RADIATION Flashcards
Q: Name two types of ionizing radiation; which one is used in H&N Ca [?]
- Photons (Xrays / Gamma rays) → H&N Use
- Particle Radiation (electrons, protons, neutrons, etc)
Q: Characteristics of Photon Radiation; which one used in H&N [?]
- Low (5MeV; H&N) or high (20MeV) energy
- Skin-sparing properties
- Depth-dose properties (Penetration)
- Isodose distribution (Beam Uniformity)
Q: Characteristics of Electron Beam XRT [?]
- Good for Superficial lesions, deep tissue sparing.
- Range of Penetration (cm) = MeV/3.
Q: Three Advantages of Brachytherapy [?]
- Better dose Localization
- Continuous Fractionation.
- Decreased dose to Adjacent normal tissue.
- Radioactive source placed in proximity to lesion.
Q: Brachytherapy source placement (3 types) [?]
1- Interstitial: radiation source directly on target tissue (i.e. prostate or breast).
2- Intracavitary: vagina, cervix.
3- Intraluminal: trachea, esophagus.
4- Surface mold: skin.
** #2-4 → “Contact brachytherapy” = radiation source in adjacent target tissue
Q: Brachytherapy time frames of use [?]
- Temporary – long-lived isotopes used (RIC = Radium, Iridium, Cesium). -Permanent – short-lived isotopes used (PIG = Palladium, Iodine, Gold).
Q: (TK) IMRT: What is it? Disadvantages [?]
- Intensity-modulated RT.
- Use of CT & Software to use multiple beams of varying intensity from different directions for accurate delivery based on tumour size & location; conformal therapy (dose distribution around tumour)
PROS:
(i)Localized radiation, (ii) high dose radiation to tumour site, (iii)sparing of normal structures (reduced toxicity).
Disadvantages:
(i) expensive, (ii) time-consuming, (iii) special immobilization required, (iv)steep
radiation falloff (limit of rads depends on tumour mapping marks by physician), (v)steep learning curve
Q: Radiotherapy Pearls [?]
- Cell Death = Inability to Proliferate; both DNA strands must be knocked out.
- Log cell kill = particular radiation Dose will kill the same Proportion of cells.
- Therapeutic Window = Dose Response curves between Tumor cell & Tissue damage; relative positions of curves determine safety of tumor control.
- Shrinking Field technique; now replaced by concomitant boost?
Q: Radiotherapy: Mechanisms of cell injury(2) & percentages of each [?]
- Direct injury – Electron from x-ray absorption causes DNA Damage (1/3)
- Indirect injury – Electron from x-ray creates an Oxygen Free Radical which then damages the DNA
(2/3)
Q: What are the 4 R’s of radiotherapy injury mechanisms [?]
- Repair – Sublethal injury will be repaired by the cell if it takes no further hits, increased fractionation = increased repair (by normal tissue, since cancer cells are worse at repair).
- Reoxygenation – Presence of oxygen increases the effects of Ionizing Radiation, radiosensitivity stays the same down to 20mmHg oxygen, below this sensitivity decreases. With fractionation hypoxic tumours reoxygenate and become more RT-sensitive.
- Redistribution – Max. radioresistance = Late S phase; max. radiosensitivity = Early M phase (late G2 also very radiosensitive); fractionation allows increased radiosensitivity, as surviving cells redistribute themselves to other phases in cycle).
- Repopulation – Tumors accelerate repopulation after cell reduction from surgery or radiation (this is one disadvantage of hyperfractionation, addressed by accelerated Fx’n).
Q: (DO): What is radiosensitivity [?]
What are the key components that make cells more sensitive [?]
Radiosensitiviy: the ability of radiation to lead to cell death via DNA damage without
adequate DNA repair.
Components:
- High cell division rate: rapidly dividing cell = increased susceptibility to DNA damage (M phase = radiosensitive vs. S phase = radioresistant)
- Low capacity for DNA repair
- High oxygen content
Q: (DO): List examples of radiosensitive (2) and radioresistant tumours [?]
- Radiosensitive: lymphoma, oral mucosa ca (SCC)
- Radioresistant: sarcomas, melanomas
Q: Definition of a Gray and Rad [?]
- Gray (Gy) = Joule/kg =The absorption of one joule of radiation energy by one kilogram of tissue
- Rad = Radiation absorbed dose; 100 rad = 1 Gy
Q: Describe Standard [?]
Standard: single dose/fraction (1.8-2.0Gy) of radiation per day, MON-FRI. (i.e 2Gy OD 5d/wk for 7 weeks (35F) = 70Gy)
Q: Describe Hyperfractionation [?]
Multiple fractions per day, lower doses per fraction, increased total dosing, increased duration of treatment (i.e. 1.2Gy BID 5d/wk for 7wks for 68F = 81.6Gy) Improved radiosensitivity of tumour cells via “Repair, reoxygenate, redistribute”
Q: Describe Accelerated fractionation [?]
Accelerated: increases amount of dose per fraction, same total dosing, decreased duration of treatment (i.e 1.6Gy/fractions/BID 5d/wk 67.2Gy/42 fractions/6wks).
- Allows decreased time for tumour “Repopulation”
- Intense: 1.5Gy TID 7d/wk x 36F = 54Gy
- Split: 1.6Gy BID 5d/wk x42F (split with 2 week gap in between) = 67.2Gy
- Concomittant Boost: 1.8Gy OD 5d/wk (30F) +1.5Gy dose BID on last 12F = 72Gy
Q: Describe Standard, Hyperfractionation, Accelerated fractionation [?]
Q: Adv / DisAdv of Hyperfractionation and Accelerated Fractionation [?]
Q Discuss “BOOST” radiation [?]
- Form of accelerated radiation
- Increase the number of fractions per day at a given time of radiation treatment (usually the end)
- Indicated for locally advanced disease → better LRC not OS
- Allows more rads to tumour due to “shrinking field”
Q: Discuss “SPLIT-COURSE” radiation [?]
- Form of accelerated radiation
- Gap midway between radiation course to allow healing of surrounding tissue
- Decide during the gap whether or not to continue with palliative or curative intent
Q: Dose limiting factors for Intensity and Total dose [?]
- Intensity (Increased # of daily fractions OR reduced duration of overall treatment → Acute toxicity = Mucositis → accelerated rads and hyperfractionated rads
- Total dose – Late toxicity = Soft tissue Fibrosis → hyperfractionated rads
Q: Time required post-XRT to call biopsy reliable [?]
- 3 months .
- Cell lysis occurs at mitosis, 4-5 divisions occur before Lysis .
- Lethally injured cells and Surviving cells are morphologically identical .
Q: (DO): Options for administration of radiations [?]
- Preoperative (neoadjuvant)
- Postoperative (adjuvant)
- Concurrent (with chemotherapy)
- Palliative (unresectable disease; stage IVb)
Q: PROS/CONS of Preoperative Radiotherapy for SCC [?]
