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Oncology Boards > RT > Flashcards

RT Flashcards

1
Q

What is the unit of absorbed dose for RT and what does it 1 equal?

A

Gray; 1 Gy= one joule absorbed per kg of tissue

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2
Q

Megavoltage/orthovoltage photons are the predominant form of RT used in vet med?

A

megavoltage

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3
Q

Megavoltage photons interact with tissue primarily by ____ that produces _____ that work via direct or indirect actions.

A

Compton effect; high energy electrons

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4
Q

How can megavoltage photons cause either direct or indirect effects

A

Direct: electrons cause ionization events to critical molecules Indirect: electrons cause ionizing events to water located near critical molecules

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5
Q

What is the most common form of cell death caused by RT?

A

Mitotic catastrophe

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6
Q

What phase are cells most resistant to RT in?k

A

Late S-phase; cells with a long G1 period also; also G0 because not cycling

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7
Q

What phase are cells most sensitive to RT

A

G2 or M; also late G1 to early S phase

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8
Q

Are normoxic or hypoxic cells more sensitive to RT?

A

Normoxic - need O2 to generate reactive oxygen spp that cause much of the damage from RT

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9
Q

How do low energy photons interact with tissues?

A

mostly photoelectric effect

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10
Q

orthovoltage x-rays distrubte the max doses in what tissue?

A

Skin –> bad acute tox in skin and SQ usually dose limiting

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11
Q

Describe the indirect effects of ionizing RT

A

Photon interacts with water, generates free radicals that damage the target

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12
Q

What are the energy ranges for orthovoltage vs. megavoltage RT?

A

Ortho: lower energy, 150-300 kVp Megavoltage: higher energy, >1MeV

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13
Q

What is the relative biologic effectiveness (RBE)

A

Measure of the amount of energy transferred per unit path length - uses LET + other factors including biologic endpoint, fractionation, RT dose rate and dose

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14
Q

What tissue/cells are particularly prone to apoptosis from RT?

A

lymphoid tissues

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15
Q

High doses of RT cause apoptosis of endothelial cells which is called _____.

A

vascular collapse

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16
Q

In vet med, what is considered the range and total dose for standard fractionation? How does this compare to human RT?

A

2.7-4.0Gy to a total of 42-57Gy Most vet med protocols considered hypofx compared to human

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17
Q

What is hyperfractionation?

A

dose per fraction reduced and the total dose is increased

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18
Q

What is accelerated RT?

A

overall time of treatment reduced

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19
Q

What are the 5Rs of RT?

A

Repair, Redistribution, Reoxygenation, Repopulation and Radiosensitivity

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20
Q

Which of the 5 R’s becomes an issue for tumor control, usually after 4-6 weeks of therapy?

A

Repopulation - after this cells may repop more quickly; this is why treatment breaks can result in poor tumor control

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21
Q

Repopulation may have a greater adverse effect on slowly/rapidly dividing tumors

A

rapidly

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22
Q

What is the only tissue that early-delayed RT effects have been found? When does this usually happen?

A

Nervous tissue - usually 2wks - 4mo

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23
Q

What are possible causes of early-delayed RT effects?

A

Demyelination Cerebral edema-associated cytokine release w/ tumor cell death

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24
Q

What cytokine is thought to play a critical role in radiation fibrosis?

A

TGF-beta

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25
Q

What strategies are used in human RT to mitigate late RT side effects?

A

use of antioxidants and free radical scavengers (superoxide dismutase, vit E, thiol radioprotectors), vascular directed therapies (clopidogrel, pentoxifylline), ACE inhibitors, antiinflammatory agents (steroids) and stem cell therapies

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26
Q

What four criteria have to be true for a tumor to be considered RT induced?

A
  1. Must arise w/in RT field 2. Sufficient latency b/w RT and development (usually ≥1yr 3. different histology than original tumor 4. tissue where new tumor forms must have been normal prior to RT exposure
27
Q

What three things does SRT require?

A
  1. Macroscopic disease 2. treatment planning and admin that allows rapid dose drop off 3. method of stereotactically verifying patient positioning
28
Q

In veterinary med, pubs show comparable STs between SRT and conventionally fractionated protocols in what tumors? What cancer may have longer STs for SRT vs. conventional fractionation?

