My story

As a kid, I was always inquisitive. I wanted to know why the sky was blue and how wind works. After being introduced to physics, I realised that it really answered a lot of those questions for me. I had a great physics teacher at school who worked as a physicist before moving into teaching, he was just so enthusiastic. I also knew I wanted to help people, but I wasn't cut out to be a doctor. So, I studied a Bachelor of Science with a physics major at the university of NSW.

My original intention was to do quantum mechanics. I loved maths, but it was conceptually just impossible to understand-I didn't want to do that for the rest of my life. I looked at astrophysics, which was fascinating to learn about-but I didn't love the research side. I wasn't cut out for academia. A woman who worked in the physics administration department (without her, the whole school would have fallen apart) was actually the one who introduced me to medical radiation physics. She suggested that it would suit me because I'd be working as an applied physicist, but I could still talk, engage and help people.

After about 10 years of study and clinical training, I'm now a radiation oncology medical physics specialist. I'm originally from the Central Coast but moved to Albury, on the New South Wales and Victorian border, to work at a cancer centre for GenesisCare. Here, we primarily treat patients with cancerous solid tumours by delivering very high energy doses of x-rays to the tumour. The x-rays are generated inside a linear particle accelerator (linac). This huge machine will speed electrons up to almost the speed of light and smash them against a heavy metal target, which produces radiation in the form of x-rays. As the resident physicist, it's our job to make sure that these machines are working correctly-to ensure that the right amount of dose is being delivered to the exact right spot. These machines are about three tonnes in weight and they rotate around a point that has to be less than one millimetre in diameter. We are also called to consult on complex patient plans. If the medical team are not sure what the best technique to use is, they will give us a call and we can make recommendations on shielding, delivery angles, what energies to use and how all of this will affect the patient. You do need to have quite a lot of training because there's no textbook for that. One of our primary responsibilities is radiation safety and protection. All staff wear radiation monitoring badges, we keep an eye on those and make sure everyone's getting the right dose and that they're wearing appropriate shielding. It's our job to make sure that nobody's getting exposed to radiation that they shouldn't be.

Radiation

It's important, especially in medical physics, to differentiate between radiation that's produced by a machine and radioactivity. We use both. For radioactivity, we know that some atoms, called radionuclides, are unstable. The instability of the atoms means that they will “decay” to another isotope and during that decay they emit radiation. There are three types of radiation that can be emitted. Alpha particles are one example, they are actually a helium nucleus. They are airborne but because they're an atom that has size and density, they're bulky and very readily stopped by skin or even a piece of paper. The second type of radiation are beta emissions, which are electrons or positrons. And the last type of radiation that can be emitted are gamma rays. These are very highly penetrating, they can travel through lead and do a lot of damage to DNA. This is both good and bad in medicine.

In radiation therapy, we predominantly use x-rays, which have similar properties to gamma rays-they are just formed in different ways. These x-rays are produced by a linear particle accelerator, called a LINAC. These are very big, three ton, machines that will accelerate electrons almost up to the speed of light. When the electrons hit a tungsten target, they produce radiation in the form of very high energy x-rays (through the photoelectric effect). This is a safer environment because the patient is not radioactive, you can think of the radiation as a wave that travels into the patients and deposits energy. We have more control given that once the machine is turned off there is no radiation at all, it doesn't linger in the air.

Using x-rays to treat cancer

We mostly deal with solid tumours in radiotherapy (as opposed to blood cancers, for example). ‘Cancer' is a bit of an umbrella term for about 200 different diseases and they are all quite different. To develop a solid tumour, something has gone wrong with a cell-it hasn't replicated properly and the DNA has been corrupted. These cells propagate that incorrect DNA very rapidly. That's why, when we treat a patient, we have to get rid of every single cell. There are trillions and trillions of cells in one small tumour and if you leave any of those behind they can re-start the process. They may hibernate for a little while and you go into remission. But a few years down the track they will come back with the same intensity as before. Depending on where that tumour is, that can cause quite a lot of damage to the things surrounding it. They can travel through the bloodstreams or through the lymphatic systems, that's when the tumour will start to spread.

