Oncology: Radiation therapy

Question 1

How does radiation kill cancer cells?

Answer 1

By directly damaging the DNA which leads to cel death during division

Question 2

How are free radicals produced during radiation therapy and whites their effect?

Answer 2

ionization of water leads to free radicals

OH radical very reactive and accounts for most of radiation damage

Question 3

What is the oxygen effect? How does this affect the efficiency of radiation therapy?

Answer 3

High O2 fixes free radical damage to DNA

Cells separated from capillary bed>100um are hypoxic,
this hypoxia protects cells from radiation damage (large tumours are less sensitive to radiation) and small tumours (with less cells to kill) are more sensitive

Question 4

What are acutely responding tissues?

Answer 4

Rapid cell proliferation

Effects develop immediately and are self-limiting

Question 5

What are late responding tissues?

Answer 5

slow rate of cell proliferation

effects develop months to years after therapy, irreversible

Question 6

What is the unit of radiation dose?

Answer 6

The gray (Gy) 
1 Gy= 1J of energy absorbed by 1Kg of tissue
Usually record dose in centiGy (1Gy=100cGy)

Question 7

How are radiation prescriptions usually given?

Answer 7

One radiation treatment is called a fraction

Radiation prescriptions include the dose per faction and the number of fraction (3Gy x19 fractions= 57Gy total dose)

Question 8

What are the types of radiation therapy available?

Answer 8

External beam radiation (Teletherapy): Delivered. (X-rays or gamma-rays)
Interstitial beam radiation (Brachytherapy): Implanted. (Gamma or beta-rays)
Systemic radiation therapy: Injected. (Gamma or beta-rays)

Question 9

Why is teletherapy the most common?

Answer 9

It gives you a non-radioactive patient

Question 10

What are the two type of teletherapy?

Answer 10

1. Orthovoltage low-medium energy range

2. Megavoltage: Linear Accelerator (x-ray beam) or Cobalt unit (gamma rays)

Question 11

What are the advantages and disadvantages of Orthovoltage?

Answer 11

Advantages: simplicity, cost, shielding requirements
Disadvantages: penetration=low; maximum dose to skin; increased absorption of dose in bone relative to soft tissue

Question 12

What are the advantages and disadvantages of Megavoltage?

Answer 12

E=6-10x orthovoltage
Advantages: skin sparing maximum dose>0.5cm below surface; less scatter =less radiation sickness; uniform dose to bone and soft tissue
Disadvantages: cost, shielding requirements, expertise

Question 13

What do we use for brachytherapy?

Answer 13

192 Iridium

Question 14

What do we use for systemic therapy?

Answer 14

131 Iodine

Question 15

Brachytherapy is often used for facial tumours in horses, what are the advantages/practicalities?

Answer 15

Radioactive horse for 7-10 days during therapy
COST as main downside (£4-5k)
Tumours shrink over weeks-months - ugly for about 6 months…

Question 16

What is sub lethal damage and fractionation?

Answer 16

Sub-lethal damage: cell damaged but not killed by a radiation event; next dose should be delivered before damage can be repaired
Fractionation is many sub-lethal events given over time

Question 17

What are the advantages of fractionation?

Answer 17

Repair of normal cells (cancer cells have limited ability to repair their DNA)
Re-oxigenation of cancer cells (well oxygenated cancer cells die with radiation)
Re-distribution of cancer cells in the cell cycle (cells in G2/M phases are most susceptible to cell death by radiation)

Question 18

What is a hypo fractionated protocol?

Answer 18

A few large doses of radiation delivered weekly for approx 4 weeks
Usually 800-900 cGy/fraction
Severe limitations on total dose because of normal tissue tolerances

Question 19

What is a fine fractionation protocol?

Answer 19

A greater number of doses given over a same time period
Usually 200-300 cGy/fraction
Able to achieve higher total dose and consequently better long-term survival for many cancer patients

Question 20

What are the most common cases referred for radiation therapy?

Answer 20

Post operative scars esp MCT, ST sarcoma, melanoma
Nasal tumours
CNS tumours

Question 21

What types of tumour are highly susceptible to radiation?

Answer 21

lymphoma
seminoma
transmissible venereal tumours
perianal adenoma/adenocarcinoma
solitary plasmacytoma
neuroblastoma

Question 22

What types of tumour are sensitive to radiation?

Answer 22

nasal adenocarcinoma
mast cell tumour
squamous cell carcinoma (skin)
metastatic lymph nodes (<1cm)
gliomas
basal cell carcinomas
pituitary tumours

Question 23

What types of tumour are moderately sensitive to radiation?

Answer 23

soft tissue sarcomas
bladder transitional cell carcinoma
prostatic carcinoma
metastatic lymph nodes (>3cm)
oral melanoma
meningioma
thyroid carcinoma

Question 24

What types of tumour are resistant to radiation?

Answer 24

bone sarcomas
metastatic lymph nodes (>6cm)
head and neck tumours (>4cm)

Question 25

What are the signs of acute radiation toxicity?

Answer 25

skin- moist desquamation, alopecia
oral cavity- mucositis, salivation, halitosis
nasal cavity- discharge
ocular- corneal ulcer
foot- pad slough, nail loss
lung- pneumonitis

Question 26

What are the signs of chronic radiation toxicity?

Answer 26

skin- fibrosis, leukotrichia, non-healing ulcer
oral- bone necrosis, periodontal disease
ocular- KCS, cataract, retinal damage
gastro-intestinal- stricture, ulcer
brain- encephalopathy, infarct
spinal cord- myelopathy, infarct
kidney- fibrosis
lung- pneumonitis

Question 27

Where should you avoid with radiation?

Answer 27

Avoid: Eyes, Nasal planum, Foot pads, Circumferential extremity, Major internal organs