Oncology: Radiation therapy Flashcards
How does radiation kill cancer cells?
By directly damaging the DNA which leads to cel death during division
How are free radicals produced during radiation therapy and whites their effect?
ionization of water leads to free radicals
OH radical very reactive and accounts for most of radiation damage
What is the oxygen effect? How does this affect the efficiency of radiation therapy?
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
What are acutely responding tissues?
Rapid cell proliferation
Effects develop immediately and are self-limiting
What are late responding tissues?
slow rate of cell proliferation
effects develop months to years after therapy, irreversible
What is the unit of radiation dose?
The gray (Gy) 1 Gy= 1J of energy absorbed by 1Kg of tissue Usually record dose in centiGy (1Gy=100cGy)
How are radiation prescriptions usually given?
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)
What are the types of radiation therapy available?
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)
Why is teletherapy the most common?
It gives you a non-radioactive patient
What are the two type of teletherapy?
- Orthovoltage low-medium energy range
2. Megavoltage: Linear Accelerator (x-ray beam) or Cobalt unit (gamma rays)
What are the advantages and disadvantages of Orthovoltage?
Advantages: simplicity, cost, shielding requirements
Disadvantages: penetration=low; maximum dose to skin; increased absorption of dose in bone relative to soft tissue
What are the advantages and disadvantages of Megavoltage?
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
What do we use for brachytherapy?
192 Iridium
What do we use for systemic therapy?
131 Iodine
Brachytherapy is often used for facial tumours in horses, what are the advantages/practicalities?
Radioactive horse for 7-10 days during therapy
COST as main downside (£4-5k)
Tumours shrink over weeks-months - ugly for about 6 months…
What is sub lethal damage and fractionation?
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
What are the advantages of fractionation?
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)
What is a hypo fractionated protocol?
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
What is a fine fractionation protocol?
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
What are the most common cases referred for radiation therapy?
Post operative scars esp MCT, ST sarcoma, melanoma
Nasal tumours
CNS tumours
What types of tumour are highly susceptible to radiation?
lymphoma seminoma transmissible venereal tumours perianal adenoma/adenocarcinoma solitary plasmacytoma neuroblastoma
What types of tumour are sensitive to radiation?
nasal adenocarcinoma mast cell tumour squamous cell carcinoma (skin) metastatic lymph nodes (<1cm) gliomas basal cell carcinomas pituitary tumours
What types of tumour are moderately sensitive to radiation?
soft tissue sarcomas bladder transitional cell carcinoma prostatic carcinoma metastatic lymph nodes (>3cm) oral melanoma meningioma thyroid carcinoma
What types of tumour are resistant to radiation?
bone sarcomas
metastatic lymph nodes (>6cm)
head and neck tumours (>4cm)
What are the signs of acute radiation toxicity?
skin- moist desquamation, alopecia oral cavity- mucositis, salivation, halitosis nasal cavity- discharge ocular- corneal ulcer foot- pad slough, nail loss lung- pneumonitis
What are the signs of chronic radiation toxicity?
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
Where should you avoid with radiation?
Avoid: Eyes, Nasal planum, Foot pads, Circumferential extremity, Major internal organs
