Medical Physics

In this short introductory video, Dr. Laura Sinclair explains the role of a medical physicist and the applications of nuclear medicine.

Radioactive material attached to a drug is known as a radiopharmaceutical. In imaging, radiopharmaceuticals may be injected, inhaled, or swallowed in capsule form. Radiopharmaceuticals can be used as tracers to see how well organs in your body are working or to find areas of disease. For a tracer, a radionuclide is mixed with a drug that collects in a particular organ in the body. Then, by detecting the radiation, we can examine that organ.

A gamma camera detects the radiation coming from the patient and produces an image of where the radioactivity is in the body. By displaying the position of each gamma ray that it detects, the gamma camera can build up a full image.  

Due to the short half-life of Tc-99m and the distance from a nuclear reactor, storage and transport of this radionuclide would be inconvenient and very expensive. Instead, the parent nuclide, Mo-99, is supplied to hospitals for use in a generator, known as a Mo-Tc generator. Within the generator the parent radionuclide, Mo-99, decays into the shorter-lived daughter radionuclide, Tc-99m. The 'daughter' radionuclide is removed by passing a solution of saline (salt-water) through the generator. This is known as elution. The saline solution elutes (removes) the soluble Tc-99m, which results in a saline solution containing the Tc-99m. The activity of the Mo-Tc generator, eluded every 24 hours, can be seen in the graph in figure 2 below: 

The Administration of Radioactive Substances Advisory Committee (ARSAC) provides a set initial activity of Tc-99m for patients known as a diagnostic reference level (DRL), which is measured in Becquerel (Bq). Typically, for diagnostic investigations, the DRL ranges from kBq to MBq depending on the type of examination. If the activity administered is too high, this may give an unnecessary radiation dose to the patient and could overwhelm the gamma camera, meaning anything relevant could be obscured. However, if the activity administered is too low, the gamma camera may not be sensitive enough and a diagnosis could be missed as a result. Therefore, the patient has to be administered the radiopharmaceutical within a set time frame to ensure the correct DRL.

Please note that the information, video, and activity below are based around radiotherapy treatments for cancer. There are many factors that affect treatment plans, and the content below is not intended to replace medical advice. If you have been personally affected by cancer and would rather not engage with this content, please let us know by emailing physics-bindingblocks@york.ac.uk (so that you may still receive your certificate).  

As described in Laura's video above, Nuclear Medicine can be used for treatment as well imaging. 

Radiotherapy is a form of cancer treatment that kills cancer cells by depositing large amounts of energy into the cells, thus ionising the atoms in the cell DNA. Photons (X-rays) or charged particles are used as sources of ionising radiation.

In the video below, Matthew Nicol from the University of York introduces radiotherapy before focusing on proton beam therapy.

Here, h is Planck’s constant (h = 6.63 x 10-34 Js) and c is the speed of light in a vacuum (c = 3 x 108 ms-1). In order to compare photon and proton energies, we can convert this energy into electron-volts (eV) where 1 eV = 1.6 x 10-19 J. 

Figure 3 (below) shows the relative dose to tissue versus the tissue depth for X-ray photons and protons.

You can see that X-rays deposit a far higher dose to healthy cells and a much lower dose to cancer cells. This is mitigated by having a rotating X-ray source. This method allows for the target area (cancer cells) to receive multiple doses whilst different areas of healthy tissue only receive two (one high when the X-Ray enters through that direction, and one lower when it leaves through that direction). The patient will also have a personalised radiotherapy treatment plan, which is broken into several treatments. This is to help healthy tissue repair and to kill cancer cells. A medical physicist or dosimetrist will plan the treatment based on the patient's CT scan (and sometimes MRI/CT and PET/CT scans are used too). There are limits of radiation dose to organs near the tumour (known as organs-at-risk). Very commonly, there is microscopic growth of tumours which can't be detected by imaging, so a volume around the tumour has to be included. Treatments can occur over 20 minutes so during this time patients move about, their chest moves when they breathe, and if they are undergoing chemotherapy or other treatments they may lose or gain weight rapidly. On a day-to-day basis, how much patients have drunk or eaten can also cause differences in the internal organs. These things can all have an effect on the treatment plan.

With all forms of radiotherapy, it is crucially important to target the radiation so that the energy is deposited in the cancer tumour, and does minimal damage to healthy tissue. One method we can take to find the correct beam energies is to simulate hitting a tumour using computer software. This allows us to safely experiment with different settings without putting a patient at risk. An example of a proton hitting a target using GEANT4 (a CERN simulation toolkit) is shown in figure 4 below:

We can pull meaningful data out from these simulations by putting them into different software which analyses the data. These software packages look at the data and allow us to see how the energy deposition changes with depth into the material and the divergence from the beam line for different particles. This analysis takes files that are over 80 gigabytes in size (each!) and turns them into graphs which are a couple of megabytes - far easier to store!

We have used these GEANT4 simulations to create a web application that allows you to simulate these ground-breaking treatment methods, as well as explore other properties of radiation, in real time. The app is simple to use, requiring only a couple of inputs to generate real data. Let's say, for example, we needed to find the range of a proton in water:

There are three different outputs from the simulation. The graph in figure 6, below, shows the energy deposited (y-axis) as you move through the material (depth of material on the x-axis). The particles in the beam will deposit energy when they are absorbed into the material. This means that the particles do not travel any further. 

In figure 7, below, we see an example of a 2D energy distribution as a function of z (depth) and r (width) for a 100 MeV proton beam in water. Colour coding shows how much energy is deposited - where red is a large amount of energy and blue is less. The energy is mostly deposited along the beam line. However, a significant portion of protons deposit their energy out to a radius of approximately 30 mm. The proton distribution is almost wedge-shaped, with the broadest part of the distribution falling near the end of the proton track. Outside this region, we see that the energy deposition becomes almost constant, but at a much lower level than the proton peak. 

The final graphical interface you can select shows the particle tracks, taken from the GEANT4 software.