How are x-rays used to treat patients?

Most people are pretty familiar with using x-rays to image a broken arm or your teeth. Those are generally taken at about a voltage of 100kV. The x-rays we use in radiotherapy are closer to 10,000kV. These are much more energetic, because in this case we're not looking to image the patient but deliver a dose of that energy. We can treat superficial conditions (such as skin cancers) with the electrons themselves. The electrons will deposit all of their energy at the surface of the patient and it will dissipate very quickly. We use x-rays instead if we are treating deep seated tumours (like a breast, prostate or brain tumour). Radiotherapy is very counterintuitive because radiation both cures and can cause cancer.

For radiotherapy, we use imaging initially to locate the tumour and to plan our radiotherapy treatment. If we're treating a prostate, for example, we'll take an image of the whole pelvis to make sure that we can see everything that's going on. We create our plan, then we will only deliver the high x-ray dose to the prostate and a small margin around it. We try our very best to avoid what we call the organs at risk, the structures around the tumour that don't respond well to radiation.

What we are looking for is enough damage to the tumour DNA to cause cell death. The x-rays go in and cause direct damage to the tumour cells. The x-rays also separate the water particles out into their components-oxygen and hydrogen in surrounding tissue. These free radicals will also cause significant damage to nearby DNA. For the most part, radiation will preferentially destroy tumour cells over healthy cells. Tumour cells can't repair themselves very well-they are much more likely to die than healthy tissue cells when they are hit with a dose of radiation. It's a very fine balance between getting enough dose in there to kill every cell that makes up the tumour without causing permanent harm to the patient. Part of this balance is not giving the patient one huge dose of radiotherapy. This would be very effective at killing the tumour, but it would also kill all the healthy tissue around it since the radiation has to travel through the body to get to the tumour.

We treat tumours using many different beam angles or arcs. If you think about the prostate being at the centre of a bike wheel, we send lots of different beams of radiation from all angles that concentrate the high dose at the centre of the wheel (i.e. where the tumour is). We avoid angles that would send radiation through important structures such as the spinal cord. We also do this over many days, in this case around 36 days, which is called fractionation. This means that the healthy tissue is getting a chance to recover in between each session so that it can repair itself while the tumour shrinks. There will be some healthy tissue that dies as well, but that's why we do really advanced imaging these days.

Other radiation therapies

How do you protect yourselves against the radiation?

In radiotherapy, the vast majority of cancer patients will be treated with x-rays. These are the same x-rays that come out of an x-ray machine to take an image or a CT scan, but the x-rays we use have much higher energies. Because they are very penetrative, they can be quite dangerous. We do these treatments inside of radiotherapy bunkers that are built with a lot of high-density concrete, where the walls of the bunker are between one to two metres thick. Radiation is attenuated exponentially and only the tiniest amount can penetrate through that much concrete. We also use things like lead and steel plates to boost that shielding.

We will not be in the room once the radiotherapy has started, we have to exit the bunker and no one is allowed inside except for the patient. That's a very strict rule. We have very good audio visual technology to monitor everything so we can stop the treatment if need be. For lower energy therapies, we don't always need that same extent of protection. We wear lead shielding for treatments such as radio nuclides therapy, or if for some reason you need to be in a CT scan or a PET scan room you could wear a lead apron. We also wear extra lead protection specifically around the thyroid.

Our radiation safety principles are time, distance, and shielding. You want to minimize the time that you are near a radioactive patient or near any type of radiation. Radiation falls off with an inverse square law, so if you distance yourself twice as far away, your dose is four times as low. Lastly, we shield with thick concrete walls, or wearing lead gowns.

Other types of radiotherapy treatments.

