Therac 25 Case Study Ppt Templates

This will be a dark and tragic lesson.

It's about the Therac-25 radiation therapy machine.

How it was poorly designed.

And how it killed patients as a result.

The events that took place happened between 1985 and 1987.

An IEEE article which analyzed the failures appeared in 1993.

Although the article is now 24 years old, it is a reminder of what

can happen if development, particularly design, is shoddy.

I think when we're done with this case study, you'll see that

the good design practices advocated previously would have prevented many,

if not all of all of the problems.

Some background, radiation therapy in cancer treatment uses X-rays to disrupt or

destroy the DNA of cancer cells, preventing them from reproducing.

The high energy photons impart some of their energy to the chemical bonds

holding the DNA together, fracturing it so that it can't be copied.

Of course the high energy would also disrupt the DNA of normal cells.

So focus, intensity, and

the distribution of the X-ray beam is critical in radiation therapy.

The Therac-25 was built by the Atomic Energy of Canada Limited and

a French company called CGR.

These two companies had collaborated since the early 1970s in building

linear accelerators for medical applications.

The previous product to the Therac-25 was the Therac-6,

a 6 million electron volt accelerator.

The Therac-25 was produced along with another machine,

the Therac-20, both being derived from the Therac-6 model.

The 20 and 25 models had 20 and 25 million electron volt accelerators respectively.

So they were much more powerful.

Also, the Therac-6, in addition to being much lower powered,

was largely a mechanical system rather than one governed by software.

As such, the settings and

safety interlocks were physical rather than virtual in nature.

My hands start to sweat at this point.

The computer hardware for the system was the DEC PDP-11.

By the mid 1980s, this was a venerable machine,

having been around since the early 1970s.

It had a 16 bit processor and a very large instruction word instruction set.

This was not uncommon for machines of this era.

But by the mid 1980s, the PDP-11 was in decline,

because it was a 16-bit architecture.

And 32-bit CPUs were coming into production.

In fairness, the PDP-11 wasn't a bad choice,

since the first Therac-25 had been prototyped in 1976.

And the first production machine was manufactured in 1982.

A single programmer produced the software for the Therac-25.

He resigned from AECL in 1986 in response to a lawsuit that was filed.

And no information about him has ever been obtained.

Let me say something about that.

In the early 1980's, I wrote C on Unix for the PDP-11 and

it's 32-bit follow on, the VAX, or virtual address extension machines.

I was familiar with PDP-11 assembly.

And the instruction set, while extremely flexible, is difficult to read.

At that time, there weren't a lot of people with programming skills.

In fact, the first PDP-11 worked on, that I had worked on, I'd built from a kit.

It didn't even come assembled.

So if you wanted to program it, you had to put the thing together.

It was also a time where there were few standards.

And nothing approaching the idea of software engineering crossed

people's minds.

I consider it a management and a design fault, however, not to have used

a standard operating system which had a proven track record of correct execution.

One of the eventually discovered, failure patterns in the Therac-25

software had to do with the operating systems having no test-and-set mechanism,

which is critical to being able to write multi-tasking, multi-threaded code.

At that time, writing operating systems was not a common skill.

Certainly, there were people who did an excellent job of it, but

they weren't running around loose as programmers for hire.

We're all familiar with the bravado that comes from the frontline programmer.

Yeah, sure, I can do that.

The six serious overdose cases occurred between 1985 and 1987.

The approval for

use of the machine was withdrawn by the Food and Drug Administration in 1987.

In each case, a machine malfunction,

which always had a software component, caused patients to receive instant

radiation burns from receiving doses hundreds of times higher than prescribed.

When a malfunction occurred, the machine would either go into a pause state,

such that treatment could be restarted by entering the letter p on the operator

screen, or the machine could also go into a reset state, which all

the prescription information, the dosing information, had to be reentered by hand.

The radiation prescription information included radiation mode,

electron beams for low energy, and X-rays for high energy levels,

dose, dose rate, time, the field size,

the gantry rotation, and data for running accessories added to the machine.

This information originally had to be entered twice.

And it had to match or the operator would have to start over.

Two of the cases, however, resulted from the operator,

upon discovering an inaccurate entry, changing the parameters quickly.

The lack of integrity in the operating system

produced raced conditions between the routine that read characters from

the operator console and the routine that recorded and processed them.

This particular error was so

subtle that a number of other false errors were found and

fixed, only to have the original overdose problem reappear.

There are a number of side stories, which you can investigate if you're interested.

One is the manufacturer refusing to believe that it was their problem.

Another was finding what they thought was the problem and fixing the wrong thing.

A third was lack of exchange of information in treatment accidents.

But insofar as the software engineering aspect is concerned,

here are the main takeaways.

Documentation should not be an afterthought.

Software quality assurance practices and standards should be established.

Designs should be kept simple.

Two more points, first about security.

I can see that some may say, well, this might have been about safety, but

it's not about security.

Security is about two things, proper function and

proper handling of malfunctions.

And that's what this case is about.

Second, about radiation treatment, in case you or your loved ones, heaven forbid,

need radiation treatment, a lot has changed since the Therac-25.

Machines are not that much more powerful, but

the systems are much better engineered.

Radiation planning is done by modeling the volume of a tumor using computer

tomography or magnetic resonance, or both.

A set a treatment parameters describing the minimum radiation to be

received by the tumor volume and

the maximum radiation to be received by surrounding tissue is put into the system.

A computer program runs simulations of the machine to discover

which of literally hundreds of beam parameters will best radiate

the affected area without radiating the surrounding tissue.

Once the treatment program has been generated, a high fidelity simulation is

run to verify that the computed parameters are correct.

Finally, a three-dimensional radiation sensing array is placed on

the treatment table.

And the radiation program is run.

The collectors and

the radiation sensing array verify that the computer model is correct.

And then treatment can proceed.

At no point during this process is data manually entered, with the possible

exception of the description of the tumor volume to be radiated.

This is generally done on high resolution screens

which overlay planned tumor volume with other imagery to ensure accuracy.

From there on, data is simply transferred from system to system and

no hand entry is performed.

So if you or someone you know is in the position of needing radiation treatment,

ask for an explanation.

The doctors and technicians should be happy to show their knowledge and

explain to you what's happening.

Okay, so, not to forget,

secure design is just good design.

Thanks.

We'll see you next time.

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presentations for free. Or use it to find and download high-quality how-to PowerPoint ppt presentations with illustrated or animated slides that will teach you how to do something new, also for free. Or use it to upload your own PowerPoint slides so you can share them with your teachers, class, students, bosses, employees, customers, potential investors or the world. Or use it to create really cool photo slideshows - with 2D and 3D transitions, animation, and your choice of music - that you can share with your Facebook friends or Google+ circles. That's all free as well!

For a small fee you can get the industry's best online privacy or publicly promote your presentations and slide shows with top rankings. But aside from that it's free. We'll even convert your presentations and slide shows into the universal Flash format with all their original multimedia glory, including animation, 2D and 3D transition effects, embedded music or other audio, or even video embedded in slides. All for free. Most of the presentations and slideshows on PowerShow.com are free to view, many are even free to download. (You can choose whether to allow people to download your original PowerPoint presentations and photo slideshows for a fee or free or not at all.) Check out PowerShow.com today - for FREE. There is truly something for everyone!


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