Lecture 16: Ratiotherapy

Question 1

What is radiotherapy?

Answer 1

Use of ionising radiation: radiation + energy, to damage target cells

Typically X-rays (can be gamma-rays)

Works in different ways:
1. damage of cell DNA
2. interrupting cell division
3. causing cell death

Commonest type: external beam therapy - teletherapy
Also: intracavitary/interstitial - brachytherapy
Radio-isotopes - molecular radiotherapy
Particle therapy - protons, electron, carbon ions

Question 2

How does radiotherapy work?

Answer 2

ionising radiation is energy that can displace electrons from atoms to form ions. Most of the energy deposited in the cell is absorbed by water (becuase cell is 80% water. This leads to free radical production such as OH+, as oxidsing agents this can cause lots of damage.

Question 3

What cells are most sensitive to radiotherapy?

Answer 3

Quiescent cells least sensitive: DNA packaged into protective proteins, cells have time to detect damage, repair DNA, or commit apoptosis.

Dividing cells most sensitive : cell death occurs not during the RT, but during the next division as cells divide before damage is detected or repaired, leading to cell cycle control loss

THEREFORE MALIGNANT CELLS MORE LIKELY TO BE KILLED

Question 4

What is the importance of radiosensitivity?

Answer 4

Not all tumour cells equally radiosensitive

Tumours can have:
1. high radio sensitivity e.g., lymphoma, leukaemia
2. intermediate radio sensitivity e.g., breast, cervix, larynx
3. low radio sensitivity e.g., giloma, melanoma

= adaptation of dosage

Question 5

What are the side effects of RT?

Answer 5

Acute:
1. damage to dividing cells within RT volume
2. starts at 10-17 days
3. worst 1-2 weeks post
4. Depends on dose
5. does get better

Late:
1. Occur > 6/12 but often years later
2. Progressive
3. Due to fibroblast, microvascular damage
4. Poorly recognised
5. Follow up
6. Lack of patient centred approach

Question 6

How can you limit the damage to normal tissue?

Answer 6

1. Exclusion of normal tissues from the RT treatment fields
   a. small fields (accuracy)
   b. Sharpened fields (conformal RT)
2. Fractionated RT
   a. delivering radiotherapy in multiple small fractions daily over 5-
   6 weeks
   b. allows normal tissue repair between fractions

Question 7

What is radiation dose-fractionation?

Answer 7

1. Biological effect is a combination of: total dose, dose per fraction, total time for treatment delivery
2. 2 Gray per fraction, one treatment per day is standard
3. 66-70Gy/33-35#6-7 weeks for curative treatments
4. Hypofractionation = higher dose per fraction = more late effects
5. Accelerated = shorter total treatment time e.g., three fractions per day = faster treatment time may be beneficial
6. Stereotactic ablative RT uses 1-5 fractions, 5-30 Gy / #

Question 8

What is external beam RT delivery?

Answer 8

1. Usually 3-7 week course of daily treatments
2. Each takes 2-5 mins (+5-10 mins setup)
3. Treatment planning starts with imaging
4. Dose is arranged to treat the required region of tissue
5. Each day’s treatment needs to be exactly the same

Question 9

What are the problems with RT

Answer 9

1. Moving body parts, e.g., breathing causes chest region to move
2. Complex, irregular shapes of tumours

Question 10

What is IMRT?

Answer 10

1. “intensity modulated RT”
2. Vary and RT dose throughout one field
3. Combine multiple such fields to reduce plan
4. Can theoretically conform dose better to target volumes and avoid critical structures
5. Can treat concave volumes wrapped around critical structure
6. Can stipulate different dose levels in one plan

Question 11

What is image guided ratiotherapy?

Answer 11

Target tumour throughout each fraction
E.g., implantable gold seeds with real time tumour tracking

Simpler methods
a. laxatives in prostate cancer
b. repeat the planning during a course
c. respiratory gating