a radiation therapy technician will outline the specific area to be treated with a marking pen indelible ink or silver nitrate Depending on the body area to be treated the patient would need to remove his or her clothing and put on a hospital gown After going to the radiation therapy room they would then either lie on a treatment table or

    Original URL path: http://computingcases.org/case_materials/therac/supporting_docs/therac_case_narr/therac_case_radtx.html (2016-04-30)
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  • Basic Principles
    space Getting the Beam into the Body Patients can be treated directly with the resulting electron beam as long as the beam is spread out by scanning magnets to produce a safe level of radiation The medical linear accelerator spreads and directs the beam at the appropriate place for treatment The picture below shows a typical medical linear accelerator in operation But a difficulty with the electron beam is that it diffuses rapidly in tissue and cannot reach deeper tissue for treatment The picture below is a simulation produced by the Stanford Linear Accelerator Center of an electron beam traveling through air and entering human tissue You can see the beam quickly diffuses and therefore does not penetrate deeply To solve this problem Therac 25 and many other machines can switch to a mode in which X ray photons are used for treatment These penetrate much more deeply without harming intervening tissue To do this the electron beam is greatly increased in intensity and a metal foil followed by a beam flattener is placed in the path of the electron beam This transforms the electron beam into an X ray called photons in some literature This process is inefficient and requires a high intensity electron beam to produce enough X ray intensity for treatment Therac 25 used a 25 MeV electron beam to produce an X ray for treatment 25 MeV is 25 million electron volts eV an eV is the energy needed to move one electron through a potential of one volt Therac 25 was what was called a dual mode machine It could produce the low energy electron beams for surface treatment and it could also produce a very high intensity electron beam that would be transformed into an X ray by placing the metal foil in the

    Original URL path: http://computingcases.org/case_materials/therac/supporting_docs/therac_case_narr/Basic_Principles.html (2016-04-30)
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  • Therac-25 Machine Design
    through the TV monitor and the intercom The intercom works both ways that is the patient can hear the operator if the operator presses a switch and the operator can hear the patient The patient however cannot see anything outside the treatment room while the operator can look in using the TV monitor Switching Between Modes The Turntable Therac 25 is a dual mode machine This means that it can treat the patient with relatively low energy electron beams or with X ray beams In addition Therac 25 had a field light position that allowed a standard light beam to shine in the path of treatment to help the operator in setting up the machine Thus there were three modes in which the Therac 25 could operate electron beam and X ray for treatment and field light for setup Even though they are relatively low energy the electron beams are too powerful in their raw form to treat the patient They need to be spread thinly enough to be the right level of energy To do this Therac 25 placed what are called scanning magnets in the way of the beam The spread of the beam and also it power could be controlled by the magnetic fields generated by these magnets Thus for electron beam therapy the scanning magnets needed to be placed in the path of the beam X ray treatment requires a very high intensity electron beam 25 MeV to strike a metal foil The foil then emits X rays photons This X ray beam is then flattened by a device below the foil and the X ray beam of an appropriate intensity is then directed to the patient Thus X ray therapy requires the foil and the flattener to be placed in the path of the electron beam The final mode of operation for Therac 25 is not a treatment mode at all It is merely a light that illuminates the field on the surface of the patient s body that will be treated with one of the treatment beams This field light required placing a mirror in place to guide the light in a path approximating the treatment beam s path This allowed accurate setup of the machine before treatment Thus for field light setup the mirror needed to be placed in the path where one of the treatment beams would eventually go In order to get each of these three assemblies scanning magnets or X ray target or field light mirror in the right place at the right time the Therac 25 designer placed them on a turntable As the name suggests this is a rotating assembly that has the items for each mode placed on it The turntable is rotated to the correct position before the beam is started up This is a crucial piece of the Therac 25 machine since incorrect matching of the turntable and the mode of operation e g scanning magnets in place but Electron beam turned on high for

    Original URL path: http://computingcases.org/case_materials/therac/supporting_docs/therac_case_narr/Machine_Design.html (2016-04-30)
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  • Software Design
    In addition when real time software has to monitor more than one thing changes in one area may occur while the software is responding to changes in another This is like the situation of trying to divide your limited attention to all the things you need to monitor when you are driving a car While you are watching a red light up ahead a car may have slipped into your blind spot without you seeing it So Therac software needed to track and respond to several things in real time without dropping any important balls What those things are is described in the next section Design of Software The main tasks for which the software is responsible include Operator Monitoring input and editing changes from an operator Updating the screen to show current status of machine Printing in response to an operator commands Machine monitoring the machine status placement of turntable strength and shape of beam operation of bending and scanning magnets setting the machine up for the specified treatment turning the beam on turning the beam off after treatment on operator command or if a malfunction is detected The Therac 25 software is designed as a real time system and implemented in machine language a low level and difficult to read language The software segregated the tasks above into critical tasks e g setup and operation of the beam and non critical tasks e g monitoring the keyboard A scheduler handled the allocation of computer time to all the processes except those handled on an interrupt basis e g the computer clock and handling of computer hardware generated errors As explained above the difficulty with this kind of software is the handling of things that might be occurring simultaneously For example the computer might be setting the magnets for

    Original URL path: http://computingcases.org/case_materials/therac/supporting_docs/therac_case_narr/Software_Design.html (2016-04-30)
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  • System Safety
    beneath the scanning magnets that spread the electron beam and one located beneath the foil that turned a high intensity electron beam into X rays These chambers monitored the amount of radiation that was being delivered to the patient in each mode electron beam or X ray and each could measure the beam intensity only within the expected range from the beam with which it was paired If the chamber detected a dose that was different from that assigned to the patient the software immediately suspended treatment If the difference was a minor amount or if the beam intensity was measured as hardly there the software might allow the operator to retry the treatment up to 5 times before shutting down completely This retry facility was added to the software because it was a regular occurrence for the beam to be slightly out of tune and for the software to suspend treatment If the beam intensity was detected to be quite different from the assigned intensity the software shut the machine down completely and required all the treatment parameters to be entered again Safety Analysis of the System In 1983 just after AECL made the Therac 25 commercially available AECL performed a safety analysis of the machine using Fault Tree Analysis This involves calculating the probabilities of the occurrence of varying hazards e g an overdose by specifying which causes of the hazard must jointly occur in order to produce the hazard In order for this analysis to work as a safety analysis one must first specify the hazards not always easy and then be able to specify the all possible causal sequences in the system that could produce them It is certainly a useful exercise since it allows easy identification of single point of failure items and the identification

    Original URL path: http://computingcases.org/case_materials/therac/supporting_docs/therac_case_narr/System_Safety.html (2016-04-30)
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