Radiation Oncology Flashcards
Photoelectric effect
Radiation less than 100kV; radiation interacts with an inner orbital electron producing a fast electron
Compton effect
Mid to high energy 1MeV to 20MeV
Radiation pushes an electron from outer ring producing a fast electron
Dose depth of cobalt
5 mm
What does penumbra resemble?
Planning target volume (include CTV and allow for movement/breathing); it is effected by type of radiation used
Penumbra means “shadow cast”
Toxicity of mediastinal radiation therapy
Pericardial effusion
What causes radiation bone necrosis?
Photoelectric effect —> highest relative uptake of radiation into bone
Pair production
Energy > 20 MeV
Radiation impact atoms nucleus and produces at positron/electron pair (2 photons)
What isotope emits gammas rays with 0.66MeV of energy?
Cesium
How do we calculate the amount of E required to get to a specific depth?
Multiple depth by 3 —> =MeV
Acute radiation effects develop at what dose?
> 2000 cGy - symptoms present within 4 weeks of starting trx
Tissue tolerance of small bowel?
45 Gy
Tissue tolerance of the rectum?
70-75 Gy
What post-radiation changes do we see in the large bowel?
Obliterating endarteritis, mucosal ischemia and fibrosis
What is most common presentation of radiation proctitis? How do we manage?
Rectal bleeding; lower GI scope indicated
Most effective trx include sucralfate enemas, flagyl enemas and hyperbaric O2; can also use topical or ablative therapies
How can we manage post-radiation fecal incontinence?
High fiber diet to increase stool bulk
Tissue tolerance of the bladder?
65-80 Gy
Tissue tolerance of the vagina?
120-140 Gy
Tissue tolerance of the vulva?
70 Gy
Tissue tolerance of the ovary?
4-20 Gy
Severity of effect depends on patient age, use of concomitant alkylating chemotherapy, and total dose received
Effects of RT on the fetus:
1-3 weeks - “all or none”
8-13 weeks - greatest effect on neural tissue - microcephaly, MR
16-25 weeks - growth retardation and microcephaly
Severe fetal anomalies RARELY seen beyond 30 weeks
How does EBRT effect bone?
Decreases osteoblasts number and fxn leading to demineralization and osteopenia; microvascular occlusion and impaired ability of BM to repopulate stem cells
Tissue tolerance of bone? Acetabulum?
65-70 Gy; 30 Gy
Goal doses of RT for CTV and GTV in cervical cancer?
CTV = 45-54 Gy GTV = 80-90 Gy
Cell kill curve
Y-axis = logarithmic scale; X-axis = dose
Snape of curve is affected by rate of RT delivered
Oxygen fixation hypothesis
Oxygen chemically modifies RT-induced DNA damage and makes it irreparable
What is the most radiosensitive phase of the cell cycle?
M phase
What type of photons are typically used?
X-rays —> produced what a stream of electrons collides with a high atomic # target (tungsten); USED in EBRT
3 mechanisms of energy transfer
All produce fast electrons
- Photoelectric effect
- Compton effect
- Pair production
Energy of iridium-192?
0.4 MeV
Energy of cesium-137?
0.6 MeV
What is LET?
Linear energy transfer - when radiation interacts with tissue, ionizing events occur along the path of energy transfer (kiloelectrons per micron)
What are high LET particles?
Heavy particles - fast neutron, heavy charged ions and pi mesons (photons)
More tissue damage
What in cell is target of RT?
DNA —> RT results in double-strand DNA breaks
What is oxygen enhancing ratio?
Ratio of doses needed to achieve the same cell surgical fraction in hypoxia vs oxic conditions; close to 1 with high LET —> thus high LET works well in hypoxic tumors!
What are the 4 Rs of RT?
Repair, reassortment, repopulation, reoxygenation
Explain sublethal damage repair…
DNA damage is repairable by cells and thus can survive —> principle of fractionation
LDR isotope
Cesium
HDR isotope
Iridium 192
Tissue tolerance of kidney
20 Gy
What is IORT?
Intra-operative RT —> biggest risk is nerve damage if administered to the pelvic sidewall
What GYN malignancy does pelvic RT predispose to the most?
Carcinosarcoma, sarcoma
What is a Martin’s flap?
bulbocavernosous fat flap —> use for low to mid rectovaginal or vesicovaginal fistulas
