Radiation Therapy Flashcards
What is the most common type of radiation used?
X-rays/ Gamma Rays (Photons)
Characteristics of X-Rays/Gamma Rays (4)
1) Low (4-6MeV) or high (15-25MeV)
2) Skin sparing
3) Depth-dose properties (Penetration)
4) Isodose distribution (Beam Uniformity)
Characteristics of External Beam Radiation (2)
1) Good for Superficial Lesions
2) Deep tissue sparing
3) Range of Penetration (cm) = MeV divided by 3
Brachytherapy (3 advantages)
1) Better dose localization
2) Continuous Fractionation
3) Decreased dose to adjacent normal tissue
Brachytherapy (3types of source placement)
1) Interstitial
2) Intracavitary
3) Surface Mold
Brachytherapy Time Frames of Use
1) Temporary - long lived isotopes used (RIC = radium, iridium, cesium)
2) Permanent - short lived isotopes used (PIG = palladium, Iodine, Gold)
What is cell death? What is required?
- Inability to proliferate
- Both DNA strands must be knocked
Log Cell Kill Def’n?
particular radiation dose will kill the same proportion of cells
Therapeutic Window Def’n?
Dose-response curves between tumour cell + tissue damage
Target Volume Definitions:
1) GTV
2) CTV
3) PTV
GTV - palpable/visible tumour
CTV - GTV + microscopic extension
PTV - CTV + uncertainty from day-to-day variations/errors
Treated Volume Def’n
volume that receives a dose that is important for local cure/palliation
Irradiated Volume
tissue volume which receive a dose that is significant in relation to normal tissue tolerance
At Risk Organs
anatomical structures with important clinical properties located in the target volume
Mechanisms of Cell Injury (2)
1) Direct Injury - electron from xray absorption causes DNA damage
2) Indirect Injury - electron from xray creates an O2 free radical which damages DNA
5 R’s of Radiotherapy
1) Repair - sublethal injury will be repaired by cells. Increased Fractionation will increase opportunity for repair
2) Reoxygenation - O2 increases the effect of ionizing radiation
3) Redistribution - cells in different phases of the cell cycle making them more or less radiosensitive
4) Repopulation - tumour cells proliferate after surgery or radiation
How does increased fractionation affect the 4 R’s of radiation?
Which property does Accelerated fractionation affect?
- Repair - increases repair
- Reoxygenation - increases affect of the radiation
- Redistribution - increases cells in more radiosensitive phases
- Accelerated fractionation affects Repopulation (reducing the chance of repopulation)
Definition of Gray
1 Gray = absorption of 1 joule of radiation energy in 1 kg of tissue
Definition of Rad
“Radiation absorbed dose”
100RAD = 1 GRAY
How much time required post chemoRT to call a biopsy for recurrence to be reliable?
3 months
Reasons:
1) Cell Lysis occurs after 4-5 mitosis rounds
2) Lethally injured cells look the same as surviving cells
Advantages of pre-op radiotherapy (5)
- Better blood supply
- Unresectable tumours become resectable
- Decrease the extent of surgical resection
- Less tumour seeding with post radiation surgery (less viable cells)
- Fewer cells in lymphatics + blood vessels so less chance of distant spread
Disadvantages of pre-op radiotherapy (4)
- Resection and Recon more difficult due to fibrosis, inflammation and reduced vascularity
- Obscured tumour margins by tumour shrinkage/inflammation
- Wound healing problems with >40Gy radiation
- Lower overall dose
Advantages of Post-op radiation therapy (6)
- Can safely give higher radiation doses
- Remove subclinical tumour
- Easier to resect non-radiated tissue
- Distinct tumour margins
- Can focus on radiating areas not amenable to surgery
- Better staging
Disadvantages of Post-op radiation (2)
- Interruption of blood flow to remaining tumour cells (less radiosensitive)
- Surgical complications may delay the start of radiation
At what timeframe post operatively to start post op chemoradiation by??
before 6 weeks
