Radiotherapy topics Flashcards
Tumor target volumes for therapy
GTV
CTV
PTV
Gross tumor volume - Visible tumor on imaging
Clinical Target volume - Target volume to cover the potential boundaries of the microscopic tumore
Planning target volume - Target volume to account for slight errors and organ movements.
Types of Teletherapy
EBRT = External Beam Radiotherapy
Uses radiation beams originating from a single point source and shaped by multileaf adjustible collimators.
Higher energy (MegaVolts) has deeper penetration than lower engergy (Kilovolts) and the peak of tissue absorption is deeper. ie, more skin sparing with higher energy.
Best described with depth dose curves
Electron vs photon radiation depths
Electron radiation is absorbed more rapidly, does not go as deep, and has less skin sparing
Radioisotopes used for brachytherapy
Cobalt 60
Cesium 137
Iridium
Iodine
Radium used to be used but not anymore .
How are patients positioned for teletherapy
With markings on the skin and/or head/body masks.
With lasers emitting from the wall striking the patient in the identical spots on the skin markings for each session.
CT or MRI/PET scans are used to taken in these immobilized positions to locate and plan the tumor target area.
Dose volume histograms
Used in the planning stage to show the expected dose deliverd to the GTV, CTV, and PTV areas.
The PTV should be significantly lower than the other two.
Types of machines and doses
Diagnostic X-ray machine - 20-150 kV
Kilovoltage X-ray machine, primarily used for superficial and semi-deep tumors
Superficial radiation 50-150 kV. Half value layer of 1-4 cm.
Semi-deep 80-120 kV. Half value layer of 2-3 cm.
Deep/orthovoltage 150-300 kV . half value of 4-7 cm.
Radionuclide irradiators emit gamma radiation.
- They are simpler and cheaper than megavoltage and linear accelerators.
- Cobalt is used to generate 1.33 MV gamma radiation, with a half layer thickness of about 10 cm.
- Good skin sparing
- Used in the Gamma knife, commonly used for intracranial RT.
- Megavoltage photon and electron radiotherapy. Electron linear accelerators.
- The most common source currently for EBRT.
- Electrons collide and generate megavoltage bremsstrahlung
- Range of 4-25 MeV.
Doses for brachytherapy
Low Dose Radiation - up to 2 Gy/hour
Idodine-125 or Palladium 103.
Medium Dose 2-12 Gy/hr
High Dose >12 Gy/hr
Pulsed dose rate - Ultra fractionated high dose in hourly pulses.
Types of brachytherapy
Interstitial
Intercavitary
Superficial Moulage
Means of implantation brachytherapy
Afterloading. - applicator is inserted first, then radiation. Protects staff during application
Usually iridium 192. Applicators are replaced every 3 months.
Permanent implantation -
Seeding the organ with LDR
Usually Iodin-125.
Left in permanently.
Brachytherapy applicators
Intracavitary - Rigid metal or flexible pl.astic catheters
- Uterus, Vagina, Cervix, Rectum
Intraluminal -
Esophagus, Bronchi, Nasopharyns.
Inserted into flexible catheters which are then secured outside or inside the body
Interstitial -
Directly into the tissue by rigid needles or flexible catheters
0 Breast, Prostate
Surface-
Inserted into a plastic/wax substance fixed to the surface of the body.
-skin, eyes.
“Conventional Fractionation” schedule for brachytherapy
2 Gy per fraction
Five fractions per week
Up to a desired total dose depending on tumor.
Types of fractioning and their benefits.
Hypofractionation -
Fewer fractions, with higher doses.
Longer time interval between fractions, but shorter total time of treatment
Hyperfractionation -
Smaller doses with more fractions.
Decreased interval but SAME total treatment time.
Accelerated fractionation.
Decreased interval and shorter total time.
Longer intervals in between fractions -
- leads to less early normal tissue damage and better tolerated, but cancer stem cells have more time to repopulate in between fractions and has less tumor control.
Increasing dose per fraction -
Increases late normal tissue damage.
