Radiotherapy

Question 1

How does ionising radiation work?

Answer 1

via the ‘Compton process’
Electrons are knocked off the outer shell and cause tissue damage with intracellular water and generation of hydroxyl radicals - leading to oxidative stress and DNA damage

Question 2

Four ‘R’s of radiotherapy

Answer 2

Repair
Repopulation
Reassortment
Reoxygenation

Question 3

Repair explained

Answer 3

Cancer cells have a reduced capacity for DNA repair. By fractionating, normal cells can repair between fractions but in the same time cancer cells can’t so you hit them again

Question 4

Repopulation

Answer 4

Consideration of tumour repopulation however, needs to be considered - so not too much time for repair

Question 5

Reassortment

Answer 5

Fractionating treatment means that it catches the cells in the most radiosensitive bit of the cell cycle G2 to M.. The cells will be at this phase at different times.

Question 6

Reoxygenation

Answer 6

Oxygen alters sensitivity to IR - chemically modifies and fixes radiation-induced DNA damage - makes it irreparable.
Hypoxia can be acute (vasospasm or changing tumour vasculature) or chronic

Question 7

What is the difference between X rays and gamma rays?

Answer 7

where they come from. Both are photons

X rays are made artificially by LINACs (bombard a tungsten target with electrons)

whereas gamma rays are a product of radioactive decay.

Question 8

When would you use electrons instead of photons for RT and why?

Answer 8

Electrons and photons both penetrate further with increasing energy. However, with electrons this causes increasing skin dose, opposite to photons.

Therefore electrons are used to treat skin and subcutaneous tissues (superficial inguinal LN/vulval cancers)

Question 9

What is IMRT

Answer 9

Intensity modulated radiation therapy

using small computer leaves in the head of the machine, the beam is modified to create better target coverage and sparing of normal tissues.

Question 10

What is IGRT

Answer 10

Image guided radiation therapy

imaging of treatment fields prior to treatment delivery in order to improve treatment accuracy.

Question 11

HDR vs LDR for brachytherapy

Answer 11

HDR - shorter so less chance of applicator displacement

Because HDR is given over a few fractions, there is time for tissue reoxygenation between

Question 12

What are the acute effects of radiation due to?

Answer 12

within three months of radiation completion

1. inherent sensitivity of the irradiated tissue and 2. the kinetics of the affected cell population as a whole

Question 13

What are the late effects of radiation due to?

Answer 13

Occur over three months after radiation completion

Damage at the capillary level where there is endothelial cell proliferation, resulting in less diffusion of oxygen in to tissues and resulting fibrosis

Question 14

Skin effects of radiation

Answer 14

Photons are skin sparing

‘folds’ more like to be affected - inguinal creases or intergluteal.

week 3 - erythema
After week 4 - dry desquamation and moist desquamation
Return of the epidermis after 2-3 weeks

also, thinning, depigmentation, fibrosis S/C, loss of hair and telangiectasia

necrosis is rare

Question 15

Vaginal effects of radiation

Answer 15

mucositis - white/yellow discharge - can last several weeks

late - vaginal narrowing and shortening

treatment with dilators, lubricants and oestrogen

Question 16

Bladder effects of radiation

Answer 16

Bladder cystitis - frequency, urgency, dysuria and reduced capacity

Need to rule out UTI

Large doses can cause chronic problems

Fistulas - more likely if tumour already involves bladder wall

Question 17

Small and large intestine effects of radiation

Answer 17

‘acute radiation enteritis’

reduced absorption leading to fluid and nutrient wasting, diarrhoea and dehydration

Rx: ‘low residue’ diet, hydration and anti-motility agents

mucosa heals within 10-14 days after finishing radiation

late: chronic diarrhoea, partial obstruction
rare: fibrosis, stenosis, perforation and fistulae

Question 18

Ovarian effects of radiation

Answer 18

Ovarian failure - if ovaries more active would need larger doses for this,

Rx: can suspend ovaries out of pelvis to try to avoid ovarian failure

Question 19

Bone marrow effects of radiation

Answer 19

Lymphocytes are the most radiosensitive cells in the bone marrow

insufficiency fracture in - sacrum, ileum, pubic bone and rarely acetabulum.

Rare: avascular necrosis of the femoral head - hip replacement

Question 20

Liver effects of radiation

Answer 20

veno-occlusive disease leading to necrosis and atrophy of the hepatic cells

radiation hepatitis - look for increase in ALP of 3-10x with ALT/AST 1-2 x

Question 21

Kidney effects of radiation

Answer 21

can lead to:

HTN, leg oedema, albuminuria
normocytic normochromic anaemia