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GYO Boards > Radiation Oncology > Flashcards

Radiation Oncology Flashcards

1
Q

Photoelectric effect

A

Radiation less than 100kV; radiation interacts with an inner orbital electron producing a fast electron

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2
Q

Compton effect

A

Mid to high energy 1MeV to 20MeV

Radiation pushes an electron from outer ring producing a fast electron

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3
Q

Dose depth of cobalt

A

5 mm

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4
Q

What does penumbra resemble?

A

Planning target volume (include CTV and allow for movement/breathing); it is effected by type of radiation used

Penumbra means “shadow cast”

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5
Q

Toxicity of mediastinal radiation therapy

A

Pericardial effusion

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6
Q

What causes radiation bone necrosis?

A

Photoelectric effect —> highest relative uptake of radiation into bone

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7
Q

Pair production

A

Energy > 20 MeV

Radiation impact atoms nucleus and produces at positron/electron pair (2 photons)

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8
Q

What isotope emits gammas rays with 0.66MeV of energy?

A

Cesium

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9
Q

How do we calculate the amount of E required to get to a specific depth?

A

Multiple depth by 3 —> =MeV

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10
Q

Acute radiation effects develop at what dose?

A

> 2000 cGy - symptoms present within 4 weeks of starting trx

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11
Q

Tissue tolerance of small bowel?

A

45 Gy

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12
Q

Tissue tolerance of the rectum?

A

70-75 Gy

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13
Q

What post-radiation changes do we see in the large bowel?

A

Obliterating endarteritis, mucosal ischemia and fibrosis

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14
Q

What is most common presentation of radiation proctitis? How do we manage?

A

Rectal bleeding; lower GI scope indicated
Most effective trx include sucralfate enemas, flagyl enemas and hyperbaric O2; can also use topical or ablative therapies

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15
Q

How can we manage post-radiation fecal incontinence?

A

High fiber diet to increase stool bulk

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16
Q

Tissue tolerance of the bladder?

A

65-80 Gy

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17
Q

Tissue tolerance of the vagina?

A

120-140 Gy

18
Q

Tissue tolerance of the vulva?

A

70 Gy

19
Q

Tissue tolerance of the ovary?

A

4-20 Gy

Severity of effect depends on patient age, use of concomitant alkylating chemotherapy, and total dose received

20
Q

Effects of RT on the fetus:

A

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

21
Q

How does EBRT effect bone?

A

Decreases osteoblasts number and fxn leading to demineralization and osteopenia; microvascular occlusion and impaired ability of BM to repopulate stem cells

22
Q

Tissue tolerance of bone? Acetabulum?

A

65-70 Gy; 30 Gy

23
Q

Goal doses of RT for CTV and GTV in cervical cancer?

A 
CTV = 45-54 Gy
GTV = 80-90 Gy
24
Q

Cell kill curve

A

Y-axis = logarithmic scale; X-axis = dose

Snape of curve is affected by rate of RT delivered

25
Q

Oxygen fixation hypothesis

A

Oxygen chemically modifies RT-induced DNA damage and makes it irreparable

26
Q

What is the most radiosensitive phase of the cell cycle?

A

M phase

27
Q

What type of photons are typically used?

A

X-rays —> produced what a stream of electrons collides with a high atomic # target (tungsten); USED in EBRT

28
Q

3 mechanisms of energy transfer

A

All produce fast electrons

  1. Photoelectric effect
  2. Compton effect
  3. Pair production
29
Q

Energy of iridium-192?

A

0.4 MeV

30
Q

Energy of cesium-137?

A

0.6 MeV

31
Q

What is LET?

A

Linear energy transfer - when radiation interacts with tissue, ionizing events occur along the path of energy transfer (kiloelectrons per micron)

32
Q

What are high LET particles?

A

Heavy particles - fast neutron, heavy charged ions and pi mesons (photons)
More tissue damage

33
Q

What in cell is target of RT?

A

DNA —> RT results in double-strand DNA breaks

34
Q

What is oxygen enhancing ratio?

A

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!

35
Q

What are the 4 Rs of RT?

A

Repair, reassortment, repopulation, reoxygenation

36
Q

Explain sublethal damage repair…

A

DNA damage is repairable by cells and thus can survive —> principle of fractionation

37
Q

LDR isotope

A

Cesium

38
Q

HDR isotope

A

Iridium 192

39
Q

Tissue tolerance of kidney

A

20 Gy

40
Q

What is IORT?

A

Intra-operative RT —> biggest risk is nerve damage if administered to the pelvic sidewall

41
Q

What GYN malignancy does pelvic RT predispose to the most?

A

Carcinosarcoma, sarcoma

42
Q

What is a Martin’s flap?

A

bulbocavernosous fat flap —> use for low to mid rectovaginal or vesicovaginal fistulas

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