A

Comparable: nasal and brain Improved w/ SRT: feline acromegaly

29
Q

What is the surviving fraction (S)?

A

The proportion of remaining cells after a dose of RT that have not been killed

30
Q

What is the equation that can be used to describe the relationship between dose and surviving fraction?

A

Linear quadratic equation: S(D) = e-(αD + βD2) (D = dose, D after β should be squared, not D2 but this program won’t let me double superscript things…)

α and β are constants:

α = cell death that increases linearly, β = cell death that increases in proportion to the square of the dose

31
Q

At low dose fractions, tissues or cells with low α/β ratio are relatively (radiosensitive/radioresistant) compared to those with a high α/β ratio

A

radioresistant

32
Q

What are tissues/cells with a low α/β ratio more radioresistant? Also what types of side effects (acute, late, etc) are these tissues more likely to have?

A

low α/β have greater capacity for repair of sublethal RT damage (i.e. bigger shoulder on a cell survival curve)

more likely to have late tox

33
Q

What are tumors that generally have lower α/β ratios?

A

Melanoma, prostatic tumors, STS, TCC and OSA

34
Q

What is the equation for biologic effective dose (BED)?

A

BED = nd[1 + d/(α/β)]

d = dose per fraction

35
Q

Most early-responding tissues have a (high/low) α/β ratio while late-responding tissues have a (high/low) α/β ratio

A

early: high

Late: low

36
Q

Orhtovoltage RT is preferentially absorbed by what tissue (other than having high skin distribution) which leads to a high rate of what side effect?

A

bone –> high probability of late effects to bone (bone necrosis) high

37
Q

What scenarios might be okay for orthovoltage?

A

Small, superficial tumors (nasal planum) or to superficial tumor beds after surgical excision

38
Q

In tx planning, what is the CTV?

A

Clinical target volume - includes GTV + expansion to account for regional microscopic disease

expansion based on tumor type - ex. sarcomas usually have wider expansion than carcinoma

39
Q

In SRT, what are the differences in GTV, CTV and PTV compared to traditional fractionation?

A

GTV same as traditional

CTV not applied (i.e. GTV = CTV)

PTV usually decreased to spare important adjacent normal tissues

40
Q

What does the shoulder of a cell kill curve represent?

A

Portion of cells that sustain sublethal doses of RT - able to repair to some degree but above a certain dose, cells begin to die

41
Q

What are the advantages and disadvantages of CRT?

A

Ad: fast plans and treatments

disad: not as good for irregular targets (i.e. harder to spare normal tissue); larger PTV (i.e. more normal tissue included in tx field

42
Q

What are the advantages and disadvantages of IMRT?

A

Ad:

  • better for irregularly shaped targets (esp if near critical structures),
  • Build up dose in areas of tumor with minimizing dose in surrounding tissue
  • more efficient treatment planning w/ correct software

disad:

  • longer treatment times
  • more complicated plans - need physics involvement
43
Q

What are the advantages of pre-op RT?

A

lower total dose

smaller tx field

no delay to start of RT

possible downstaging - extent of sx needed may decrease pending reponse to RT

easier to plan - have a target

44
Q

What are the disadvantages of pre-op RT?

A
  • Lack of initial surgical staging
  • Post-surgical wound healing complications possible
  • Hypoxic regions in bulky tumors may decrease response to RT
45
Q

What are the advantages of post-op RT

A
  • Surgical staging - tailoring of RT target volumes and doses
  • RT most effective in microscopic disease
  • No negative impact on post-sx wound healing
  • No delay in surgery
  • Have histo of entire mass - could change dx
46
Q

What are the disadvantages of post-op RT?

A
  • Volume of normal tissue irradiated is large
  • Increased risk of tumor cell dissemination at surgery
  • alteration in blood supply to tumor cells
  • Delay to start of RT if sx complications with wound healing occur
  • Higher overall dose
47
Q

What does 60Cobalt produce and what is its relative energy?

A

Produces gamma-rays as it decays to nickel

Averagy energy relatively low (1.25 MeV), only 0.5cm of skin sparing so difficult to treat deep-seated tumors

48
Q

What depth of skin sparing dose a 6MeV photon bean have? Does this depth increase or decrease with increasing photon energy?

