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A safety-critical software system for treating cancer patients has two main components:

A radiation therapy machine that delivers controlled doses of radiation to tumor sites. This machine is controlled by an embedded software system.

A treatment database that includes details of the treatment given to each patient. Treatment requirements are entered in this database and are automatically downloaded to the radiation therapy machine.

Identify three hazards that may arise in this system. For each hazard, suggest a defensive requirement that will reduce the probability that these hazards will result in an accident. Explain why your suggested defense is likely to reduce the risk associated with the hazard.

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Below are some of the hazards that may arise in a safety-critical software system for treating cancer patients


1. Incorrect dosage of radiation computed 

2. Radiation delivered to the wrong site on patient’s body 

3. Data for wrong patient used to control machine 

4. Data transfer failure between database and therapy machine 

Software protection: 

1. Comparison with previous doses delivered. Establishment of a maximum monthly dose which may never be exceeded. Feasibility checks (e.g. for negative dosages). Confirmation of dose to be delivered by operator. Continuous visual display of dose being delivered. 

2. Comparison with delivery site in previous treatment. Light used to illuminate site of radiation delivery. Operator confirmation of site before machine can operate. 

3. Patient asked to verify name, address and age before machine starts by pressing button. Issue patient with a personal treatment card which is handed over to identify patient. Maintain separate list of patients to be treated each day and correlate with patient databases. Force machine operator to verify list and database consistency before starting machine. 

4. Dual display of information in therapy machine and database. Highlighting of differences in operator display. Locking of machine until information is consistent. Use of check digits and other error checking codes in the data. Duplicate communication channels between machine and database. 

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Hazard 1: High dosage of radiation

Use a sensor to observe the radiation levels. Whenever the dosage levels of radiation is high or not as per the prescription, the sensor intimates, so that the radiation levels can be adjusted.

Hazard 2: Radiation leakage.

A sensor must be placed to determine any leakage of the radiation. As it monitors the leakage, it invokes an alarm whenever a leakage occurs. So the doctors can be notified and become alert.

Hazard 3: Misuse the patient information from the database.

An unauthorized user may change the patient information in the database records. It may sometimes mislead to wrong treatment. By using a restriction on the system, this can be avoided. Whenever an unauthorized user tries to access the system, it has to generate a mail and send to the administrator. It prevents the unauthorized access of patient records.

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