The problem with x-rays is that you are treating everything between the skin and the target. For some patients, instead of external beam radiotherapy we can use brachytherapy. That's generally when we insert a radiation source into the patient. For high dose rates, we use the isotope Iridium-198, for low dose rates we most often use Iodine-125. These are mainly used for prostate and gynaecological cancers. For high dose rate brachytherapy, it will only sit inside the patient for about 20 minutes before we take it out. Because radiation drops off steeply with distance, brachytherapy is effective because you are only treating that particular region without exposing all of the area outside of it to high radiation doses. However this treatment too has pros and cons. The pellet is basically just a spear, so you can't shape the dose at all. The organs at risk will just get the dose they get. They are very good for what we call a boost dose. Sometimes patients will have quite a large area treated, but so we can provide a boost dose to where we know the actual tumour is.

Another use of radiation in medicine include CT scans. This is a 3D x-ray that gives much better tissue resolution than a normal x-ray image. The nice thing about CT's is that they are very fast. You can do a CT scan between 10 seconds to two minutes tops.

Another type of imaging we use are PET scans. PET scans give you quite a big dose of radiation because especially these days, most pet scans will be a hybrid PET/CT. A PET scan is a physiological scan, you will be injected with a radio nuclide (most often Fluoride-18, a slightly radioactive positron emitter). That nuclide gets labelled with a tracer, which is basically sugar. When that liquid goes into the body, anything with a higher metabolic rate will uptake the sugar, which means it's up taking the fluoride. Uptake will always be high in the brain because the brain uses a lot of sugar and you'll also always see it in the kidneys and the bladder. Those regions will consequently always appear hot. Tumours also have quite a high amount of metabolic activity, so they will show up much more clearly than on any other type of imaging mechanisms. However, even though you can see this really bright, hot spot, it's quite hard to actually localize exactly where that is within the body. That's where the CT scan comes in, we can lay the beautiful anatomical image (where you can see everything in the body) over the top of the PET scan. The PET scan gives you a really bright spot where the tumour is, the CT scan tells you exactly where that is in the body. That's very often used for both cancer diagnosis and then for follow-up to see whether the treatments have worked.

Our radiotherapy specialist team

In radiation oncology, we have a multidisciplinary team where there are four key medical players. Firstly, the radiation oncologists, which are different to a medical oncologist who works more in chemotherapy. They're responsible for prescribing the dose to the patient and they're responsible for the overall care of the patients. Once the plan comes through, it's up to them to approve it. As physicists, we'll also check plans, but we are checking the accuracy and the deliverability of the plan. It's the doctor's responsibility to assess the quality of the plan.

We then have radiation therapists and they're very important. They are the people that actually treat the patients. I can't treat the patient, the doctors are not allowed to treat the patients either. It's the job of the radiation therapist. They have radiation therapy degrees, they'll do the CT scanning of the patients and use very complex computer software to create the plans. The doctors will approve it, physics will check it and then that gets transferred to the machines and the radiation therapists actually treat the patients on a day to day basis.

Then there is the nursing team. These nurses aren't specially trained in radiation, but they do need to do more training when they come into a radiotherapy department. We also work closely with the engineers. They're specifically linac engineers. They have to do quite a lot of training. We also have our administration team as well, that takes care of all the patient bookings and sending the right patient around.

In physics, our main jobs are quality assurance and radiation safety and protection. It's our job to make sure that nobody's getting exposed to radiation that they shouldn't be. Everyone wears radiation badges, we monitor those and make sure everyone's getting the right dose and making sure everyone's wearing appropriate shielding where needed. It's our job to make sure the linear accelerators are working correctly. A physicist needs to ensure that the right amount of dose is being delivered to the exact right spot. These machines are three tonnes and they rotate around a point that has to be less than one millimetre in diameter. The engineers will basically keep them in tip top shape and then we'll do a lot of quality assurance to make sure that the dose is correct and that all the mechanics are correct.

We are also called to consult on complex patient plans. If they are not sure what the best technique to use is, the doctors and radiation therapists can give you a call and you can make recommendations on shielding, delivery angles, what energies to use and how that will affect the patients. You do need to have quite a lot of training because there's no textbook for that. This is what really appealed to me, because I wanted to do applied physics. I make use of the knowledge I've learned every day.

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