Medical Physics

Course requirements for Module 4: Medical Physics

• 4.1 Medical Imaging (Questions)

• 4.2 Medical Treatments (Questions)

• 4.3 Modelling Radiotherapy (Online Simulation)

• 4.4 Modelling Radiotherapy (Questions)

The radionuclide technetium-99m (Tc-99m) is the most commonly used radionuclide in medicine, and it can be attached to a range of pharmaceuticals. The m in Tc-99m stands for metastable, (explained below). It is suited to this role as it's relatively short-lived (half life of 6.022 hours) and it emits readily detectable gamma rays. This allows for rapid data collection/image taking and keeps the total patient radiation exposure low.

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, technetium-99m is produced in hospitals by the decay of molybdenum-99 (Mo-99), a radionuclide with a longer half-life of 66 hours. Mo-99 is usually created commercially by fission of highly enriched uranium. 

Molybdenum-99 decays via beta-minus decay 87.6% of the time into the excited, metastable state, Tc-99m. Metastable states are almost always excited states (there are a few weird exceptions, but let’s not go into that!). This means that it has excess energy that it needs to get rid of.

In the case of Tc-99m, it gets rid of some of its energy by emitting an atomic electron first: a process called internal conversion. This helps it get rid of about 2.2 keV of energy. It then decays by giving out a gamma-ray with an energy of 140.5 keV. It is this gamma-decay that medical physicists use. The resulting nuclide is Tc-99, which has a half-life of 211,100 years!

The process is shown in the simplified diagram below:

As described in Dr Laura Sinclair'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. This ionises the atoms in the cell DNA. High energy X-rays or gamma rays, or charged particles such as protons, can be used as sources of ionising radiation.

The X-rays used for radiotherapy are highly penetrating and highly ionising forms of electromagnetic radiation. They have enough energy to radicalise (tear an electron away from) oxygen and hydrogen pairs (known as hydroxy groups) in cell DNA. This radicalisation eventually leads to the cell not being able to replicate or repair itself, and the cell dies.

This is desirable when the cell is a cancer cell, but one of the problems with X-ray radiotherapy is that the ionisation (or radicalisation) is not exclusive to cancer cells: the X-rays damage healthy cells as well. We can help to prevent this 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 higher dose 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 scans. There are limits of radiation dose to organs near the tumour (known as organs-at-risk). 

Many things can have an effect on the treatment plan. Treatments can occur over about 20 minutes, and during this time, patients move about and their chest moves when they breathe. If they are undergoing chemotherapy or other treatments, they may also lose or gain weight rapidly. On a day-to-day basis, how much the patients have drunk or eaten can also cause differences in the internal organs. 

When considering the physics of how much energy is deposited into cells, it is useful to describe X-rays as composed of packets of energy called photons. We can calculate the energy of a photon (E) based on its wavelength (λ) using the equation:  

E = hc / λ 

Here, h is Planck’s constant (h = 6.63 x 10-34 Js) and c is the speed of light in a vacuum (c = 3.00 x 108 ms-1). 

Proton beam therapy involves firing highly energetic protons at target cancer cells. These protons directly damage the cancer cell by depositing a large amount of energy onto them. Due to their relatively high mass, protons scatter only a very short distance before stopping. This minimises the damage to the surrounding, healthy cells. 

Just like X-rays, high energy proton beams ionise atoms as they pass through the body but, unlike X-rays, the doses are relatively low until right before they stop (where they will deposit the majority of their energy). 

However, there are disadvantages to proton therapy. The high precision of the dose means that treatment plans have to be meticulously planned. It also makes it difficult to use for cancers in areas where there is a lot of body or organ movement - for example, in the lungs, where the motion of breathing varies the depth of a tumour. The high precision of a single energy proton beam will also not target the entire tumour. For that, you need several proton beams at different energies to overlay each other. 

Matt Nicol explains proton beam therapy in the video below.