Radiation Oncology

Question 1

Photoelectric effect

Answer 1

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

Question 2

Compton effect

Answer 2

Mid to high energy 1MeV to 20MeV

Radiation pushes an electron from outer ring producing a fast electron

Question 3

Dose depth of cobalt

Answer 3

5 mm

Question 4

What does penumbra resemble?

Answer 4

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

Penumbra means “shadow cast”

Question 5

Toxicity of mediastinal radiation therapy

Answer 5

Pericardial effusion

Question 6

What causes radiation bone necrosis?

Answer 6

Photoelectric effect —> highest relative uptake of radiation into bone

Question 7

Pair production

Answer 7

Energy > 20 MeV

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

Question 8

What isotope emits gammas rays with 0.66MeV of energy?

Answer 8

Cesium

Question 9

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

Answer 9

Multiple depth by 3 —> =MeV

Question 10

Acute radiation effects develop at what dose?

Answer 10

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

Question 11

Tissue tolerance of small bowel?

Answer 11

45 Gy

Question 12

Tissue tolerance of the rectum?

Answer 12

70-75 Gy

Question 13

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

Answer 13

Obliterating endarteritis, mucosal ischemia and fibrosis

Question 14

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

Answer 14

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

Question 15

How can we manage post-radiation fecal incontinence?

Answer 15

High fiber diet to increase stool bulk

Question 16

Tissue tolerance of the bladder?

Answer 16

65-80 Gy

Question 17

Tissue tolerance of the vagina?

Answer 17

120-140 Gy

Question 18

Tissue tolerance of the vulva?

Answer 18

70 Gy

Question 19

Tissue tolerance of the ovary?

Answer 19

4-20 Gy

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

Question 20

Effects of RT on the fetus:

Answer 20

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

Question 21

How does EBRT effect bone?

Answer 21

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

Question 22

Tissue tolerance of bone? Acetabulum?

Answer 22

65-70 Gy; 30 Gy

Question 23

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

Answer 23

CTV = 45-54 Gy
GTV = 80-90 Gy

Question 24

Cell kill curve

Answer 24

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

Snape of curve is affected by rate of RT delivered

Question 25

Oxygen fixation hypothesis

Answer 25

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

Question 26

What is the most radiosensitive phase of the cell cycle?

Answer 26

M phase

Question 27

What type of photons are typically used?

Answer 27

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

Question 28

3 mechanisms of energy transfer

Answer 28

All produce fast electrons 1. Photoelectric effect 2. Compton effect 3. Pair production

Question 29

Energy of iridium-192?

Answer 29

0.4 MeV

Question 30

Energy of cesium-137?

Answer 30

0.6 MeV

Question 31

What is LET?

Answer 31

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

Question 32

What are high LET particles?

Answer 32

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

Question 33

What in cell is target of RT?

Answer 33

DNA —> RT results in double-strand DNA breaks

Question 34

What is oxygen enhancing ratio?

Answer 34

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!

Question 35

What are the 4 Rs of RT?

Answer 35

Repair, reassortment, repopulation, reoxygenation

Question 36

Explain sublethal damage repair...

Answer 36

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

Question 37

LDR isotope

Answer 37

Cesium

Question 38

HDR isotope

Answer 38

Iridium 192

Question 39

Tissue tolerance of kidney

Answer 39

20 Gy

Question 40

What is IORT?

Answer 40

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

Question 41

What GYN malignancy does pelvic RT predispose to the most?

Answer 41

Carcinosarcoma, sarcoma

Question 42

What is a Martin’s flap?

Answer 42

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