Radiotherapy

Question 1

What is the general procedure for a patient undergoing radiotherapy, and what role does a medical physicist play?

Answer 1

• Patient has a CT scan and doctors isolate tumour region on scan
• This is given to medical physicists to come up with patient specific treatment plan
• The plan aims to deliver a high dose to the tumour area and minimise the dose to healthy tissue
• Medical physicists may also be involved in making immobilisation equipment to ensure a patient’s body is in the same place during treatment, and every time they have treatment

Question 2

How does radiotherapy work?

Answer 2

• Ionising radiation is incident in a beam towards the tumour site. This radiation ionises the DNA of cancerous tissue, causing it to be unable to replicate
• Radiation may also cause indirect ionisation - water is ionised creating free radicals which damage DNA
• To space healthy tissue, beams of radiation are aimed at several angles which intersect at the tumour, giving a much larger absorbed dose to the tumour site.

Question 3

What is dose fractionation and why is it done?

Answer 3

• The dose may also be fractionated where the total dose is divided into several smaller doses over several days. This results in less harm to healthy tissue.
• Fractionation can also help if the tumour is hypoxic and resistant to radiation as reoxygenation occurs between doses

Question 4

How are heavy charged particles used in radiotherapy?

Answer 4

• Charged particles may be protons or boron/carbon/neon ions
• They cause direct damage to cancer cell DNA through high linear energy transfer
• Their effect is mostly independent of tumour oxygen supply as they interact via direct energy transfer causing double stranded DNA breaks
• Their relatively large mass means they have little lateral side scatter in tissue, limiting dose to surrounding tissue
• The Bragg peak effect is also used: heavy charged particles deposit a large amount of energy just before the end of their range. This is due to an increase in the particles interaction cross section at lower energies (energy lost is inversely proportional to square of the velocity)

Question 5

What are some advantages and disadvantages of therapy with heavy charged particles?

Answer 5

• Advantages: easy to collimate, dose is delivered via Bragg peak to better ratio of dose to tumour vs tisssue, good for treating localised tumours close to critical organs
• Disadvantages: still some dose to surrounding tissue, requires 60 MeV protons which is expensive

Question 6

Describe boron neutron capture therapy

Answer 6

• Boron is enriched in tumour cells by attaching it to a compound which targets the tumour site. Boron carrying antibodies are used to get B-10 to the malignant tissue
• Boron has a very high thermal-neutron reaction cross section so the number of neutron reactions in the tumour site is increased.
• Thermal neutrons are incident on the tumour site and unstable B-11 nuclei are formed
• The resulting compound nucleus B-11 breaks up into two fragments which release energy over a short distance to cancerous cells nearby

Question 7

Why is photon therapy not a good idea for a hypoxic tumour?

Answer 7

• In photon therapy, most of the radiation effect is through free radicals (particularly oxygen FRs)
• Solid tumors can outgrow their blood supply, causing a low-oxygen state known as hypoxia.
• If there is low oxygen, there is low potential for damage from oxygen free radicals
• Tumour cells in a hypoxic environment may be 2-3 times more resistant to radiation than in a normal oxygen environment
• Dose fractionation can help with hypoxic tumours as they take some time to re-oxygenate