A

6MeV has 1.4cm skin sparing

skin sparing increases as photon energy increases

49
Q

Chemotherapy drugs that are used with concurrent RT ideally kill cells in what phase?

A

S phase - kill in most resistant phase and redistribute more cells into G2-M phase (most sensitive)

50
Q

What was the rate of late complications in dogs treated with >45gy total for pelvic region irradiation? (Arthur VRU 2008)

A

39% had late complications, recommended reducing dose per fraction to < 3Gy

51
Q

What was the outcome of combining RT + gemcitabine as a radiosensitizer for dogs and cats with head/neck carcinomas? (LeBlanc VRU 2004)

A

High dose of tox - 12/15 dogs and 5/10 cats needed dose reduction or treatment delatys due to heme tox or normal tissue tox

conclusion: not a good plan to use this combo

52
Q

What were the AEs associated with concurrent carbo and RT in dogs? What dose of carbo was used? (Hume JVIM 2009)

A

200mg/m2 median dose

21% grade 3-4 neutropenia

20% grade 3-4 thrombocytopenia

10% grade 3-5 GI tox

53
Q

What was the residual setup error for dogs with intracranial tumors for megavoltage, kilovoltage or CBCT image guidance set up? (Morimoto VCO 2019)

A

MV: 1.7mm

kV: 1.5mm

CBCT: 2.2 mm

conclusion - likely only need mm margins for PTV for intracranial region tumors

54
Q

What margin shoud be used w/in the PTV when MV or kV planar imaging is used for RT set up by matching bony landmarks of the skull compared to CBCT? (Magestro VCO 2019)

A

at least 1mm for Mv or kV planar imaging used

55
Q

What was the rate of complications for VAPS in dogs treated with RT? (Mayer JAVMA 2008)

A

17% malposition of catheter tip

30% seroma formation

13% breakage of port-anchoring sutures

1 dog (4%) fatal septicemia

7% of anesthetic episodes had temporary withdrawal occlusion

56
Q

What are the recommendations regarding VAPs and RT based on the Mayer JAVMA 2008 study?

A

Fluoroscopy for positioning the catheter tip

Removal of VAP at completion of RT unless needed

57
Q

What was the rate of complications when surgical flaps were irradiated? (Seguin Vet Surg 2005)

A

77% had complications - necrosis, infection, dehiscence, ulceration

6/26 needed second flap procedure

4 /26 had unresolved complications

58
Q

What was the response rate of sialoceles in dogs to RT? (Poirier JVIM 2018)

A

54% CR, 45% PR

conclusion: RT useful for treating recurrent sialocele refractory to surgery

59
Q

What total dose of RT is effective in treatment for sialoceles in dogs? (Poirier JVIM 2018)

A

16-20 Gy

all originally had at least 12Gy but 2 dogs needed additional RT to 20Gy total

60
Q

What was significantly associated with developing pneumonia in dogs receiving repeated anesthesia for RT? (Baetge VRU 2018)

A

Presence of a neuro tumor

Presence of respiratory disease

Presence of megaesophagus

# of fractions of RT completed

61
Q

What changes in anesthetic protocol resulted in decreased risk of pneumonia in dogs treated with RT and repeated anesthesia (Baetge VRU 2018)

A
  • decrease in anticholinergic (glycopy.) and pure-mu opioid use (hydro, changed to torb if needed)
  • change in positioning during intubation and recovery (sternal w/ head raised)
  • prophylactic treatment of nausea (metoclopramide)
  • timing of cuff inflation and deflation
  • aseptic handling of intubation equipment
62
Q

What was the result of treatment with prophylactic cephalexin in dogs starting halfway through definitive RT protocols? (Keyerleber Vet Derm, 2018)

A
  • No sig difference in prevalence of bacterial infection, but MDR infections significantly increased for dogs with cephalexin compared to controls
  • Clinician and client-perceived severity of tox greater and median duration of moist desquamation was sig longer in dogs getting cephalexin
  • conclusion: no prophylactic cephalexin
63
Q
A

Oncology Boards (26 decks)

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